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You use a whilestatement to continually execute a block of statements while a condition remains true. The general syntax of the
while
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'.The value printed by the last line is: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); } }Copy this strin
.Java provides another statement that is similar to the
while
statement--the do-whilestatement. The general syntax of the
do
-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
:
The value printed by the last line is: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); } }Copy this strin
.
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