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Thinking in Java
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Iteration

Looping is controlled by while, do-while and for, which are sometimes classified as iteration statements. A statement repeats until the controlling Boolean-expression evaluates to false. The form for a while loop is

while(Boolean-expression)
  statement


The Boolean-expression is evaluated once at the beginning of the loop and again before each further iteration of the statement.

Here’s a simple example that generates random numbers until a particular condition is met:

//: c03:WhileTest.java
// Demonstrates the while loop.
import com.bruceeckel.simpletest.*;

public class WhileTest {
  static Test monitor = new Test();
  public static void main(String[] args) {
    double r = 0;
    while(r < 0.99d) {
      r = Math.random();
      System.out.println(r);
      monitor.expect(new String[] {
        "%% \\d\\.\\d+E?-?\\d*"
      }, Test.AT_LEAST);
    }
  }
} ///:~


This uses the static method random( ) in the Math library, which generates a double value between 0 and 1. (It includes 0, but not 1.) The conditional expression for the while says “keep doing this loop until the number is 0.99 or greater.” Each time you run this program, you’ll get a different-sized list of numbers.

In the expect( ) statement, you see the Test.AT_LEAST flag following the expected list of strings. The expect( ) statement can include several different flags to modify its behavior; this one says that expect( ) should see at least the lines shown, but others may also appear (which it ignores). Here, it says “you should see at least one double value.”
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire