Java Tutorial - Java While

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The while loop repeats a statement or block while its controlling condition is true.

Java while Loop

Here is its general form:

while(condition) { 
   // body of loop 
}

• The condition can be any Boolean expression.
• The body of the loop will be executed as long as the conditional condition is true.
• The curly braces are unnecessary if only a single statement is being repeated.

Here is a while loop that counts down from 10, printing exactly ten lines of "tick":

 
public class Main {
  public static void main(String args[]) {
    int n = 10;
    while (n > 0) {
      System.out.println("n:" + n);
      n--;
    }
  }
}

When you run this program, you will get the following result:

Example

The following code shows how to use the while loop to calculate sum.

public class Main {
  public static void main(String[] args) {
    int limit = 20;
    int sum = 0;/*from   ww  w.j a  va 2  s. c om*/
    int i = 1;

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

The code above generates the following result.

Example 2

The body of the while loop will not execute if the condition is false. For example, in the following fragment, the call to println() is never executed:

 
public class Main {
  public static void main(String[] argv) {
    int a = 10, b = 20;
    while (a > b) {
      System.out.println("This will not be displayed");
    }
    System.out.println("You are here");
  }
}

The output:

Example 3

The body of the while can be empty. For example, consider the following program:

 
public class Main {
  public static void main(String args[]) {
    int i, j;//from  w ww. ja v a  2  s . co  m
    i = 10;
    j = 20;

    // find midpoint between i and j
    while (++i < --j)
      ;
    System.out.println("Midpoint is " + i);
  }
}

The while loop in the code above has no loop body and i and j are calculated in the while loop condition statement. It generates the following output:

Java do while loop

To execute the body of a while loop at least once, you can use the do-while loop.

The syntax for Java do while loop is:

do { 
   // body of loop 
} while (condition);

Here is an example to show how to use a do-while loop.

 
public class Main {
  public static void main(String args[]) {
    int n = 10;
    do {
      System.out.println("n:" + n);
      n--;
    } while (n > 0);
  }
}

The output:

The loop in the preceding program can be written as follows:

public class Main {
  public static void main(String args[]) {
    int n = 10;
    do {
      System.out.println("n:" + n);
    } while (--n > 0);
  }
}

The output is identical the result above:

Example 4

The following program implements a very simple help system with do-while loop and switch statement.

 
public class Main {
  public static void main(String args[]) throws java.io.IOException {
    char choice;// ww  w.  j  av  a  2  s .  c  om
    do {
      System.out.println("Help on:");
      System.out.println(" 1. A");
      System.out.println(" 2. B");
      System.out.println(" 3. C");
      System.out.println(" 4. D");
      System.out.println(" 5. E");
      System.out.println("Choose one:");
      choice = (char) System.in.read();
    } while (choice < '1' || choice > '5');
    System.out.println("\n");
    switch (choice) {
      case '1':
        System.out.println("A");
        break;
      case '2':
        System.out.println("B");
        break;
      case '3':
        System.out.println("C");
        break;
      case '4':
        System.out.println("D");
        break;
      case '5':
        System.out.println("E");
        break;
    }
  }
}

Here is a sample run produced by this program:

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