Java Review

Java Review
(Essentials of Java for Hadoop)

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What is Java ?
• Java
- Java is not just a programming language but it is a complete
platform for object oriented programming.
• JRE
- Java standard class libraries which provide Application
Programming Interface and JVM together form JRE (Java Runtime
Environment).
• JDK
- JDK (Java development kit) provides all the needed support for
software development in Java.

Java Virtual Machine (JVM)
• Runs the Byte Code.
• Makes Java platform independent.
• Handles Memory Management.

How Java works ?
• Java compilers convert your code from human readable to
so ethi g alled bytecode i the Ja a orld.
• Bytecode is i terpreted y a JVM, which operates much
like a physical CPU to actually execute the compiled code.
• Just-in-time (JIT) compiler is a program that turns
Java bytecode into instructions that can be sent directly to
the processor.

Data Types in Java
• Data types define the nature of a value
• We need different data-types to handle real-world information
Name Size (in bits) Range
long 64 -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
int 32 –2,147,483,648 to 2,147,483,647
Short 16 –32,768 to 32,767
byte 8 –128 to 127
double 64 4.9e–324 to 1.8e+308
float 32 1.4e–045 to 3.4e+038
char 16 0 to 65,536
boolean ??
true/false

Naming Convention of Variables
• Can start with a letter, an underscore(_), or a dollar sign ($)
• Cannot start with a number.
 long _LongNumber = 9999999;
 String firstName = Joh ;
 float $Val = 2.3f;
 int i, index = 2;
 double gamma = 1.2;
 boolean value2 = false;

Operators
• Provide a way to perform different operations on
variables
• Categories of Java Operators
Assignment Operators =
Arithmetic Operators - + * / %
Relational Operators > < >= <= == !=
Logical Operators && || & | ^
Unary Operators + - ++ -- !

Assignment and Arithmetic Operators
• Used to assign a value to a variable
• Syntax
– <variable> = <expression>
• Java provides eight Arithmetic operators:
– for addition, subtraction, multiplication, division, modulo (or
remainder), increment (or add 1), decrement (or subtract 1),
and negation.
Assignment Operator =

Relational Operators
• Used to compare two values.
• Binary operators, and their operands are numeric
expressions.
Relational Operators > < >= <= == !=

Logical Operators
• Return a true or false value based on the state of
the variables
• There are six logical operators
Conditional AND Conditional OR AND OR NOT Exclusive OR
Logical Operators && || & | ! ^

Static versus Non-static Variables
• Static variables are shared across all the objects of a
class
– There is only one copy
• Non-Static variables are not shared
– There is a separate copy for each individual live object.
• Static variables cannot be declared within a
method.

Statements & Blocks
• A simple statement is a command terminated by a semi-colon:
a e = Fred ;
• A block is a compound statement enclosed in curly brackets:
{
a e = Fred ; a e = Bill ;
}
• Blocks may contain other blocks.

Flow of Control
• Java executes one statement after the other in the
order they are written.
• Many Java statements are flow control statements:
Alternation:
if, if else, switch
Looping:
for, while, do while

Java Basic Constructs
• If…else
• Switch Statement
• For Loop
• While Loop
• Do...While Loop

If else– Syntax
if ( <condition> )
{
// Execute these statements if <condition> is TRUE
}
else
{
// Execute these statements if < condition> is FALSE
}

switch– Syntax
switch (expression)
{
case cond1:
block_1;
break;
case cond2:
block_2;
break;
...
default:
block_default;
}

For– Syntax
for (initialization; condition; increment/decrement)
{
statement 1;
statement 2; . .
}
Sample:
for( int i=0; i<5; i++ )
{
System.out.println(i);
}

For loop - Working
For(int i=0; i<5; i++){
Statement 1;
Statement 2;
}

While – Syntax
while (condition)
{
statement 1;
statement 2; . .
}
Sample:
int i=0;
while (i<5)
{
i++;
}

While Loop - Working
int i=0;
while(i < 5){
Statement 1;
Statement 2;
i++;
}

Do While – Syntax
do
{
statement 1;
statement 2; . .
} while (condition) ;
Sample:
int i=0;
do
{
i++;
} while (i<5);

Do While Loop - Working
int i=0;
do{
Statement 1;
Statement 2;
i++;
} while(i < 5)

Arrays
• An array is a list of similar things.
• An array has a fixed:
– name
– type
– length
• These must be declared when the array is created.
• Array size cannot be changed during the execution of the
code.

Example of an Array
int array [] = new int[5];
for(int i=0; i<5 ; i++)
{
array[i] = i+1;
}
1
2
3
4
5
array[0]
array[1]
array[2]
array[3]
array[4]

An Example class
package com.edureka.entity; // package
public class Car{ //class declaration
String name;
String color;
float weight;
...
public void move(){ //
method
...
}
}

Methods
• A method is a named sequence of code that can be
invoked by other Java code.
• A method takes some parameters, performs some
computations and then optionally returns a value (or
object).
Ex:
public float convert_to _Celsius( float temp) {
return(((temp * 9.0f) / 5.0f) + 32.0f );
}

return_type method_name (arg1, arg2, arg3)
{
____
____
}
Methods
Methods have five components:
• Modifiers
• The return type
• The method name
• The parameter list in parenthesis
• The method body, enclosed
between braces
Body of the
Method
Any Number of
Parameters
Name of the
MethodValue to be
returned

Modifiers
• public: any method (in any class) can access the
field.
• protected: any method in the same package or any
derived class can access the field.
• private: only methods within the class can access the
field.
• default is that only methods in the same package
can access the field.
Java Programming: OOP 32

Java Review

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