Loops in Java

There may be a situation when we need to execute a block of code several number of times, and is often referred to as a loop.

Java has very flexible three looping mechanisms. You can use one of the following three loops:

  • while Loop
  • do...while Loop
  • for Loop

As of Java 5, the enhanced for loop was introduced. This is mainly used for Arrays.

The while Loop:

A while loop is a control structure that allows you to repeat a task a certain number of times.

public class Test {
  public static void main(String args[]) {
     int x = 10;

      while( x < 20 ) {
      System.out.print("value of x : " + x );
      x++;
      System.out.print("\n");

      }

   }

}	

Output :


value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value of x : 17
value of x : 18
value of x : 19


The do...while Loop:

A do...while loop is similar to a while loop, except that a do...while loop is guaranteed to execute at least one time.

public class Test {
 public static void main(String args[]){
     int x = 10;
     do{
       System.out.print("value of x : " + x );
        x++;
       System.out.print("\n");
      }while( x < 20 );

   }
}

The for Loop:

A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.

A for loop is useful when you know how many times a task is to be repeated.

Here is the flow of control in a for loop:

  • The initialization step is executed first, and only once. This step allows you to declare and initialize any loop control variables. You are not required to put a statement here, as long as a semicolon appears.
  • Next, the Boolean expression is evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop does not execute and flow of control jumps to the next statement past the for loop.
  • After the body of the for loop executes, the flow of control jumps back up to the update statement. This statement allows you to update any loop control variables. This statement can be left blank, as long as a semicolon appears after the Boolean expression.
  • The Boolean expression is now evaluated again. If it is true, the loop executes and the process repeats itself (body of loop, then update step, then Boolean expression). After the Boolean expression is false, the for loop terminates.
 public class Test {
   public static void main(String args[]) {
 
      for(int x = 10; x < 20; x = x+1) {
           System.out.print("value of x : " + x );
        System.out.print("\n");
     }
   }
}