A repetition control structure executes a statement of code multiple times in succession.
The while statement has a termination condition, implemented as a boolean expression, that will continue as long as the expression evaluates to true.
Curly braces required for block of multiple statements, optional for single statement
while(booleanExpression) {
}
A while loop is similar to an if-then statement in that it is composed of a boolean expression and a statement, or block of statements.
The boolean expression is evaluated before each iteration of the loop and exits if the evaluation returns false.
A while loop may terminate after evaluation of the boolean expression for the first time. In this manner, the statement block may never be executed.
Consider the following segment of code:
The boolean expression that is evaluated prior to each loop iteration is never modified, so the expression (x < 10) will always evaluate to true.
The result is that the loop will never end.
int x = 1;
int y = 1;
while(x < 10)
y++;
A do-while loop is a repetition control structure with a termination condition and statement, or block of statements.
A do-while loop guarantees that the statement or block will be executed at least once.
Curly braces required for block of multiple statements, optional for single statement.
do { } while (booleanExpression);
For example, take a look at the output of the following statements:
int x = 0; do { x++; } while(false); System.out.println(x); // Outputs 1
Java will execute the statement block first, and then check the loop condition.