While in Java

While Loop

The while loop is Java's most fundamental loop statement.It repeats the statement or block of code while its controlling expression is true.In while loop, condition is given before the statement.  It can execute the statements 0 or more times.

Syntax of the while loop:

while(condition)
{
   //statements
}

The condition can be any boolean expression.The body of the loop will be executed as long as the conditional expression is true.

When condition becomes false , control passes to the next line of code immediately following the loop.

If loop have only one statement then , curly braces are not necessary.

Flow Chart of the while loop:

Simple program of the while loop:

public class Test
{
 public static void main(String[] args)
 {
  int i=5;
  while(i>0)
  {
   System.out.println("Count " + i );
   i--;
  }
 }
}

OUTPUT

Count 5
Count 4
Count 3
Count 2
Count 1

Some important points about while loop:

1.Since the while loop evaluates its conditional expression at the beginning the loop , the body of the loop        will not execute if condition is false at the beginning.

public class Test
{
 public static void main(String[] args)
 {
  int i=5 , j=10;
  while(i>j)
  {
   System.out.println("While loop body");
  }
  System.out.println("Out of While loop");
 }
}

OUTPUT

Out of While loop

2. The body of the while loop can be empty.This is because a null statement is syntactically valid in java.

public class Test
{
 public static void main(String[] args)
 {
  int i=5 , j=10;
  while(++i<--j)
  System.out.println(i);
 }
}

OUTPUT

8

3.If condition will always true then it will become infinite loop.

while(true) //infinite loop

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For Loop in Java

The for loop is used to iterate the statement or a part of the program several times.
There are two forms of the for loop in java.
1. Traditional for loop
2. For-each loop

1. Traditional For Loop
Syntax of the traditional for loop is:

for(initialization ; condition ; increment/decrements)
{
       //statements
}

When the loop first starts , the initialization portion of the loop is executed.It is important to know that the initialization statement is only executed once.Next , condition is evaluated.This must be a Boolean expression.It checks the  loop control variable against a target value.If this expression is true , then the body of loop is executed otherwise terminates the loop.Next, the increment/decrements portion executed which is increments or decrements the loop control variable.
After first pass the loop iterates , first evaluated the conditional statements ,then executing the body of loop and then executes the increments/decrements portion.This process repeats until the condition false.


Flow Chart Of  Traditional For Loop:

Simple program of traditional for loop:

package org.modi;
public class Test
{
	public static void main(String[] args)
	{
		for(int i=0;i<5;i++)
		{
			System.out.println("loop body " + i );
		}
	}
}

OUTPUT

loop body 0
loop body 1
loop body 2
loop body 3
loop body 4

2. For-each Loop

The for-each loop was introduce in JDK 5(1.5)  version of java.The for-each loop is also know as enhanced for loop.The for-each specially designed for iterate the collection of objects , such as an array.It iterate all the elements from starting element to last element.We can con control the number of iteration it iterate all the elements one by one.

Syntax of for-each loop:

for(type variable : collection)
{
   //statements
}

Here, type specifies the type (e.g. String , int ) and variable specifies the name of the iteration variable that will receive the elements form the collection , one at a time from begin to end.The collection is the collection of elements which will be iterated by the loop.

Simple program of the for-each loop:

package org.modi;
public class Test
{
   public static void main(String[] args)
   {
	int[] arr={0,1,3,4};
	for(int itr : arr)
	{
	   System.out.println("loop body " + itr);
	}
    }
}

OUTPUT

loop body 0
loop body 1
loop body 3
loop body 4

Some Important points:

1. If for loop body contains only one statement then , no need for curly braces.
E.g.
for(int i=0 ; i< 5 ; i++)
      System.out.println("Hello World!");
2. If we declare a variable inside for loop , then the scope of the variable ends with ends of for loop.
3. To create infinite loop leave all the parts of for loop empty.
E.g.
for(  ;   ;  )

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