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Iteration allows you to repeat a sequence of steps.  Java provides three different loops the allow you to implement an iterative algorithm: for loops, while loops, and do while loops.


The for loop is typically used as a counting loop.  If you want to execute a sequence of steps a fixed number of times (e.g., you know you need to do the same operation 10 times) you should use a for loop.  The syntax for a for loop is as follows:
for(initialization; condition; update) {
The keyword for is followed by parentheses.  Inside of the parentheses is the initialization of the control variable followed by a semi-colon, the condition followed by a semi-colon, and the update of the control variable.  The body of the loop is surrounded by curly braces.  A common for loop might look as follows:

for(int i = 0; i < 10; i++) {

You may omit the braces in the event that your loop has only one statement in the body, but this is not recommended.


The while loop is used in situations when the loop should execute as long as some condition is true.  For example, you might use a while loop to indicate that you want to continue to get input from a user until that user enters a sentinel value (e.g, enter 0 to end input).  The general syntax for a while loop is as follows:

//control variable initialization
while(condition) {
    //update control variable

The control variable is initialized before the loop, the condition is in parentheses following the while keyword, and the body of the loop is surrounded by curly braces.  The body of the loop must contain the update of the control variable.  Failing to update the control variable results in an infinite loop.

A while loop can be translated to a for loop and vice versa.  Below are two examples of while loops: the first is identical to the for loop above and the second is a better example of a while loop that executes until a sentinel value is entered.

int i = 0;
while(i < 10) {

System.out.println("Enter an int (0 to end input): ");
int i = scanner.nextInt();
while(i != 0) {
    System.out.println("Enter another int (0 to end input): ");
    i = scanner.nextInt();
do while

While a for loop can be translated to a while and vice versa, a do while loop operates differently.  A do while loop always executes at least once.  The syntax for a do while loop is as follows:
do {
    //control variable initialization/update
} while(condition);
Because you know the body of the loop will execute at least once, you can initialize your control variable in the same place where you update it.