break continue do while for Modulo Random

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.

Loop details

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.

A program

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.

And We increment "i" upwards and decrement "z" downwards. When "i" is no longer smaller than "z," the loop ceases.

Thus In programs, for-loops are more often useful. But in cases where the loop termination is unknown, a while-loop is better.

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.

However The break statement terminates the loop. No further iterations are run.

Here This program will continue looping in the while-loop until it reaches a random number greater than 0.8.

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.

Detail A do-while loop can make programs faster by reducing the number of checks done.

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.

Detail We also use a modulo test to see if a number is evenly divisible by 2. We use the continue statement to stop the current iteration.

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

Summary

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