Think Java

Chapter 7. Loops

Computers are often used to automate repetitive tasks. Repeating tasks without making errors is something that computers do well and people do poorly.

Running the same code multiple times is called iteration. We have seen methods, like countdown and factorial, that use recursion to iterate. Although recursion is elegant and powerful, it takes some getting used to. Java provides language features that make iteration much easier: the while and for statements.

The while Statement

Using a while statement, we can rewrite countdown like this:

public static void countdown(int n) {
    while (n > 0) {
        System.out.println(n);
        n = n - 1;
    }
    System.out.println("Blastoff!");
}

You can almost read the while statement like English: “While n is greater than zero, print the value of n and then reduce the value of n by 1. When you get to zero, print Blastoff!”

The expression in parentheses is called the condition. The statements in braces are called the body. The flow of execution for a while statement is:

1. Evaluate the condition, yielding true or false.

2. If the condition is false, skip the body and go to the next statement.

3. If the condition is true, execute the body and go back to step 1.

This type of flow is called a loop, because the last step loops back around to the first.

The body of the loop should change the value of one or more variables so that, eventually, the condition becomes false and the loop terminates. Otherwise the loop will repeat forever, which is called an infinite loop ...