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Trail: Learning the Java Language
Lesson: Language Basics

## 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 its block until the expression returns `false`.

> The example program shown below, called `WhileDemo`, uses a `while` statement to step through the characters of a string, appending each character from the string to the end of 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`.

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

> >     do {
> >         statement(s)
> >     } 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 rewritten 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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