"do" Statements

Java Tutorials - Herong's Tutorial Examples

∟"do" Statements

This section describes 'do' statement, which is a looping statement that executes the contained statement immediately, then repeat it while a given condition is true.

What Is "do" Statement? - A "do" statement is a looping statement that executes the contained statement immediately, then repeat it while a given condition is true.

Here is the syntax for a "do" statement.

do contained_statement while (looping_condition)

When a "do" statement is executed, the flow of control can be described as:

Step 1. Run "contained_statement".

Step 2. Evaluate "looping_condition". If result is "false", end the "while" statement.

Step 3. Run "contained_statement".

Step 4. Go back to Step 2.

The contained statement can be empty, a simple single statement, or a complex statement block. Here is a sample program that shows you how to use "do" statements:

/* DoStatementTest.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
class DoStatementTest {
   private static int rank = 5;
   private static int downgrade() {
      rank--;
      System.out.println("   Rank downgraded to: "+rank);
      return rank;
   }

   public static void main(String[] arg) {
      java.io.PrintStream out = System.out;

      out.println("\"do\" statement with no contained statement:");
      do ; while (downgrade() > 0);

      int max = 20;
      int sum = 0;
      int i = 0;
      out.println("\"do\" statement with a contained statement:");
      do sum += ++i; while (i < max);
      out.println("   Sum of 1 to 20: "+sum);

      boolean isPrime;
      i = 3;
      out.println("\"do\" statement with a contained statement block:");
      do {
         isPrime = true;
         int j = 2;
         do {
            isPrime = i%j > 0;
            if (!isPrime) break;
            j++;
         } while (j < i);
         if (isPrime) out.println("   "+i+" is a prime number.");
         i++;
      } while (i < max);
   }
}

If you compile and run the above program, you will see:

herong> java DoStatementTest.java

"do" statement with no contained statement:
   Rank downgraded to: 4
   Rank downgraded to: 3
   Rank downgraded to: 2
   Rank downgraded to: 1
   Rank downgraded to: 0

"do" statement with a contained statement:
   Sum of 1 to 20: 210

"do" statement with a contained statement block:
   3 is a prime number.
   5 is a prime number.
   7 is a prime number.
   11 is a prime number.
   13 is a prime number.
   17 is a prime number.
   19 is a prime number.