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You use awhile
statement to continually execute a block of statements while a condition remains true. The general syntax of thewhile
statement is:First, thewhile (expression) { statement }while
statement evaluates expression, which must return a boolean value. If the expression returns true, then thewhile
statement executes the statement(s) associated with it. Thewhile
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'.Java provides another statement that is similar to thepublic 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); } }while
statement--thedo
-while
statement. The general syntax of thedo
-while
is:Instead of evaluating the expression at the top of the loop,do { statement } while (expression);do
-while
evaluates the expression at the bottom. Thus the statements associated with ado
-while
are executed at least once.Here's the previous program re-written to use
do
-while
and renamed toDoWhileDemo
:
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); } }
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