While Loop

The while Statement

One way to create a loop is by using the while statement. Another way is to use the for statement

The while statement has the following syntax.

while (booleanExpression) {     statement (s) }

1. statement(s) will be executed as long as booleanExpression evaluates to true.
2. If there is only a single statement inside the braces, you may omit the braces.
3. For clarity, you should always use braces even when there is only one statement.

As an example of the while statement, the following code prints integer numbers that are
less than three.

public class MainClass {   public static void main(String[] args) {     int i = 0;     while (i < 3) {       System.out.println(i);       i++;     }   } }

To produce three beeps with an interval of 500 milliseconds, use this code:

public class MainClass {   public static void main(String[] args) {     int j = 0;     while (j < 3) {       java.awt.Toolkit.getDefaultToolkit().beep();       try {         Thread.currentThread().sleep(500);       } catch (Exception e) {       }       j++;     }   } }

Sometimes, you use an expression that always evaluates to true

(such as the boolean literal true) but relies on the break statement to escape from the loop.

public class MainClass {

  public static void main(String[] args) {
    int k = 0;
    while (true) {
      System.out.println(k);
      k++;
      if (k > 2) {
        break;
      }
    }
  }

}

 

 

Using the while loop to calculate sum







public class MainClass {
  public static void main(String[] args) {
    int limit = 20;
    int sum = 0;
    int i = 1;

    while (i <= limit) {
      sum += i++;
    }
    System.out.println("sum = " + sum);
  }
}

 

Compile

sum = 210




While loop with double value

 

public class MainClass {
  public static void main(String[] args) {
    double r = 0;
    while(r < 0.99d) {
      r = Math.random();
      System.out.println(r);
    }
  }
}

 

Compile

0.4377278997305387
0.2688654455422754
0.36392537297385574
0.15254413511361042
0.6688621030424611
0.143156733550304
0.3867695752401421
0.6348496031126075
0.8262243996358971
0.057290108917235516
0.48887104413122007
0.9181002018325309
0.03580026338719999
0.6915245271034702
0.06281957479185263
0.7484170566879976
0.08309381253696246
0.7708638307798187
0.6586973690207539
0.3606321940413979
0.17404632324149583
0.1007512703731156
0.7971297767971545
0.4461084890266951
0.8627269068871244
0.04984737498714209
0.2597921665157922
0.5410470601800582
0.5779541886682865
0.1847556794086489
0.32127067165147705
0.12266607697656651
0.62729812639505
0.12191275953517977
0.72308829958122
0.5766115831015479
0.03444891261877858
0.3034596140603594
0.058872682884219985
0.85618745813345
0.07472991871327217
0.32676415639620604
0.6390549075075913
0.09246846051500635
0.9135703200463736
0.661365290176903
0.834153688687532
0.6340862529502882
0.3240420706550172
0.07246065490188025
0.35761285323105796
0.9964556875646731

Java's 'labeled while' loop 

 

public class MainClass {
  public static void main(String[] args) {
    int i = 0;
    outer: while (true) {
      System.out.println("Outer while loop");
      while (true) {
        i++;
        System.out.println("i = " + i);
        if (i == 1) {
          System.out.println("continue");
          continue;
        }
        if (i == 3) {
          System.out.println("continue outer");
          continue outer;
        }
        if (i == 5) {
          System.out.println("break");
          break;
        }
        if (i == 7) {
          System.out.println("break outer");
          break outer;
        }
      }
    }
  }
}

 

Compile

Outer while loop
i = 1
continue
i = 2
i = 3
continue outer
Outer while loop
i = 4
i = 5
break
Outer while loop
i = 6
i = 7
break outer

 




http://www.java2s.com/Tutorial/Java/0080__Statement-Control/Catalog0080__Statement-Control.htm