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The while and do-while Statements

You use a while statement to continually execute a block of statements while a condition remains true. The general syntax of the while statement is:

while (expression) {
    statement
}

First, the while statement evaluates expression, which must return a boolean value. If the expression returns true, then the while statement executes the statement(s) associated with it. The while statement continues testing the expression and executing statements until the expression returns false.

The example program shown below, called WhileDemo, uses a while statement to iterate over a string, copying the characters from the string into a string buffer until it encounters the letter 'g'.

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

        String copyFromMe = "Copy this string until you encounter the letter 'g'.";
        StringBuffer copyToMe = new StringBuffer();

        int i = 0;
        char c = copyFromMe.charAt(i);

        while (c != 'g') {
            copyToMe.append(c);
            c = copyFromMe.charAt(++i);
        }
        System.out.println(copyToMe);
    }
}

The value printed by the last line is: Copy this strin.

Java provides another statement that is similar to the while statement--the do-while statement. The general syntax of the do-while is:

do {
    statement
} while (expression);

Instead of evaluating the expression at the top of the loop, do-while evaluates the expression at the bottom. Thus the statements associated with a do-while are executed at least once.

Here's the previous program re-written to use do-while and renamed to DoWhileDemo:

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

        String copyFromMe = "Copy this string until you encounter the letter 'g'.";
        StringBuffer copyToMe = new StringBuffer();

        int i = 0;
        char c = copyFromMe.charAt(i);

        do {
            copyToMe.append(c);
            c = copyFromMe.charAt(++i);
        } while (c != 'g');
        System.out.println(copyToMe);
    }
}

The value printed by the last line is: Copy this strin.

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