While

In Java several loops are available—each has its benefits. A while-loop tends to be best when there is no simple exit condition.

With the while-loop, a block of statements repeat infinitely—until the terminating condition (or a break or return) is reached. With break or return, we can exit the loop.

Example

In this example, we control a loop based on both "i" and "z." This could be expressed with a for-loop, but it might be less clear.

public class Program {
    public static void main(String[] args) {

        int i = 0;
        int z = 10;

        // Loop with two variables.
        while (i < z) {
            i++;
            z--;

            // Display the values.
            System.out.println(i + "/" + z);
        }
    }
}
1/9
2/8
3/7
4/6
5/5

Break, while-true

Break is often useful in while-loops. But it can be used in any kind of loop. Here we show a while-true loop—it would be infinite if there were no break statement.

import java.lang.Math;

public class Program {
    public static void main(String[] args) {

        // Loop infinitely.
        while (true) {

            // Get random number between 0 and 1.
            double value = Math.random();
            System.out.println(value);

            // Break if greater than 0.8.
            if (value >= 0.8) {
                break;
            }
        }
    }
}
0.16129889659284657
0.0977987643977064
0.859556475501672

Do-while

This loop is just like a while-loop, except its condition is not checked before the first iteration is reached. So we must ensure the loop works with its initial value.

public class Program {
    public static void main(String[] args) {

        int i = 0;
        // Loop while the variable is less than 3.
        // ... It is not checked on the first iteration.
        do {
            System.out.println(i);
            i++;
        } while (i < 3);
    }
}
0
1
2

Post-increment, continue

This while-loop uses several syntax forms. It uses a post-increment in the while-loop condition. This means the test is done before the increment is done.

public class Program {
    public static void main(String[] args) {

        int index = 0;
        // Use post increment in while-loop expression.
        while (index++ < 10) {

            // Continue if even number.
            if ((index % 2) == 0) {
                continue;
            }
            System.out.println("Element: " + index);
        }
    }
}
Element: 1
Element: 3
Element: 5
Element: 7
Element: 9

Loops seem simple—but we find the most complex algorithms are based mainly on loops. It is important to clearly understand Java loops.