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Java While Loop

A while loop is a loop that repeats its block while its condition stays true. The condition is checked before each pass.

A counter loop

The counter starts at 0. Each pass prints the counter and then adds 1 to it.

public class Main {
  public static void main(String[] args) {
    int i = 0;
    while (i < 5) {
      System.out.println(i);
      i++;
    }
  }
}

A countdown

The counter starts at 3 and falls by 1 on each pass. The loop stops when the counter reaches 0.

public class Main {
  public static void main(String[] args) {
    int i = 3;
    while (i > 0) {
      System.out.println(i);
      i--;
    }
    System.out.println("Go");
  }
}

A do/while loop runs once

The condition here is false after the first pass. The block still runs once before that check.

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

Example

public class Main {
  public static void main(String[] args) {
    int i = 1;
    int total = 0;
    while (i <= 5) {
      total += i;
      i++;
    }
    System.out.println(total);
  }
}

Key points

  • The while keyword starts the loop.
  • The condition sits in parentheses after while.
  • The block repeats while the condition is true.
  • A value tested by the condition must change, or the loop never stops.
  • A do/while loop checks its condition after its block.
  • A do/while loop always runs its block at least once.

In this example, i starts at 1 and total starts at 0. Each pass adds i to total and then adds 1 to i. The total passes through 1, 3, 6, and 10. The last pass adds 5, and the printed total is 15.

Note

A while loop runs zero times when its condition starts false.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

What does a do/while loop guarantee?