Java Notes

'while' Statement

Purpose - to repeat statements

The purpose of the while statement is to repeat a group of Java statements many times. It's written just like an if statement, except that it uses the while keyword.

General Form

The while statement has this form:

   while (condition) {
      statements to repeat while the condition is true
   }

Condition is `true` or `false`

The value of condition must be true or false (ie, a boolean value). It is often a comparison (see example below).

Example: Loop continues while the condition is true

public void paintComponent(Graphics g) {
    super.paintComponent(g);
    int count = 0;
    while (count < 50) {
       g.drawLine(20, count*5, 80, count*5);
       count = count + 1;
    }
    g.drawString("Loop is finished.  count="+count, 10, 300);
}

This repeats the drawLine() call 50 times. The first time the while condition is tested, it is true because the value of count is 0, which is less than 50. After the statements in the body are done, the while loop comes back to the top of the loop and makes the test again. Each time the value of count is larger. Finally, count is 50, and the value of the condition will be false.

When the loop stops, the program continues with the statement after the end of the loop (the drawLine() call). This will display the string "Loop is finished. count=50".

Example with flowchart

The following shows some code and the equivalent flowchart.

   int n = 0;
   int i = 1;
   while (i < 4) {
      n++;
      i++;
   }
   . . .

Here is another example of code and an equivalent flowchart

   int n = 0;
   int i = 1;
   while (i < 4) {
      int j = 1;
      while (j<=i) {
         n += 1;
         j++;
      }
      i = i + 1;
   }