Java basic

JAVA
Fundamental of
java
Prepared by
Miss. Arati A. Gadgil

2
Java
Java was developed in the early 90s by Sun Microsystems
Java is a high-level language.
There are many features of java. They are also known as
java buzzwords.
Simple
Java omits many rarely used, poorly understood, confusing
features of C++. Say : No Pointer! No dynamic delete.

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Object Oriented
Object –oriented design is a technology that focuses design
on the data (object) and on the interfaces to it.
Everything is an object, everything will become a class in
Java. Every java program, in top- level view, is classes.
Robust
The single biggest difference between Java and C/C++ is
that Java has “a inner safe pointer-model”, therefore it
eliminates the possibility of overwriting memory and
corrupting data, so programmers feel very safe in coding.

4
Distributed
Basically, Java is for Net-Work application, for WEB
project. Java can open and access “objects” across the Net via
URLs (Uniform Resource Locator)
eg. “http//:gamut.neiu.edu/~ylei/home.html”, with the same
ease as when accessing a local file system
Platform Independent
Java code can be run on multiple platforms e.g. Windows,
Linux, Sun Solaris,Mac/OS etc. Java code is compiled by the
compiler and converted into bytecode.This bytecode is a
platform independent code because it can be run on multiple
platforms i.e. Write Once and Run Anywhere(WORA).

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Secured
Java is secured because:
No explicit pointer,Programs run inside virtual machine
sandbox also,
•Classloader- adds security by separating the package for the
classes of the local file system from those that are imported
from network sources.
•Bytecode Verifier- checks the code fragments for illegal code
that can violate access right to objects.
•Security Manager- determines what resources a class can
access such as reading and writing to the local disk

6
Architecture-neutral
The compiler generates an architecture-neutral object file
format. The compile code can run on many processors ,given
the presence of the runtime system. The java compiler does
this by generting bytecode instruction which have nothing to
do with a particular computer architecture.
Portable
We may carry the java bytecode to any platform.
High-performance
Java is faster than traditional interpretation since byte code is
"close" to native code still somewhat slower than a compiled
language (e.g., C++)

7
Multi-threaded
A thread is like a separate program, executing concurrently.
We can write Java programs that deal with many tasks at once
by defining multiple threads. The main advantage of multi-
threading is that it shares the same memory. Threads are
important for multi-media, Web applications etc.
Dynamic
In number of ways, java is more dynamic language than C or
C++.It was designed to adapt to an evolving environment.
Libraries can freely add new methods and instance variables
without any effect on their clients. In java finding out runtime
type information is straightforward.

8
Java Virtual Machine
The .class files generated by the compiler are not executable
binaries, Java combines compilation and interpretation.
Instead, they contain “byte-codes” to be executed by the Java
Virtual Machine.
This approach provides platform independence, and greater
security.
The JVM performs following main tasks:
•Loads code
•Verifies code
•Executes code
•Provides runtime environment

9
JRE
JRE is an acronym for Java Runtime Environment. It is used
to provide runtime environment.It is the implementation of
JVM. It physically exists.It contains set of libraries + other
files that JVM uses at runtime.
JDK
JDK is an acronym for Java Development Kit. It physically
exists. It contains JRE + development tools

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JVM
Java Virtual Machine (JVM) is an abstract computing
machine. Java Runtime Environment (JRE) is an
implementation of the JVM. Java Development Kit (JDK)
contains JRE along with various development tools like Java
libraries, Java source compilers, Java debuggers, bundling and
deployment tools.

11
Relations between JVM JRE JDK

12
JIT
JIT refers to execution engine in few of JVM
implementations, one that is faster but requires more memory,
is a just-in-time compiler. In this scheme, the bytecodes of a
method are compiled to native machine code the first time the
method is invoked.
JIT refers to execution engine in few of JVM
implementations, one that is faster but requires more
memory,is a just-in-time compiler. In this scheme, the
bytecodes of a method are compiled to native machine code
the first time the method is invoked.

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Java Bytecode
Java bytecode is the instruction set of the Java virtual
machine. Eachbytecode is composed by one, or in some cases
two, bytes that represent the instruction (opcode), along with
zero or more bytes for passing parameters..
Bytecode is nothing but the intermediate representation of
Java source code which is produced by the Java compiler by
compiling that source code. This byte code is an machine
independent code.It is not an completely a compiled code but
it is an intermediate code somewhere in the middle which is
later interpreted and executed by JVM

14
HotSpot, released as the "Java HotSpot Performance Engine"is
a Java virtual machine for desktops and servers, maintained and
distributed by Oracle Corporation. It features techniques such as just-
in-time compilation and adaptive optimization designed to improve
performance.
Sun intended to write a new just-in-time (JIT) compiler for the
newly developed virtual machine.This new compiler would give rise
to the name "HotSpot", which derives from the fact that, as the
software runs Java bytecode, it continually analyzes the program's
performance for "hot spots" which are frequently or repeatedly
executed. These are then targeted for optimization, leading to high-
performance execution with a minimum of overhead for less
performance-critical code
Java HotSpot

15
Sun's JRE features two virtual machines, one called Client and the
other Server. The Client version is tuned for quick loading. It makes use
of interpretation. The Server version loads more slowly, putting more
effort into producing highly optimized JI compilations, that yield higher
performance. Both VMs compile only often-run methods, using a
configurable invocation-count threshold to decide which methods to
compile.
The HotSpot Java virtual machine is written in C++. As stated on the
HotSpot web page, the source contains approximately 250,000 lines of
code.
Hotspot provides:
•A class loader
•A bytecode interpreter
•Client and Server virtual machines, optimized for their respective uses
•Several garbage collectors
•A set of supporting runtime libraries

16
The main() method
public static void main(String args[])
{
...
}
public--- the interpreter can call it.
static ----It is a static method belonging to the class.
void -----It does not return a value.
String----It always has an array of String objects as its formal parameter.
the array contains any arguments passed to the program on the
command line.The source file’s name must match the class name which
main method is in.

17
Source File Declaration Rules
There can be only one public class per source code file.
Comments can appear at the beginning or end of any line in the source
code file; they are independent of any of the positioning rules discussed
here.
If there is a public class in a file, the name of the file must match the
name of the public class.
For example, a class declared as public class Dog { }must be in a source
code file named Dog.java.
If the class is part of a package, the package statement must be the first
line in the source code file, before any import statements that may be
present.

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If there are import statements, they must go between the package
statement (if there is one) and the class declaration. If there isn't a
package statement, then the import statement(s) must be the first line(s)
in the source code file.
If there are no package or import statements, the class declaration must
be the first line in the source code file.
import and package statements apply to all classes within a source
code file. In other words, there's no way to declare multiple classes in a
file and have them in different packages, or use different imports.
A file can have more than one nonpublic class.
Files with no public classes can have a name that does not match any of
the classes in the file

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Comments
/* This kind of comment can span multiple lines */
// This kind is to the end of the line
/**
* This kind of comment is a special
* ‘javadoc’ style comment
*/

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Primitive types
•int 4 bytes
•short 2 bytes
•long 8 bytes
•byte 1 byte
•float 4 bytes
•double 8 bytes
•char Unicode encoding (2 bytes)
•boolean {true,false}
Behaviors is
exactly as in
C++

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• Constants
37 integer
37.2 float
42F float
0754 integer (octal)
0xfe integer (hexadecimal)
• Comparison operators
== equal
!= not equal
< less than
> greater than
<= less than or equal
>= greater than or equal

22
Variable declaration
type variable-name;
Meaning: variable <variable-name> will be a variable of type <type>
Where type can be:
int //integer
double //real number
Example:
int a, b, c;
double x;
int sum;

23
String is an Object
Constant strings as in C, does not exist
 The function call show(“Hello”) creates a String object, containing
“Hello”, and passes reference to it to show.
 The String object is a constant. It can’t be changed using a reference
to
it.

24
Writing to user (output)
System.out.println(variable-name);
prints the value of variable <variable-name> to the user
System.out.println(“any message “);
prints the message within quotes to the user
System.out.println(“hello” + “world” + a + “plus“ + b);
assuming the value of a is 3 and of b is 7, it prints
helloworld3plus7
Note: System.out.println() always prints on a new line.

25
Example
/*
This program illustrates the System.out.println command.
*/
public class Hello
{
public static void main (String args[])
{
System.out.println(“This is my first Java program!”);
System.out.print(“I like Java.”);
System.out.print(“I think Java is cool.”);
} // end of main
} // end of class

26
Example
/*
Printing ages.
*/
public class MyFirstJavaProgram
{
public static void main (String args[])
{
int myAge, myFriendAge; /* declare two integer
variables */
myAge = 20;
myFriendAge = myAge + 1; //one year older
System.out.println(“Hello, I am “ +myAge + “years old,
and my friend is “ + myFriendAge + “ years old”);
System.out.println(“Goodbye”);
} // end of main
} // end of class

27
Flow control
if/else
do/while
for
switch
If(x==4) {
// act1
} else {
// act2
}
int i=5;
do {
// act1
i--;
} while(i!=0);
int j;
for(int i=0;i<=9;i++)
{
j+=i;
}
char
c=IN.getChar();
switch(c) {
case ‘a’:
case ‘b’:
// act1
break;
default:
// act2
}

28
Big Numbers
If the precision of the basic integer and floating-point types is not
sufficient, you can turn to a couple of handy classes in
the java.math package, called BigInteger and BigDecimal. These are
classes for manipulating numbers with an arbitrarily long sequence of
digits. The BigInteger class implements arbitrary precision integer
arithmetic, and BigDecimal does the same for floating-point numbers.
Use the static valueOf method to turn an ordinary number into a big
number:
BigInteger a = BigInteger.valueOf(100);
BigInteger c = a.add(b); // c = a + b
BigInteger d = c.multiply(b.add(BigInteger.valueOf(2)));
// d = c * (b + 2)

29
Array
•Array is an object
• Array size is fixed
A[] arr; // nothing yet …
arr = new A[4]; // only array of pointers
for(int i=0 ; i < arr.length ; i++)
arr[i] = new Animal();

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Arrays - Multidimensional
• In C++
Animal arr[2][2]
Is:
• In Java
What is the type of
the object here ?
A[][] arr=
new A[2][2]

Java basic

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