Programming Tutorials

while loop in java

By: Jagan in Java Tutorials on 2007-09-07  

The while loop is Java's most fundamental looping statement. It repeats a statement or block while its controlling expression is true. Here is its general form:

while(condition) {
// body of loop
}

The condition can be any Boolean expression. The body of the loop will be executed as long as the conditional expression is true. When condition becomes false, control passes to the next line of code immediately following the loop. The curly braces are unnecessary if only a single statement is being repeated. Here is a while loop that counts down from 10, printing exactly ten lines of "tick":

// Demonstrate the while loop.
class While {
    public static void main(String args[]) {
        int n = 10;
        while (n > 0) {
            System.out.println("tick " + n);
            n--;
        }
    }
}

When you run this program, it will "tick" ten times:

tick 10
tick 9
tick 8
tick 7
tick 6
tick 5
tick 4
tick 3
tick 2
tick 1

Since the while loop evaluates its conditional expression at the top of the loop, the body of the loop will not execute even once if the condition is false to begin with. For example, in the following fragment, the call to println() is never executed:

int a = 10, b = 20;
while(a > b)
System.out.println("This will not be displayed");

The body of the while (or any other of Java's loops) can be empty. This is because a null statement (one that consists only of a semicolon) is syntactically valid in Java. For example, consider the following program:

// The target of a loop can be empty.
class NoBody {
    public static void main(String args[]) {
        int i, j;
        i = 100;
        j = 200;
        // find midpoint between i and j
        while (++i < --j)
            ; // no body in this loop
        System.out.println("Midpoint is " + i);
    }
}

This program finds the midpoint between i and j. It generates the following output:

Midpoint is 150

Here is how the while loop works. The value of i is incremented, and the value of j is decremented. These values are then compared with one another. If the new value of i is still less than the new value of j, then the loop repeats. If i is equal to or greater than j, the loop stops. Upon exit from the loop, i will hold a value that is midway between the original values of i and j. (Of course, this procedure only works when i is less than j to begin with.) As you can see, there is no need for a loop body; all of the action occurs within the conditional expression, itself. In professionally written Java code, short loops are frequently coded without bodies when the controlling expression can handle all of the details itself.