Exception handling, Stream Classes, Multithread Programming

Lecture By,
Prabu.U
Assistant Professor,
Department of Computer Science and Engineering.
DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING,
V R Siddhartha Engineering College.
20ES3102 – JAVA PROGRAMMING
UNIT 3

UNIT III:
Exception handling: Exception handling fundamentals, exception types,
uncaught exceptions, using try and catch, multiple catch clauses, throw,
throws, finally, creating your own exception subclasses.
Stream Classes: Byte Streams- InputStream, OutputStream,
FileInputStream, FileOutputStream, Character Streams- Reader, Writer,
FileReader, FileWriter.
Multithread Programming: The Java Thread Model, creating a thread:
Implementing Runnable, Extending Thread, creating multiple threads,
Thread Priorities, Synchronization: Using Synchronized methods, The
synchronized Statement.
20ES3102 – JAVA PROGRAMMING

1. Exception-Handling Fundamentals
2. Exception Types
3. Uncaught Exceptions
4. Using try and catch
5. Multiple catch Clauses
6. throw, throws, finally
7. Creating Your Own Exception Subclasses
EXCEPTION HANDLING

1. Byte Streams
 InputStream
 OutputStream
 FileInputStream
 FileOutputStream
2. Character Streams
 Reader
 Writer
 FileReader
 FileWriter
STREAM CLASSES

1. The Java Thread Model
2. Creating a Thread
 Implementing Runnable
 Extending Thread
3. Creating Multiple Threads
4. Thread Priorities
5. Synchronization
 Using Synchronized methods
 The synchronized Statement
MULTITHREADED PROGRAMMING

1. Exception-Handling Fundamentals
 A Java exception is an object that describes an exceptional (that is,
error) condition that has occurred in a piece of code.
 When an exceptional condition arises, an object representing that
exception is created and thrown in the method that caused the
error.
 That method may choose to handle the exception itself or pass it
on. Either way, at some point, the exception is caught and
processed.
 Exceptions can be generated by the Java run-time system, or they
can be manually generated by your code

 Exceptions thrown by Java relate to fundamental errors that violate
the rules of the Java language or the constraints of the Java
execution environment.
 Manually generated exceptions are typically used to report some
error condition to the caller of a method.
 Java exception handling is managed via five keywords: try, catch,
throw, throws, and finally.
 Program statements that you want to monitor for exceptions are
contained within a try block.

 If an exception occurs within the try block, it is thrown.
 Your code can catch this exception (using catch) and handle it in
some rational manner.
 System-generated exceptions are automatically thrown by the Java
runtime system.
 To manually throw an exception, use the keyword throw. Any
exception that is thrown out of a method must be specified as such
by a throws clause.
 Any code that absolutely must be executed after a try block
completes is put in a finally block.

This is the general form of an exception-handling block:
try
{
// block of code to monitor for errors
}
catch (ExceptionType1 exOb)
{
// exception handler for ExceptionType1
}
catch (ExceptionType2 exOb)
{
// exception handler for ExceptionType2
}
// ...
finally
{
// block of code to be executed after try block ends
}

2. Exception Types
 All exception types are subclasses of the built-in class Throwable.
Thus, Throwable is at the top of the exception class hierarchy.
 Immediately below Throwable are two subclasses that partition
exceptions into two distinct branches. One branch is headed by
Exception.
 This class is used for exceptional conditions that user programs
should catch.
 This is also the class that you will subclass to create your own
custom exception types.

 There is an important subclass of Exception, called
RuntimeException.
 Exceptions of this type are automatically defined for the programs
that you write and include things such as division by zero and
invalid array indexing.
 The other branch is topped by Error, which defines exceptions that
are not expected to be caught under normal circumstances by your
program.
 Exceptions of type Error are used by the Java run-time system to
indicate errors having to do with the run-time environment, itself.
Stack overflow is an example of such an error.

Exception handling, Stream Classes, Multithread Programming

3. Uncaught Exceptions
 This small program includes an expression that intentionally
causes a divide-by-zero error:
class Exc0
{
public static void main(String args[])
{
int d = 0;
int a = 42 / d;
}
}

 When the Java run-time system detects the attempt to divide by zero, it
constructs a new exception object and then throws this exception.
 This causes the execution of Exc0 to stop, because once an exception has
been thrown, it must be caught by an exception handler and dealt with
immediately.
 In this example, we haven’t supplied any exception handlers of our own,
so the exception is caught by the default handler provided by the Java run-
time system.
 Any exception that is not caught by your program will ultimately be
processed by the default handler.
 The default handler displays a string describing the exception, prints a
stack trace from the point at which the exception occurred, and terminates
the program.

 Here is the exception generated when this example is executed:
java.lang.ArithmeticException: / by zero
at Exc0.main(Exc0.java:4)

4. Using try and catch
 Although the default exception handler provided by the Java run-
time system is useful for debugging, you will usually want to
handle an exception yourself.
 Doing so provides two benefits. First, it allows you to fix the error.
Second, it prevents the program from automatically terminating.
 To handle a run-time error, simply enclose the code that you want
to monitor inside a try block.
 Immediately following the try block, include a catch clause that
specifies the exception type that you wish to catch.

class Exc2
{
{
int d, a;
// monitor a block of code
try
{
d = 0;
a = 42 / d;
System.out.println("This will not be printed.");
}

// catch divide-by-zero error
catch (ArithmeticException e)
{
System.out.println("Division by zero.");
}
System.out.println("After catch statement.");
}
}
Division by zero.
After catch statement.
 Once an exception is thrown, program control transfers out of the try
block into the catch block.

 A try and its catch statement form a unit.
 The scope of the catch clause is restricted to those statements
specified by the immediately preceding try statement.
 A catch statement cannot catch an exception thrown by another try
statement (except in the case of nested try statements, described
shortly).
 The statements that are protected by try must be surrounded by
curly braces (That is, they must be within a block.)

Java Random Class Methods
nextBoolean() - This method returns next pseudorandom which is a
boolean value from random number generator sequence.
nextDouble() - his method returns next pseudorandom which is double
value between 0.0 and 1.0.
nextFloat() - This method returns next pseudorandom which is float value
between 0.0 and 1.0.
nextInt() - This method returns next int value from random number
generator sequence.
nextInt(int n) - This method return a pseudorandom which is int value
between 0 and specified value from random number generator sequence.

import java.util.Random;
public class RandomNumberExample
{
public static void main(String[] args)
{
//initialize random number generator
Random random = new Random();
//generates boolean value
System.out.println(random.nextBoolean());
//generates double value
System.out.println(random.nextDouble());

//generates float value
System.out.println(random.nextFloat());
//generates int value
System.out.println(random.nextInt());
//generates int value within specific limit
System.out.println(random.nextInt(20));
}
}

false
0.21218821288158107
0.96327835
930869598
6

class HandleError
{
{
int a=0, b=0, c=0;
Random r = new Random();
for(int i=0; i<10; i++)
{
try
{
b = r.nextInt();
c = r.nextInt();
a = 12345 / (b/c);
}

{
System.out.println("Division by zero.");
a = 0; // set a to zero and continue
}
System.out.println("a: " + a);
}
}
}

a: -12345
Division by zero.
a: 0
Division by zero.
a: 0
Division by zero.
a: 0
Division by zero.
a: 0
Division by zero.
a: 0
a: -2469
a: -12345
Division by zero.
a: 0
a: 4115

 In the next program each iteration of the for loop obtains two
random integers.
 Those two integers are divided by each other, and the result is used
to divide the value 12345.
 The final result is put into a. If either division operation causes a
divide-by-zero error, it is caught, the value of a is set to zero, and
the program continues.

Displaying a Description of an Exception
 Throwable overrides the toString( ) method (defined by Object) so
that it returns a string containing a description of the exception.
 You can display this description in a println( ) statement by simply
passing the exception as an argument.

class HandleErrorDescription
{
{
int a=0, b=0, c=0;
Random r = new Random();
for(int i=0; i<10; i++)
{
try
{
b = r.nextInt();
c = r.nextInt();
a = 12345 / (b/c);
}

{
System.out.println("Exception: " + e);
a = 0; // set a to zero and continue
}
System.out.println("a: " + a);
}
}
}

Output:
Exception: java.lang.ArithmeticException: / by zero
a: 0
a: 0
a: 0
a: -6172
a: 12345
a: 12345
a: 0
a: 6172
a: 12345
a: 12345

5. Multiple catch Clauses
 In some cases, more than one exception could be raised by a single
piece of code.
 To handle this type of situation, you can specify two or more catch
clauses, each catching a different type of exception.
 When an exception is thrown, each catch statement is inspected in
order, and the first one whose type matches that of the exception is
executed.
 After one catch statement executes, the others are bypassed, and
execution continues after the try / catch block.

class MultipleCatches
{
{
try
{
int a = args.length;
System.out.println("a = " + a);
int b = 42 / a;
int c[] = { 1 };
c[42] = 99;
}

catch(ArithmeticException e)
{
System.out.println("Divide by 0: " + e);
}
catch(ArrayIndexOutOfBoundsException e)
{
System.out.println("Array index oob: " + e);
}
System.out.println("After try/catch blocks.");
}
}

a = 0
Divide by 0: java.lang.ArithmeticException: / by zero
After try/catch blocks.

6. throw, throws, finally
throw
 So far, you have only been catching exceptions that are thrown by
the Java run-time system.
 However, it is possible for your program to throw an exception
explicitly, using the throw statement. The general form of throw is
shown here:
throw ThrowableInstance;

 Here, ThrowableInstance must be an object of type Throwable or a
subclass of Throwable.
 Primitive types, such as int or char, as well as non-Throwable
classes, such as String and Object, cannot be used as exceptions.
 There are two ways you can obtain a Throwable object: using a
parameter in a catch clause or creating one with the new operator.
 The flow of execution stops immediately after the throw statement;
any subsequent statements are not executed.

 The nearest enclosing try block is inspected to see if it has a catch
statement that matches the type of exception.
 If it does find a match, control is transferred to that statement.
 If not, then the next enclosing try statement is inspected, and so on.
 If no matching catch is found, then the default exception handler
halts the program and prints the stack trace.

public class TestThrow
{
static void validate(int age)
{
if(age<18)
throw new ArithmeticException("not valid");
else
System.out.println("welcome to vote");
}
{
validate(13);
System.out.println("rest of the code...");
}
}

Exception in thread "main" java.lang.ArithmeticException: not valid
at TestThrow.validate(TestThrow.java:6)
at TestThrow.main(TestThrow.java:12)

throws
 The throws keyword is used in the method declaration to declare
the type of exceptions that might occur within it.
 Its syntax is:
accessModifier returnType methodName() throws
ExceptionType1, ExceptionType2 …
{
// code
}

import java.io.*;
class TestThrows
{
public static void findFile() throws IOException
{
// code that may produce IOException
File newFile=new File("test.txt");
FileInputStream stream=new FileInputStream(newFile);
}

{
try
{
findFile();
}
catch(IOException e)
{
System.out.println(e);
}
}
}
java.io.FileNotFoundException: test.txt (The system cannot find the file
specified)

Differences between throw and throws

finally
 finally creates a block of code that will be executed after a try /catch
block has completed and before the code following the try/catch
block.
 The finally block will execute whether or not an exception is thrown.
 If an exception is thrown, the finally block will execute even if no
catch statement matches the exception.
 Any time a method is about to return to the caller from inside a
try/catch block, via an uncaught exception or an explicit return
statement, the finally clause is also executed just before the method
returns.

 This can be useful for closing file handles and freeing up any other
resources that might have been allocated at the beginning of a
method with the intent of disposing of them before returning.
 The finally clause is optional. However, each try statement requires
at least one catch or a finally clause.

public class TestFinally
{
{
try
{
int data=25/0;
System.out.println(data);
}
catch(ArithmeticException e)
{
System.out.println(e);
}

finally
{
System.out.println("finally block is always executed");
}
System.out.println("rest of the code...");
}
}
java.lang.ArithmeticException: / by zero
finally block is always executed
rest of the code...

7. Creating Your Own Exception Subclasses
 Although Java’s built-in exceptions handle most common errors,
you will probably want to create your own exception types to
handle situations specific to your applications.
 This is quite easy to do: just define a subclass of Exception (which
is, of course, a subclass of Throwable).
 Your subclasses don’t need to actually implement anything—it is
their existence in the type system that allows you to use them as
exceptions.

 The Exception class does not define any methods of its own. It
does, of course, inherit those methods provided by Throwable.
 Thus, all exceptions, including those that you create, have the
methods defined by Throwable available to them.
 You may also wish to override one or more of these methods in
exception classes that you create.
 Exception defines four public constructors. Two support chained
exceptions, described in the next section. The other two are shown
here:
 Exception( )
 Exception(String msg)

 The first form creates an exception that has no description. The
second form lets you specify a description of the exception.
 Although specifying a description when an exception is created is
often useful, sometimes it is better to override toString( ). Here’s
why: The version of toString( ) defined by Throwable (and
inherited by Exception) first displays the name of the exception
followed by a colon, which is then followed by your description.
 By overriding toString( ), you can prevent the exception name and
colon from being displayed.
 This makes for a cleaner output, which is desirable in some cases.

class MyException extends Exception
{
private int detail;
MyException(int a)
{
detail = a;
}
public String toString()
{
return "MyException[" + detail + "]";
}
}

class ExceptionDemo
{
static void compute(int a) throws MyException
{
System.out.println("Called compute(" + a + ")");
if(a > 10)
throw new MyException(a);
System.out.println("Normal exit");
}

{
try
{
compute(1);
compute(20);
}
catch (MyException e)
{
System.out.println("Caught " + e);
}
}
}

Called compute(1)
Normal exit
Called compute(20)
Caught MyException[20]

Chained Exceptions
 To allow chained exceptions, two constructors and two methods were
added to Throwable. The constructors are shown here:
 Throwable(Throwable causeExc)
 Throwable(String msg, Throwable causeExc)
 In the first form, causeExc is the exception that causes the current
exception. That is, causeExc is the underlying reason that an exception
occurred.
 The second form allows you to specify a description at the same time
that you specify a cause exception.
 These two constructors have also been added to the Error, Exception,
and RuntimeException classes.

 The chained exception methods supported by Throwable are
getCause( ) and initCause( ).
 Throwable getCause( )
 Throwable initCause(Throwable causeExc)
 The getCause( ) method returns the exception that underlies the
current exception.
 If there is no underlying exception, null is returned. The
initCause( ) method associates causeExc with the invoking
exception and returns a reference to the exception.
 Thus, you can associate a cause with an exception after the
exception has been created.

 However, the cause exception can be set only once.
 Thus, you can call initCause( ) only once for each exception object.
 Furthermore, if the cause exception was set by a constructor, then
you can’t set it again using initCause( ).
 In general, initCause( ) is used to set a cause for legacy exception
classes that don’t support the two additional constructors described
earlier.

class ChainExcDemo
{
static void demoproc()
{
// create an exception
NullPointerException e = new NullPointerException("top layer");
// add a cause
e.initCause(new ArithmeticException("cause"));
throw e;
}

{
try
{
demoproc();
}
catch(NullPointerException e)
{
// display top level exception
System.out.println("Caught: " + e);
// display cause exception
System.out.println("Original cause: " +e.getCause());
}
}
}

Caught: java.lang.NullPointerException: top layer
Original cause: java.lang.ArithmeticException: cause

Multi-catch Features
 The multi-catch feature allows two or more exceptions to be caught
by the same catch clause.
 It is not uncommon for two or more exception handlers to use the
same code sequence even though they respond to different
exceptions.
 Instead of having to catch each exception type individually, you can
use a single catch clause to handle all of the exceptions without
code duplication.

 To use a multi-catch, separate each exception type in the catch
clause with the OR operator.
 Each multi-catch parameter is implicitly final. (You can explicitly
specify final, if desired, but it is not necessary.)
 Because each multi-catch parameter is implicitly final, it can’t be
assigned a new value.

class MultiCatch
{
{
int a=10, b=0;
int vals[] = { 1, 2, 3 };
try
{
// generates an ArithmeticException
int result = a / b;
// generates an ArrayIndexOutOfBoundsException
vals[10] = 19;
// This catch clause catches both exceptions.
}

catch(ArithmeticException | ArrayIndexOutOfBoundsException e)
{
System.out.println("Exception caught: " + e);
}
System.out.println("After multi-catch.");
}
}
Exception caught: java.lang.ArithmeticException: / by zero
After multi-catch.

Overview of I/O Streams
 An I/O Stream represents an input source or an output destination.
 A stream can represent many different kinds of sources and
destinations, including disk files, devices, other programs, and
memory arrays.
 Streams support many different kinds of data, including simple
bytes, primitive data types, localized characters, and objects.
 Some streams simply pass on data; others manipulate and
transform the data in useful ways.

 No matter how they work internally, all streams present the same
simple model to programs that use them: A stream is a sequence of
data.
 A program uses an input stream to read data from a source, one
item at a time:

 A program uses an output stream to write data to a destination, one
item at time:

What is a Stream?
 Abstraction that consumes or produces information
 Linked to source and destination
 Implemented within class hierarchies defined in java.io.package
 An input stream acts as a source of data
 An output stream acts as a destination of data

Different Types of I/O Streams
Byte Streams:
 They provide a convenient means for handling input and output of
bytes.
 Programs use byte streams to perform input and output of 8-bit
bytes.
 All byte stream classes descend from InputStream and
OutputStream class.
Character Streams:
 They provide a convenient means for handling input and output of
characters.
 They use Unicode and therefore, can be internationalized.

Buffered Streams:
 Buffered input streams read data from a memory area known as a
buffer; the native input API is called only when the buffer is empty.
 Similarly, buffered output streams write data to a buffer, and the native
output API is called only when the buffer is full.
Data Streams:
 Data streams support binary I/O of primitive data type values (Boolean,
char, byte, short, int, long, float, and double) as well as String values.
Object Streams:
 Just as data streams support I/O of primitive data types, object streams
support I/O of objects.
Scanning and Formatting:
 It allows a program to read and write formatted text.

The Predefined Streams
 The java.lang.System class encapsulates several aspects of the run-
time environment.
 Contains three predefined stream variables: in,out, and err.
 These fields are declared as public and static within System.
 System.out: refers to the standard output stream
 System.err: refers to standard error stream
 System.in: refers to standard input
 out and err are objects of type PrintStream class and in is an object
of the InputStream class.

Streams Classes
 Java’s stream-based I/O is built upon four abstract classes:
InputStream, OutputStream, Reader, and Writer.
 InputStream and OutputStream are designed for byte streams.
Reader and Writer are designed for character streams.
 The byte stream classes and the character stream classes form
separate hierarchies.
 In general, you should use the character stream classes when
working with characters or strings and use the byte stream classes
when working with bytes or other binary objects.

1. Byte Streams
 The byte stream classes provide a rich environment for handling
byte-oriented I/O.
 A byte stream can be used with any type of object, including binary
data.
 This versatility makes byte streams important to many types of
programs.

InputStream
 InputStream is an abstract class that defines Java’s model of
streaming byte input.
 It implements the AutoCloseable and Closeable interfaces.
 Most of the methods in this class will throw an IOException when
an I/O error occurs. (The exceptions are mark( ) and
markSupported( ).)
 The methods in InputStream is shown in the table

OutputStream
 OutputStream is an abstract class that defines streaming byte
output.
 It implements the AutoCloseable, Closeable, and Flushable
interfaces.
 Most of the methods defined by this class return void and throw an
IOException in the case of I/O errors.
 The methods in OutputStream is shown in the table.

FileInputStream
 The FileInputStream class creates an InputStream that you can use
to read bytes from a file.
 Two commonly used constructors are shown here:
 FileInputStream(String filePath)
 FileInputStream(File fileObj)
 Either can throw a FileNotFoundException. Here, filePath is the full
path name of a file, and fileObj is a File object that describes the file.

 The FileInputStream class of the java.io package can be used to read
data (in bytes) from files.
 It extends the InputStream abstract class.

Create a FileInputStream
 In order to create a file input stream, we must import the
java.io.FileInputStream package first.
 Once we import the package, here is how we can create a file input
stream in Java.
1. Using the path to file
FileInputStream input = new FileInputStream(stringPath);
Here, we have created an input stream that will be linked to the file
specified by the path.

2. Using an object of the file
FileInputStream input = new FileInputStream(File fileObject);
Here, we have created an input stream that will be linked to the file
specified by fileObject.
Methods of FileInputStream
 read() - reads a single byte from the file
 read(byte[] array) - reads the bytes from the file and stores in the
specified array
 read(byte[] array, int start, int length) - reads the number of bytes
equal to length from the file and stores in the specified array
starting from the position start

import java.io.*;
public class FIS
{
{
try
{
FileInputStream input = new FileInputStream("Sample.txt");
System.out.println("Data in the file: ");
// Reads the first byte
int i = input.read();
while(i != -1)
{
System.out.print((char)i);
// Reads next byte from the file
i = input.read();
}

input.close();
}
catch(IOException e)
{
System.out.println("Error:" +e);
}
}
}
Data in the file:
Hello World

available() Method
 To get the number of available bytes, we can use the available() method.
import java.io.*;
public class AvailableExample
{
{
try
{
// Returns the number of available bytes
System.out.println("Available bytes at the beginning: " +
input.available());

// Reads 2 bytes from the file
input.read();
input.read();
// Returns the number of available bytes
System.out.println("Available bytes at the end: " + input.available());
input.close();
}
catch (Exception e)
{
}
}
}
Available bytes at the beginning: 11
Available bytes at the end: 9

skip() Method
 To discard and skip the specified number of bytes, we can use the skip()
method.
import java.io.*;
public class SkipExample
{
{
try
{
// Skips the 2 bytes
input.skip(2);
System.out.println("Input stream after skipping 2 bytes:");

// Reads the first byte
int i = input.read();
while (i != -1)
{
System.out.print((char) i);
i = input.read();
}
input.close();
}
catch (Exception e)
{
}
}
}

Input stream after skipping 2 bytes:
llo World

FileOutputStream
 FileOutputStream creates an OutputStream that you can use to
write bytes to a file.
 It implements the AutoCloseable, Closeable, and Flushable
interfaces.
 Four of its constructors are shown here:
 FileOutputStream(String filePath)
 FileOutputStream(File fileObj)
 FileOutputStream(String filePath, boolean append)
 FileOutputStream(File fileObj, boolean append)

 They can throw a FileNotFoundException.
 Here, filePath is the full path name of a file, and fileObj is a File
object that describes the file.
 If append is true, the file is opened in append mode.
 Creation of a FileOutputStream is not dependent on the file already
existing.
 FileOutputStream will create the file before opening it for output
when you create the object.
 In the case where you attempt to open a read-only file, an exception
will be thrown.

 The FileOutputStream class of the java.io package can be used to
write data (in bytes) to the files.
 It extends the OutputStream abstract class.

Create a FileOutputStream
 In order to create a file output stream, we must import the
java.io.FileOutputStream package first.
 Once we import the package, here is how we can create a file
output stream in Java.
1. Using the path to file
// Including the boolean parameter
FileOutputStream output = new FileOutputStream(String path,
boolean value);
// Not including the boolean parameter
FileOutputStream output = new FileOutputStream(String path);

 Here, we have created an output stream that will be linked to the
file specified by the path.
 Also, value is an optional boolean parameter.
 If it is set to true, the new data will be appended to the end of the
existing data in the file.
 Otherwise, the new data overwrites the existing data in the file.

FileOutputStream output = new FileOutputStream(File fileObject);
Here, we have created an output stream that will be linked to the file
specified by fileObject.
Methods of FileOutputStream
write() - writes the single byte to the file output stream
write(byte[] array) - writes the bytes from the specified array to the
output stream
write(byte[] array, int start, int length) - writes the number of bytes
equal to length to the output stream from an array starting from the
position start

import java.io.*;
public class FOS
{
{
String data = "Output File";
try
{
FileOutputStream output = new FileOutputStream("outfile.txt");
byte[] b = data.getBytes();
// Writes byte to the file
output.write(b);
output.close();
}

catch(Exception e)
{
}
}
}
outfile.txt
Output File

flush() Method
 To clear the output stream, we can use the flush() method. This method forces
the output stream to write all data to the destination.
import java.io.*;
public class FlushExample
{
public static void main(String[] args) throws IOException
{
FileOutputStream out = null;
String data = "Printed by flush method";
try
{
out = new FileOutputStream("flush.txt");
// Using write() method
out.write(data.getBytes());

// Using the flush() method
out.flush();
out.close();
}
catch(Exception e)
{
}
}
}
flush.txt
Printed by flush method

ByteArrayInputStream
 The ByteArrayInputStream class of the java.io package can be used
to read an array of input data (in bytes).

Create a ByteArrayInputStream
 In order to create a byte array input stream, we must import the
java.io.ByteArrayInputStream package first.
// Creates a ByteArrayInputStream that reads entire array
ByteArrayInputStream input = new ByteArrayInputStream(byte[] arr);
// Creates a ByteArrayInputStream that reads a portion of array
ByteArrayInputStream input = new ByteArrayInputStream(byte[] arr,
int start, int length);

import java.io.ByteArrayInputStream;
public class ByteArrayInputExample
{
{
// Creates an array of byte
byte[] array = {1, 2, 3, 4};
//byte[] array ={'A','B','C'};
try
{
ByteArrayInputStream input = new ByteArrayInputStream(array);
System.out.print("The bytes read from the input stream: ");

for(int i= 0; i < array.length; i++)
{
// Reads the bytes
int data = input.read();
System.out.print(data + ", ");
//System.out.print((char)data + ", ");
}
input.close();
}
catch(Exception e)
{
e.getStackTrace();
}
}
}
The bytes read from the input stream: 1, 2, 3, 4,

ByteArrayOutputStream
 The ByteArrayOutputStream class of the java.io package can be
used to write an array of output data (in bytes).
 ByteArrayOutputStream maintains an internal array of bytes to
store the data.

Create a ByteArrayOutputStream
 In order to create a byte array output stream, we must import the
java.io.ByteArrayOutputStream package first.
// Creates a ByteArrayOutputStream with default size
ByteArrayOutputStream out = new ByteArrayOutputStream();
 Here, we have created an output stream that will write data to an
array of bytes with default size 32 bytes. However, we can change
the default size of the array.
// Creating a ByteArrayOutputStream with specified size
ByteArrayOutputStream out = new ByteArrayOutputStream(int size);
 The size specifies the length of the array.

Access Data from ByteArrayOutputStream
toByteArray() - returns the array present inside the output stream
toString() - returns the entire data of the output stream in string form
import java.io.ByteArrayOutputStream;
class ByteArrayOutputExample
{
{
String data = "This is data.";
try
{
// Creates an output stream
ByteArrayOutputStream output = new ByteArrayOutputStream();
// Writes data to the output stream
output.write(data.getBytes());

// Returns an array of bytes
byte[] byteData = output.toByteArray();
System.out.print("Data using toByteArray(): ");
for(int i=0; i<byteData.length; i++)
{
System.out.print((char)byteData[i]);
}
// Returns a string
String stringData = output.toString();
System.out.println("nData using toString(): " + stringData);
output.close();
}

catch(Exception e)
{
e.getStackTrace();
}
}
}
Data using toByteArray(): This is data.
Data using toString(): This is data.

ObjectInputStream
 The ObjectInputStream class of the java.io package can be used to
read objects that were previously written by ObjectOutputStream.

Working of ObjectInputStream
 The ObjectInputStream is mainly used to read data written by the
ObjectOutputStream.
 Basically, the ObjectOutputStream converts Java objects into
corresponding streams. This is known as serialization. Those
converted streams can be stored in files or transferred through
networks.
 Now, if we need to read those objects, we will use the
ObjectInputStream that will convert the streams back to
corresponding objects. This is known as deserialization.

Create an ObjectInputStream
 In order to create an object input stream, we must import the
java.io.ObjectInputStream package first.
// Creates a file input stream linked with the specified file
FileInputStream fileStream = new FileInputStream(String file);
// Creates an object input stream using the file input stream
ObjectInputStream objStream = new ObjectInputStream(fileStream);
 An object input stream named objStream is created that is linked
with the file input stream named fileStream.
 Now, the objStream can be used to read objects from the file.

Methods of ObjectInputStream
 The ObjectInputStream class provides implementations of different
methods present in the InputStream class.
read() Method
read() - reads a byte of data from the input stream
readBoolean() - reads data in boolean form
readChar() - reads data in character form
readInt() - reads data in integer form
readObject() - reads the object from the input stream

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
class ObjectInputExample
{
{
int data1 = 200;
String data2 = "Two Hundred";
try
{
FileOutputStream file = new FileOutputStream("Objectinfile.txt");
ObjectOutputStream output = new ObjectOutputStream(file);
// Writing to the file using ObjectOutputStream
output.writeInt(data1);
output.writeObject(data2);

FileInputStream fileStream = new FileInputStream("Objectinfile.txt");
// Creating an object input stream
//Using the readInt() method
System.out.println("Integer data :" + objStream.readInt());
// Using the readObject() method
System.out.println("String data: " + objStream.readObject());
output.close();
objStream.close();
}

catch (Exception e)
{
e.getStackTrace();
}
}
}
Integer data :200
String data: Two Hundred

ObjectOutputStream
 The ObjectOutputStream class of the java.io package can be used to
write objects that can be read by ObjectInputStream.

Working of ObjectOutputStream
 Basically, the ObjectOutputStream encodes Java objects using the
class name and object values. And, hence generates corresponding
streams. This process is known as serialization.
 Those converted streams can be stored in files and can be
transferred among networks.
 The ObjectOutputStream class only writes those objects that
implement the Serializable interface. This is because objects need to
be serialized while writing to the stream.

Create an ObjectOutputStream
 In order to create an object output stream, we must import the
java.io.ObjectOutputStream package first.
// Creates a FileOutputStream where objects from
ObjectOutputStream are written
FileOutputStream fileStream = new FileOutputStream(String file);
// Creates the ObjectOutputStream
ObjectOutputStream objStream = new ObjectOutputStream(fileStream);
 An object output stream named objStream is created which is linked
with the file output stream named fileStream.

Methods of ObjectOutputStream
 The ObjectOutputStream class provides implementations for
different methods present in the OutputStream class.
write() Method
write() - writes a byte of data to the output stream
writeBoolean() - writes data in boolean form
writeChar() - writes data in character form
writeInt() - writes data in integer form
writeObject() - writes object to the output stream

import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
class ObjectOutputExample
{
{
int data1 = 300;
String data2 = "Three Hundred";
try
{
FileOutputStream file = new FileOutputStream("ObjectOutfile.txt");
// Creates an ObjectOutputStream
ObjectOutputStream output = new ObjectOutputStream(file);
// writes objects to output stream
output.writeInt(data1);
output.writeObject(data2);

// Reads data using the ObjectInputStream
FileInputStream fileStream = new FileInputStream("ObjectOutfile.txt");
System.out.println("Integer data :" + objStream.readInt());
System.out.println("String data: " + objStream.readObject());
output.close();
objStream.close();
}
catch (Exception e)
{
e.getStackTrace();
}
}
}

Integer data :300
String data: Three Hundred

BufferedInputStream
 The BufferedInputStream class of the java.io package is used with
other input streams to read the data (in bytes) more efficiently.
 It extends the InputStream abstract class

Working of BufferedInputStream
 The BufferedInputStream maintains an internal buffer of 8192 bytes.
 During the read operation in BufferedInputStream, a chunk of bytes
is read from the disk and stored in the internal buffer. And from the
internal buffer bytes are read individually.
 Hence, the number of communication to the disk is reduced. This is
why reading bytes is faster using the BufferedInputStream.

Create a BufferedInputStream
 In order to create a BufferedInputStream, we must import the
java.io.BufferedInputStream package first.
// Creates a FileInputStream
FileInputStream file = new FileInputStream(String path);
// Creates a BufferedInputStream
BufferedInputStream buffer = new BufferInputStream(file);
 A BufferdInputStream named buffer with the FileInputStream
named file.
 Here, the internal buffer has the default size of 8192 bytes. However,
we can specify the size of the internal buffer as well.

// Creates a BufferedInputStream with specified size internal buffer
BufferedInputStream buffer = new BufferInputStream(file, int size);
 The buffer will help to read bytes from the files more quickly.

Methods of BufferedInputStream
 The BufferedInputStream class provides implementations for
different methods present in the InputStream class.
read() Method
read() - reads a single byte from the input stream
read(byte[] arr) - reads bytes from the stream and stores in the
specified array
read(byte[] arr, int start, int length) - reads the number of bytes equal
to the length from the stream and stores in the specified array starting
from the position start

import java.io.BufferedInputStream;
class BufferedInputExample
{
{
try
{
FileInputStream file = new FileInputStream("BufIn.txt");

// Creates a BufferedInputStream
BufferedInputStream input = new BufferedInputStream(file);
// Reads first byte from file
int i = input .read();
while (i != -1)
{
System.out.print((char) i);
i = input.read();
}
input.close();
}

catch (Exception e)
{
e.getStackTrace();
}
}
}
This line is printed from BufIn File

PrintStream
 The PrintStream class of the java.io package can be used to write
output data in commonly readable form (text) instead of bytes.
 It extends the abstract class OutputStream.

Create a PrintStream
 In order to create a PrintStream, we must import the
java.io.PrintStream package first.
1. Using other output streams
// Creates a FileOutputStream
FileOutputStream file = new FileOutputStream(String file);
// Creates a PrintStream
PrintStream output = new PrintStream(file, autoFlush);

 Here, we have created a print stream that will write formatted data
to the file represented by FileOutputStream
 the autoFlush is an optional boolean parameter that specifies
whether to perform auto flushing or not.
2. Using filename
// Creates a PrintStream
PrintStream output = new PrintStream(String file, boolean autoFlush);
 We have created a print stream that will write formatted data to the
specified file.
 autoFlush is an optional boolean parameter that specifies whether to
perform autoflush or not.

Methods of PrintStream
The PrintStream class provides various methods that allow us to print
data to the output.
print() Method
print() - prints the specified data to the output stream
println() - prints the data to the output stream along with a new line
character at the end

Example: print() method with System class
class Main
{
{
String data = "Hello World.";
System.out.print(data);
}
}
Hello World.

Example: print() method with PrintStream class
import java.io.PrintStream;
class Main
{
{
String data = "This is a text written to Print file.";
try
{
PrintStream output = new PrintStream("Print.txt");
output.print(data);
output.close();
}

catch(Exception e)
{
e.getStackTrace();
}
}
}
This is a text written to Print file.

2. Character Streams
 Reader
 Writer
 FileReader
 FileWriter
STREAM CLASSES

Reader
 The Reader class of the java.io package is an abstract superclass that
represents a stream of characters.
 Since Reader is an abstract class, it is not useful by itself. However,
its subclasses can be used to read data.
Subclasses of Reader
In order to use the functionality of Reader, we can use its subclasses.
Some of them are:
 BufferedReader
 InputStreamReader
 FileReader
 StringReader

Create a Reader
 In order to create a Reader, we must import the java.io.Reader package
first.
// Creates a Reader
Reader input = new FileReader();
Methods of Reader
ready() - checks if the reader is ready to be read
read(char[] array) - reads the characters from the stream and stores in the
specified array
read(char[] array, int start, int length) - reads the number of characters equal
to length from the stream and stores in the specified array starting from the
start
mark() - marks the position in the stream up to which data has been read
reset() - returns the control to the point in the stream where the mark is set
skip() - discards the specified number of characters from the stream

import java.io.Reader;
import java.io.FileReader;
class ReaderExample
{
{
// Creates an array of character
char[] array = new char[100];
try
{
// Creates a reader using the FileReader
Reader input = new FileReader("ReaderInput.txt");

// Checks if reader is ready
System.out.println("Is there data in the stream? " + input.ready());
// Reads characters
input.read(array);
System.out.println("Data in the stream:");
System.out.println(array);
// Closes the reader
input.close();
}
catch(Exception e)
{
e.getStackTrace();
}
}
}
This line is printed from the ReaderInput File.

Writer
 The Writer class of the java.io package is an abstract superclass that
represents a stream of characters.
Subclasses of Writer
 In order to use the functionality of the Writer, we can use its
subclasses. Some of them are:
 BufferedWriter
 OutputStreamWriter
 FileWriter
 StringWriter

Create a Writer
 In order to create a Writer, we must import the java.io.Writer
package first.
// Creates a Writer
Writer output = new FileWriter();
 Here, we have created a writer named output using the FileWriter
class. It is because the Writer is an abstract class. Hence we cannot
create an object of Writer.

The Writer class provides different methods that are implemented by
its subclasses. Here are some of the methods:
write(char[] array) - writes the characters from the specified array to
the output stream
write(String data) - writes the specified string to the writer
append(char c) - inserts the specified character to the current writer
flush() - forces to write all the data present in the writer to the
corresponding destination
close() - closes the writer

import java.io.FileWriter;
import java.io.Writer;
public class WriterExample
{
{
String data = "This line is printed from FileWriterOut File";
try
{
// Creates a Writer using FileWriter
Writer output = new FileWriter("FileWriterOut.txt");
// Writes string to the file
output.write(data);

// Closes the writer
output.close();
}
catch (Exception e)
{
e.getStackTrace();
}
}
}

InputStreamReader
 The InputStreamReader class of the java.io package can be used to
convert data in bytes into data in characters.
 It extends the abstract class Reader.

 The InputStreamReader class works with other input streams. It is
also known as a bridge between byte streams and character streams.
 This is because the InputStreamReader reads bytes from the input
stream as characters.
 For example, some characters required 2 bytes to be stored in the
storage.
 To read such data we can use the input stream reader that reads the
2 bytes together and converts into the corresponding character.

Create an InputStreamReader
 In order to create an InputStreamReader, we must import the
java.io.InputStreamReader package first.
// Creates an InputStream
FileInputStream file = new FileInputStream(String path);
// Creates an InputStreamReader
InputStreamReader input = new InputStreamReader(file);

Methods of InputStreamReader
 The InputStreamReader class provides implementations for different
methods present in the Reader class.
read() Method
read() - reads a single character from the reader
read(char[] array) - reads the characters from the reader and stores in
the specified array
read(char[] array, int start, int length) - reads the number of characters
equal to length from the reader and stores in the specified array
starting from the start

import java.io.InputStreamReader;
class IPStreamReaderEx
{
{
try
{
FileInputStream file = new FileInputStream("IPStreamReaderFile.txt");

// Creates an InputStreamReader
InputStreamReader input = new InputStreamReader(file);
// Reads characters from the file
input.read(array);
System.out.println("Data in the stream:");
input.close();
}
catch(Exception e)
{
e.getStackTrace();
}
}
}

Data in the stream:
This line is printed from IPStreamReaderFile

OutputStreamWriter
 The OutputStreamWriter class of the java.io package can be used to
convert data in character form into data in bytes form.
 It extends the abstract class Writer.

 The OutputStreamWriter class works with other output streams. It is
also known as a bridge between byte streams and character streams.
 This is because the OutputStreamWriter converts its characters into
bytes.
 For example, some characters require 2 bytes to be stored in the
storage.
 To write such data we can use the output stream writer that converts
the character into corresponding bytes and stores the bytes together.

Create an OutputStreamWriter
 In order to create an OutputStreamWriter, we must import the
java.io.OutputStreamWriter package first.
// Creates an OutputStream
FileOutputStream file = new FileOutputStream(String path);
// Creates an OutputStreamWriter
OutputStreamWriter output = new OutputStreamWriter(file);

write() Method
write() - writes a single character to the writer
write(char[] array) - writes the characters from the specified array to
the writer

import java.io.OutputStreamWriter;
public class OPStreamWriterEx
{
{
String data = "This line is written to OPStreamWriterFile";
try
{
FileOutputStream file = new FileOutputStream("OPStreamWriterFile.txt");
// Creates an OutputStreamWriter
OutputStreamWriter output = new OutputStreamWriter(file);

// Writes string to the file
output.write(data);
output.close();
}
catch (Exception e)
{
e.getStackTrace();
}
}
}

FileReader
 The FileReader class of the java.io package can be used to read data (in
characters) from files.
 It extends the InputSreamReader class.

Create a FileReader
 In order to create a file reader, we must import the java.io.FileReader
package first.
1. Using the name of the file
FileReader input = new FileReader(String name);
FileReader input = new FileReader(File fileObj);

read() Method
read() - reads a single character from the reader
read(char[] array) - reads the characters from the reader and stores in the
specified array
equal to length from the reader and stores in the specified array starting

class FileReaderEx
{
{
try
{
// Creates a reader using the FileReader
FileReader input = new FileReader("FreaderFile.txt");
// Reads characters
input.read(array);

input.close();
}
catch(Exception e)
{
e.getStackTrace();
}
}
}
Data in the file:
This line is written in File Reader

FileWriter
 The FileWriter class of the java.io package can be used to write data
(in characters) to files.
 It extends the OutputStreamWriter class.

Create a FileWriter
 In order to create a file writer, we must import the Java.io.FileWriter
package first.
1. Using the name of the file
FileWriter output = new FileWriter(String name);
FileWriter input = new FileWriter(File fileObj);

Methods of FileWriter
The FileWriter class provides implementations for different methods
present in the Writer class.
write() Method
write() - writes a single character to the writer
write(char[] array) - writes the characters from the specified array to the
writer

public class FileWriterEx
{
{
String data = "This line is written to FWriter File";
try
{
// Creates a FileWriter
FileWriter output = new FileWriter("FWriterFile.txt");

// Writes the string to the file
output.write(data);
output.close();
}
catch (Exception e)
{
e.getStackTrace();
}
}
}

BufferedReader
 The BufferedReader class of the java.io package can be used with
other readers to read data (in characters) more efficiently.

Working of BufferedReader
 The BufferedReader maintains an internal buffer of 8192 characters.
 During the read operation in BufferedReader, a chunk of characters is
read from the disk and stored in the internal buffer. And from the
internal buffer characters are read individually.
why reading characters is faster using BufferedReader.

Create a BufferedReader
 In order to create a BufferedReader, we must import the
java.io.BuferedReader package first.
// Creates a FileReader
FileReader file = new FileReader(String file);
// Creates a BufferedReader
BufferedReader buffer = new BufferedReader(file);
// Creates a BufferdReader with specified size internal buffer
BufferedReader buffer = new BufferedReader(file, int size);

Methods of BufferedReader
 The BufferedReader class provides implementations for different
methods present in Reader.
read() Method
read() - reads a single character from the internal buffer of the reader
specified array

import java.io.BufferedReader;
class BufferReaderEx
{
{
try
{
// Creates a FileReader
FileReader file = new FileReader("BufferReaderFile.txt");
// Creates a BufferedReader
BufferedReader input = new BufferedReader(file);

// Reads characters
input.read(array);
input.close();
}
catch(Exception e)
{
e.getStackTrace();
}
}
}
Data in the file:
This line is printed from BufferReaderFile

BufferedWriter
 The BufferedWriter class of the java.io package can be used with other
writers to write data (in characters) more efficiently.

Working of BufferedWriter
 The BufferedWriter maintains an internal buffer of 8192 characters.
 During the write operation, the characters are written to the internal
buffer instead of the disk. Once the buffer is filled or the writer is
closed, the whole characters in the buffer are written to the disk.
why writing characters is faster using BufferedWriter.

Create a BufferedWriter
 In order to create a BufferedWriter, we must import the
java.io.BufferedWriter package first.
FileWriter file = new FileWriter(String name);
// Creates a BufferedWriter
BufferedWriter buffer = new BufferedWriter(file);
// Creates a BufferedWriter with specified size internal buffer
BufferedWriter buffer = new BufferedWriter(file, int size);

Methods of BufferedWriter
 The BufferedWriter class provides implementations for different
methods present in Writer.
write() Method
write() - writes a single character to the internal buffer of the writer
write(char[] array) - writes the characters from the specified array to the
writer

import java.io.BufferedWriter;
public class BufferedWriterEx
{
{
String data = "This line is written to BufferedWriterFile";
try
{
FileWriter file = new FileWriter("BufferedWriterFile.txt");

// Creates a BufferedWriter
BufferedWriter output = new BufferedWriter(file);
// Writes the string to the file
output.write(data);
output.close();
}
catch (Exception e)
{
e.getStackTrace();
}
}
}
This line is written to BufferedWriterFile

StringReader
 The StringReader class of the java.io package can be used to read data
(in characters) from strings.
 In StringReader, the specified string acts as a source from where
characters are read individually.

Create a StringReader
 In order to create a StringReader, we must import the
java.io.StringReader package first.
// Creates a StringReader
StringReader input = new StringReader(String data);
Methods of StringReader
 The StringReader class provides implementations for different

Methods of StringReader
 The StringReader class provides implementations for different
read() Method
read() - reads a single character from the string reader
specified array

import java.io.StringReader;
public class StringReaderEX
{
{
String data = "This is the text read from StringReader.";
// Create a character array
try
{
// Create a StringReader
StringReader input = new StringReader(data);
//Use the read method
input.read(array);

System.out.println("Data read from the string:");
input.close();
}
catch(Exception e)
{
e.getStackTrace();
}
}
}
Data read from the string:
This is the text read from StringReader.

StringWriter
 The StringWriter class of the java.io package can be used to write data (in
characters) to the string buffer.

Create a StringWriter
 In order to create a StringWriter, we must import the java.io.StringWriter
package first.
// Creates a StringWriter
StringWriter output = new StringWriter();
// Creates a StringWriter with specified string buffer capacity
StringWriter output = new StringWriter(int size);

Methods of StringWriter
write() Method
write() - writes a single character to the string writer
write(char[] array) - writes the characters from the specified array to the writer

import java.io.StringWriter;
public class StringWriterEx
{
{
String data = "This is the text in the string.";
try
{
// Create a StringWriter with default string buffer capacity
// Writes data to the string buffer
output.write(data);

// Prints the string writer
System.out.println("Data in the StringWriter: " + output);
output.close();
}
catch(Exception e)
{
e.getStackTrace();
}
}
}
Data in the StringWriter: This is the text in the string.

Access Data from StringBuffer
getBuffer() - returns the data present in the string buffer
toString() - returns the data present in the string buffer as a string

import java.io.StringWriter;
public class StringWriterBufferEx
{
{
String data = "This is the original data";
try
{
// Create a StringWriter with default string buffer capacity
// Writes data to the string buffer
output.write(data);

// Returns the string buffer
StringBuffer stringBuffer = output.getBuffer();
System.out.println("StringBuffer: " + stringBuffer);
// Returns the string buffer in string form
String string = output.toString();
System.out.println("String: " + string);
output.close();
}
catch(Exception e)
{
e.getStackTrace();
}
}
}
StringBuffer: This is the original data
String: This is the original data

PrintWriter
 The PrintWriter class of the java.io package can be used to write output
data in a commonly readable form (text).

Working of PrintWriter
 Unlike other writers, PrintWriter converts the primitive data (int, float,
char, etc.) into the text format. It then writes that formatted data to the
writer.
 Also, the PrintWriter class does not throw any input/output exception.
Instead, we need to use the checkError() method to find any error in it.
 The PrintWriter class also has a feature of auto flushing. This means it
forces the writer to write all data to the destination if one of the println() or
printf() methods is called.

Create a PrintWriter
 In order to create a print writer, we must import the java.io.PrintWriter
package first.
1. Using other writers
FileWriter file = new FileWriter("output.txt");
// Creates a PrintWriter
PrintWriter output = new PrintWriter(file, autoFlush);
Here,
 we have created a print writer that will write data to the file represented by
the FileWriter
 autoFlush is an optional parameter that specifies whether to perform auto
flushing or not

2. Using other output streams
FileOutputStream file = new FileOutputStream("output.txt");
PrintWriter output = new PrintWriter(file, autoFlush);
Here,
 We have created a print writer that will write data to the file represented by
the FileOutputStream
 The autoFlush is an optional parameter that specifies whether to perform
auto flushing or not

3. Using filename
PrintWriter output = new PrintWriter(String file, boolean autoFlush);
Here,
 We have created a print writer that will write data to the specified file
 The autoFlush is an optional boolean parameter that specifies whether to
perform auto flushing or nor
 In all the above cases, the PrintWriter writes data to the file using some
default character encoding. However, we can specify the character
encoding (UTF8 or UTF16) as well.

Methods of PrintWriter
 The PrintWriter class provides various methods that allow us to print data
to the output.
print() Method
print() - prints the specified data to the writer
println() - prints the data to the writer along with a new line character at the
end

import java.io.PrintWriter;
class PrintWriterEx
{
{
String data = "This is a text inside the file.";
try
{
PrintWriter output = new PrintWriter("output.txt");
output.print(data);
output.close();
}

catch(Exception e)
{
e.getStackTrace();
}
}
}

Multithreaded Programming
 Java provides built-in support for multithreaded programming. A
multithreaded program contains two or more parts that can run
concurrently.
 Each part of such a program is called a thread, and each thread
defines a separate path of execution. Thus, multithreading is a
specialized form of multitasking.
 You are almost certainly acquainted with multitasking because it is
supported by virtually all modern operating systems. However,
there are two distinct types of multitasking: process-based and
thread-based.

 It is important to understand the difference between the two. For
many readers, process-based multitasking is the more familiar
form.
 A process is, in essence, a program that is executing. Thus, process-
based multitasking is the feature that allows your computer to run
two or more programs concurrently.
 For example, process-based multitasking enables you to run the
Java compiler while you are using a text editor or visiting a web
site.
 In process-based multitasking, a program is the smallest unit of
code that can be dispatched by the scheduler.

 In a thread-based multitasking environment, the thread is the
smallest unit of dispatchable code.
 This means that a single program can perform two or more tasks
simultaneously. For instance, a text editor can format text at the
same time that it is printing, as long as these two actions are being
performed by two separate threads.
 Thus, process-based multitasking deals with the “big picture,” and
thread-based multitasking handles the details.
 Multitasking threads require less overhead than multitasking
processes. Processes are heavyweight tasks that require their own
separate address spaces.

 Interprocess communication is expensive and limited. Context
switching from one process to another is also costly.
 Threads, on the other hand, are lighter weight. They share the same
address space and cooperatively share the same heavyweight
process.
 Interthread communication is inexpensive, and context switching
from one thread to the next is lower in cost.
 While Java programs make use of process-based multitasking
environments, process-based multitasking is not under Java’s
control. However, multithreaded multitasking is.

 Multithreading enables you to write efficient programs that make
maximum use of the processing power available in the system.
 One important way multithreading achieves this is by keeping idle
time to a minimum. This is especially important for the interactive,
networked environment in which Java operates because idle time is
common.
 For example, the transmission rate of data over a network is much
slower than the rate at which the computer can process it.
 Even local file system resources are read and written at a much
slower pace than they can be processed by the CPU. And, of course,
user input is much slower than the computer.

 In a single-threaded environment, your program has to wait for
each of these tasks to finish before it can proceed to the next one—
even though most of the time the program is idle, waiting for input.
 Multithreading helps you reduce this idle time because another
thread can run when one is waiting.

1. The Java Thread Model
 The Java run-time system depends on threads for many things, and
all the class libraries are designed with multithreading in mind.
 The value of a multithreaded environment is best understood in
contrast to its counterpart.
 Single-threaded systems use an approach called an event loop with
polling. In this model, a single thread of control runs in an infinite
loop, polling a single event queue to decide what to do next.

 Once this polling mechanism returns with, say, a signal that a
network file is ready to be read, then the event loop dispatches
control to the appropriate event handler.
 Until this event handler returns, nothing else can happen in the
program. This wastes CPU time.
 It can also result in one part of a program dominating the system
and preventing any other events from being processed.
 In general, in a single-threaded environment, when a thread blocks
(that is, suspends execution) because it is waiting for some
resource, the entire program stops running.

 The benefit of Java’s multithreading is that the main loop/polling
mechanism is eliminated.
 One thread can pause without stopping other parts of your
program. For example, the idle time created when a thread reads
data from a network or waits for user input can be utilized
elsewhere.
 Multithreading allows animation loops to sleep for a second
between each frame without causing the whole system to pause.
 When a thread blocks in a Java program, only the single thread that
is blocked pauses. All other threads continue to run.

 Threads exist in several states. Here is a general description. A
thread can be running.
 It can be ready to run as soon as it gets CPU time. A running thread
can be suspended, which temporarily halts its activity.
 A suspended thread can then be resumed, allowing it to pick up
where it left off. A thread can be blocked when waiting for a
resource.
 At any time, a thread can be terminated, which halts its execution
immediately. Once terminated, a thread cannot be resumed.

The Main Thread
 When a Java program starts up, one thread begins running
immediately.
 This is usually called the main thread of your program, because it is
the one that is executed when your program begins. The main thread
is important for two reasons:
 It is the thread from which other “child” threads will be spawned.
 Often, it must be the last thread to finish execution because it
performs various shutdown actions.
 Although the main thread is created automatically when your
program is started, it can be controlled through a Thread object.

 To do so, you must obtain a reference to it by calling the method
currentThread( ), which is a public static member of Thread. Its
general form is shown here:
static Thread currentThread( )
 This method returns a reference to the thread in which it is called.
Once you have a reference to the main thread, you can control it just
like any other thread.

class CurrentThreadDemo
{
{
Thread t = Thread.currentThread();
System.out.println("Current thread: " + t);
// change the name of the thread
t.setName("My Thread");
System.out.println("After name change: " + t);

try
{
for(int n = 6; n > 0; n--)
{
System.out.println(n);
Thread.sleep(1000);
}
}
catch (InterruptedException e)
{
System.out.println("Main thread interrupted");
}
}
}

Output:
Current thread: Thread[main,5,main]
After name change: Thread[My Thread,5,main]
6
5
4
3
2
1

 In this program, a reference to the current thread (the main thread, in
this case) is obtained by calling currentThread( ), and this reference is
stored in the local variable t.
 Next, the program displays information about the thread. The
program then calls setName( ) to change the internal name of the
thread.
 Information about the thread is then redisplayed. Next, a loop counts
down from five, pausing one second between each line.
 The pause is accomplished by the sleep( ) method. The argument to
sleep( ) specifies the delay period in milliseconds. Notice the try/catch
block around this loop.

 The sleep( ) method in Thread might throw an InterruptedException. This
would happen if some other thread wanted to interrupt this sleeping one.
 Notice the output produced when t is used as an argument to println( ).
This displays, in order: the name of the thread, its priority, and the name
of its group.
 By default, the name of the main thread is main. Its priority is 5, which is
the default value, and main is also the name of the group of threads to
which this thread belongs.
 A thread group is a data structure that controls the state of a collection of
threads as a whole.
 After the name of the thread is changed, t is again output. This time, the
new name of the thread is displayed.

 The sleep( ) method causes the thread from which it is called to
suspend execution for the specified period of milliseconds. Its general
form is shown here:
static void sleep(long milliseconds) throws InterruptedException
 The number of milliseconds to suspend is specified in milliseconds.
This method may throw an InterruptedException.
 The sleep( ) method has a second form, shown next, which allows
you to specify the period in terms of milliseconds and nanoseconds:
static void sleep(long milliseconds, int nanoseconds) throws InterruptedException
 This second form is useful only in environments that allow timing
periods as short as nanoseconds.

 The name of a thread can be set by using setName( ).
 The name of a thread can be obtained by calling getName( ) (but note
that this is not shown in the program).
 These methods are members of the Thread class and are declared like
this:
final void setName(String threadName)
final String getName( )
 Here, threadName specifies the name of the thread.

2. Creating a Thread
 A thread can be created by instantiating an object of type Thread.
 Java defines two ways in which this can be accomplished:
 Implement the Runnable interface.
 Extend the Thread class, itself.

Implementing Runnable
 The easiest way to create a thread is to create a class that implements
the Runnable interface.
 Runnable abstracts a unit of executable code. A thread can be
constructed on any object that implements Runnable.
 To implement Runnable, a class need only implement a single
method called run( ), which is declared like this:
public void run( )
 Inside run( ), the code is defined that constitutes the new thread.

 It is important to understand that run( ) can call other methods, use
other classes, and declare variables, just like the main thread can.
 The only difference is that run( ) establishes the entry point for
another, concurrent thread of execution within your program.
 This thread will end when run( ) returns.
 After you create a class that implements Runnable, you will
instantiate an object of type Thread from within that class.
 Thread defines several constructors. The one that we will use is
shown here:
Thread(Runnable threadOb, String threadName)

 In this constructor, threadOb is an instance of a class that implements
the Runnable interface.
 This defines where execution of the thread will begin. The name of
the new thread is specified by threadName.
 After the new thread is created, it will not start running until you call
its start( ) method, which is declared within Thread. In essence, start(
) executes a call to run( ).
 The start( ) method is shown here:
void start( )

class NewThread implements Runnable
{
Thread t;
NewThread()
{
// Create a new, second thread
t = new Thread(this, "Demo Thread");
System.out.println("Child thread: " + t);
// Start the thread
t.start();
}

// This is the entry point for the second thread.
public void run()
{
try
{
for(int i = 5; i > 0; i--)
{
System.out.println("Child Thread: " + i);
Thread.sleep(500);
}
}
{
System.out.println("Child interrupted.");
}
System.out.println("Exiting child thread.");
}
}

class RunnableExample
{
public static void main(String args[ ] )
{
// create a new thread
new NewThread();
try
{
for(int i = 5; i > 0; i--)
{
System.out.println("Main Thread: " + i);
Thread.sleep(1000);
}
}

{
System.out.println("Main thread interrupted.");
}
System.out.println("Main thread exiting.");
}
}

Output:
Child thread: Thread[Demo Thread,5,main]
Child Thread: 5
Main Thread: 5
Child Thread: 4
Main Thread: 4
Child Thread: 3
Child Thread: 2
Main Thread: 3
Child Thread: 1
Exiting child thread.
Main Thread: 2
Main Thread: 1
Main thread exiting.

 Passing this as the first argument indicates that you want the new
thread to call the run( ) method on this object.
 Next, start( ) is called, which starts the thread of execution beginning
at the run( ) method.
 This causes the child thread’s for loop to begin.
 After calling start( ), NewThread’s constructor returns to main( ).
 When the main thread resumes, it enters its for loop. Both threads
continue running, sharing the CPU in singlecore systems, until their
loops finish.

Extending Thread
 The second way to create a thread is to create a new class that
extends Thread, and then to create an instance of that class.
 The extending class must override the run( ) method, which is the
entry point for the new thread.
 It must also call start( ) to begin execution of the new thread.

class NewThread extends Thread
{
NewThread()
{
// Create a new, second thread
super("Demo Thread");
System.out.println("Child thread: " + this);
// Start the thread
start();
}

class ExtendThread
{
{
// create a new thread
new NewThread();
try
{
for(int i = 5; i > 0; i--)
{
System.out.println("Main Thread: " + i);
Thread.sleep(1000);
}
}

 The child thread is created by instantiating an object of NewThread,
which is derived from Thread.
 Notice the call to super( ) inside NewThread. This invokes the
following form of the Thread constructor:
public Thread(String threadName)
 Here, threadName specifies the name of the thread

3. Creating Multiple Threads
 So far, only two threads : the main thread and one child thread
have been used.
 However, the program can spawn as many threads as it needs.

class NewThread implements Runnable
{
// name of thread
String name;
Thread t;
NewThread(String threadname)
{
name = threadname;
t = new Thread(this, name);
System.out.println("New thread: " + t);
// Start the thread
t.start();
}

// This is the entry point for thread.
public void run()
{
try
{
for(int i = 5; i > 0; i--)
{
System.out.println(name + ": " + i);
Thread.sleep(1000);
}
}
{
System.out.println(name + "Interrupted");
}
System.out.println(name + " exiting.");
}
}

class MultiThreadDemo
{
{
// start threads
new NewThread("One");
new NewThread("Two");
new NewThread("Three");
try
{
// wait for other threads to end
Thread.sleep(10000);
}

{
System.out.println("Main thread Interrupted");
}
}
}

Output:
New thread: Thread[One,5,main]
New thread: Thread[Two,5,main]
New thread: Thread[Three,5,main]
One: 5
Three: 5
Two: 5
One: 4
Three: 4
Two: 4
One: 3
Three: 3
Two: 3
One: 2
Three: 2
Two: 2
One: 1
Three: 1
Two: 1
One exiting.
Three exiting.
Two exiting.

 Once started, all three child threads share the CPU.
 Notice the call to sleep(10000) in main( ).
 This causes the main thread to sleep for ten seconds and ensures that
it will finish last.

Using isAlive( ) and join( )
 Two ways exist to determine whether a thread has finished. First, you
can call isAlive( ) on the thread.
 This method is defined by Thread, and its general form is shown
here:
final boolean isAlive( )
 The isAlive( ) method returns true if the thread upon which it is
called is still running. It returns false otherwise.
 While isAlive( ) is occasionally useful, the method that you will more
commonly use to wait for a thread to finish is called join( ), shown
here:
final void join( ) throws InterruptedException

// This is the entry point for thread.
public void run()
{
try
{
for(int i = 5; i > 0; i--)
{
System.out.println(name + ": " + i);
Thread.sleep(1000);
}
}
{
System.out.println(name + " interrupted.");
}
System.out.println(name + " exiting.");
}
}

class DemoJoin
{
{
NewThread ob1 = new NewThread("One");
NewThread ob2 = new NewThread("Two");
NewThread ob3 = new NewThread("Three");
System.out.println("Thread One is alive: "+ ob1.t.isAlive());
System.out.println("Thread Two is alive: "+ ob2.t.isAlive());
System.out.println("Thread Three is alive: "+ ob3.t.isAlive());
// wait for threads to finish
try
{
System.out.println("Waiting for threads to finish.");
ob1.t.join();
ob2.t.join();
ob3.t.join();
}

{
System.out.println("Main thread Interrupted");
}
System.out.println("Thread One is alive: "+ ob1.t.isAlive());
System.out.println("Thread Two is alive: "+ ob2.t.isAlive());
System.out.println("Thread Three is alive: "+ ob3.t.isAlive());
}
}

Output:
New thread: Thread[One,5,main]
New thread: Thread[Two,5,main]
New thread: Thread[Three,5,main]
Thread One is alive: true
Two: 5
One: 5
Thread Two is alive: true
Three: 5
Thread Three is alive: true
Waiting for threads to finish.
Three: 4
Two: 4
One: 4
Three: 3
One: 3
Two: 3

Three: 2
One: 2
Two: 2
Three: 1
Two: 1
One: 1
Three exiting.
Two exiting.
One exiting.
Thread One is alive: false
Thread Two is alive: false
Thread Three is alive: false

4. Threads Priorities
 Thread priorities are used by the thread scheduler to decide when each
thread should be allowed to run.
 In theory, over a given period of time, higher-priority threads get more
CPU time than lower-priority threads.
 In practice, the amount of CPU time that a thread gets often depends on
several factors besides its priority. (For example, how an operating
system implements multitasking can affect the relative availability of
CPU time.)
 In theory, threads of equal priority should get equal access to the CPU.
But you need to be careful.

 To set a thread’s priority, use the setPriority( ) method, which is a
member of Thread. This is its general form:
final void setPriority(int level)
 Here, level specifies the new priority setting for the calling thread.
The value of level must be within the range MIN_PRIORITY and
MAX_PRIORITY. Currently, these values are 1 and 10, respectively.
To return a thread to default priority, specify NORM_PRIORITY,
which is currently 5.
 These priorities are defined as static final variables within Thread.
 You can obtain the current priority setting by calling the getPriority(
) method of Thread, shown here:
final int getPriority( )

public class ThreadPriorityEx extends Thread
{
public void run()
{
System.out.println("Thread Running... "+Thread.currentThread().getName());
}
{
ThreadPriorityEx t1=new ThreadPriorityEx();

t1.start();
t2.start();
t3.start();
t4.start();
t5.start();
t1.setPriority(Thread.MIN_PRIORITY);
t2.setPriority(Thread.NORM_PRIORITY);
t3.setPriority(Thread.MAX_PRIORITY);
t4.setPriority(9);

System.out.println("Priority of thread t1 is: " + t1.getPriority()); //1
try
{
t5.setPriority(11);
}
catch (Exception e)
{
System.out.println("Illegal Arguement");
}
}
}

Output:
Thread Running... Thread-2
Priority of thread t1 is: 1
Illegal Arguement

5. Synchronization
 When two or more threads need access to a shared resource, they
need some way to ensure that the resource will be used by only one
thread at a time. The process by which this is achieved is called
synchronization.
 Key to synchronization is the concept of the monitor. A monitor is
an object that is used as a mutually exclusive lock. Only one thread
can own a monitor at a given time.
 When a thread acquires a lock, it is said to have entered the
monitor.

 All other threads attempting to enter the locked monitor will be
suspended until the first thread exits the monitor.
 These other threads are said to be waiting for the monitor. A thread
that owns a monitor can reenter the same monitor if it so desires.

Using Synchronized Methods
 Synchronization is easy in Java, because all objects have their own
implicit monitor associated with them.
 To enter an object’s monitor, just call a method that has been
modified with the synchronized keyword.
 While a thread is inside a synchronized method, all other threads
that try to call it (or any other synchronized method) on the same
instance have to wait.
 To exit the monitor and relinquish control of the object to the next
waiting thread, the owner of the monitor simply returns from the
synchronized method.

class Callme
{
void call(String msg)
{
System.out.print("[" + msg);
try
{
Thread.sleep(1000);
}
{
System.out.println("Interrupted");
}
System.out.println("]");
}
}

class Caller implements Runnable
{
String msg;
Callme target;
Thread t;
public Caller(Callme targ, String s)
{
target = targ;
msg = s;
t = new Thread(this);
t.start();
}

// synchronize calls to call()
public void run()
{
synchronized(target)
{
// synchronized block
target.call(msg);
}
}
}

class Synch1
{
{
Callme target = new Callme();
Caller ob1 = new Caller(target, "Hello");
Caller ob2 = new Caller(target, "Synchronized");
Caller ob3 = new Caller(target, "World");
// wait for threads to end
try
{
ob1.t.join();
ob2.t.join();
ob3.t.join();
}

catch(InterruptedException e)
{
System.out.println("Interrupted");
}
}
}
Output:
[Hello]
[World]
[Synchronized]

The Synchronized Statement
The general form of the synchronized statement:
synchronized(objRef)
{
// statements to be synchronized
}
 Here, objRef is a reference to the object being synchronized.
 A synchronized block ensures that a call to a synchronized
method that is a member of objRef’s class occurs only after the
current thread has successfully entered objRef’s monitor.

References
1. Herbert Schildt, “Java The Complete Reference”, 9th Edition,
McGraw-Hill Education, New Delhi, 2017.

Exception handling, Stream Classes, Multithread Programming

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