Java: Loops - 'while' and 'for'

Purpose

The purpose of loop statements is to repeat Java statements many times. There are several kinds of loop statements in Java.

while statement - Test at beginning

The while statement is used to repeat a block of statements while some condition is true. The condition had better become false somewhere in the loop, otherwise it will never terminate.

int i = 0;
while {i < names.length) (
    System.out.println(names[i]);
    i++;
}

for statement - traditional three part

If you want to repeat a block of statements a fixed number of times, the for loop is the right choice. It's alos used in many other cases where there the intitialization of a loop, the loop condition, and the loop increment can be combined.

for (int i = 0; i < names.length; i++) (
    System.out.println(names[i]);
}

do..while statement - Test at end

When you want to test at the end to see whether something should be repeated, the do..while statement is the natural choice.

do (
    . . .
    String ans = JOptionPane.showInputDialog(null, "Do it again (Y/N)?");
} while (ans.equals("Y"));

for statement - Java 5 data structure iterator

Java 5 introduced what is sometimes called a "for each" statement that accesses each successive element of an array, List, or Set without the bookkeeping associated with iterators or indexing.

for (String s : names) (
    System.out.println(s);
}

Similar to the 'if' statement

There are three general ideas that you will see in many parts of Java.

• use of braces {} to enclose multiple statements.
• indentation
• boolean (true/false) conditions.

Scope of loop indicated with braces {}

If the body of a loop has more than one statement, you must put the statements inside braces. If there is only one statement, it is not necessary to use braces {}. However, many programmers think it is a good idea to always use braces to indicate the scope of statements. Always using braces allows the reader to relax and not worry about the special single statement case.

Indentation of loops

All statements inside a loop should be indented 2-4 spaces (the same as an if statement.

boolean true/false conditions

True/false (boolean) expressions control both loops and the if statement.