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Control Statements > Iteration statements (Loops) >
Siva Nookala - 11 Apr 2016
The while loop is Java’s most fundamental looping statement. It repeats statement or block while its controlling expression (termination condition) is true.

The while statement continually executes a block of statements while a particular condition is true. Its syntax can be expressed as:

while (expression) {
     statement;
}
The while statement evaluates expression, which must return a boolean value. If the expression is true, then the while loop executes the statement in the while block. The while loop continues testing the expression and executing its block until the controlling expression evaluates to false.
while loop can be used to print the values from 1 through 5. It is shown in the following WhileDemo program.
While Demo
class WhileDemo
{
    public static void main(String arg[])
    {
        int x = 1;
        while( x <= 5 ) // LINE A
        {
            System.out.println("x = "+ x);
            x++;
        }
    
    }
}
OUTPUT

x = 1
x = 2
x = 3
x = 4
x = 5

DESCRIPTION

Here x is initialized to 1. In the while condition in LINE A, it checks for the condition 1 <= 5, since it is true it goes into the block and prints x = 1. Then it increments the x value by one. With x value as 2 and again it checks for the condition 2 <= 5 and prints the output as x = 2. This repeats for 3 more times, then x will become 6. Since the condition 6 <= 5 becomes false, the control comes out of while loop.

THINGS TO TRY
  • Change the above program such that the below shown output is printed.
    x = 5
    x = 4
    x = 3
    x = 2
    x = 1
  • Try to print the even numbers between 20 and 30 using a while loop.
  • Try to print the numbers which are divisible by 5 between 10 to 50.
The major differences between for and while are:
  • for loop is used when we know the number of iterations we have to perform i.e. we know how many times we need to execute a loop.
  • while is used when you are not sure about the iterations but you know what the condition is and then you can loop that block until the condition is false.
  • Although for and while are different, every for loop can be rewritten using while and every while can be rewritten using for.

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