Java Do While Loop

Java do-while loop is an Exit control loop. Unlike for or while loop, a do-while check for the condition after executing the statements of the loop body.

Example:

public class GFG {
  
    public static void main(String[] args) {
      
        int c = 1;

      // Using do-while loop
        do {
            System.out.println("GeeksforGeeks: " + c);
            c++;
        } while (c <= 5);
    }
}

GeeksforGeeks: 1
GeeksforGeeks: 2
GeeksforGeeks: 3
GeeksforGeeks: 4
GeeksforGeeks: 5

Explanation:

• do-while loop executes the code block at least once because the condition is checked after the loop body runs.
• In each iteration, the value of c is printed and then incremented using c++.
• The loop continues until the condition c <= 5 becomes false, after which the loop terminates.

Syntax: 

do
{
// Loop Body
Update_expression
}

// Condition check
while (test_expression);

Parameters:

• test_expression: boolean condition that controls the loop
• update_expression: updates the loop variable

Note: The test_expression in a do-while loop must evaluate to a boolean value; otherwise, a compile-time error occurs.

Working of Do-While Loop

Example:

class GFG {

    public static void main(String[] args)
    {
        // initial counter variable
        int c = 0;

        do {

            // Body of loop that will execute minimum 1 time for sure no matter what
            System.out.println("Print Statement");
            c++;
        }

        // Checking condition
        // Note: It is being checked after inimum 1 iteration
        while (c < 0);
    }
}

Print Statement

Explanation:

• The do block executes once before the condition is checked, so "Print Statement" is printed even though the condition is false.
• The variable c starts at 0 and is incremented to 1 inside the loop.
• The condition c < 0 is false, so the loop terminates after the first iteration

Flowchart do-while loop:

Example 1: This program will try to print "Hello World" 5 times.  

class GFG {
    public static void main(String args[])
    {
        // Declaring and initialization expression
        int c = 1;

        // Do-while loop
        do {
            // Body of do-while loop
            // Print statement
            System.out.println("Hello World");

            // Update expression
            c++;
        }
        // Test expression
        while (c < 6);
    }
}

Hello World
Hello World
Hello World
Hello World
Hello World

Explanation:

• The variable c is initialized to 1 before entering the loop.
• The do block executes first and prints "Hello World", ensuring the loop runs at least once.
• After printing, c is incremented using c++.
• The loop continues as long as the condition c < 6 is true, resulting in the message being printed 5 times.

Example 2: do-while loop without curly braces {}

import java.io.*;

class GFG {
    public static void main(String[] args) {
        int c = 1;

        do {
            // Only single statement in do block
            System.out.println("Hello GFG!");
        } 
        // This condition is false, so the loop will execute only once
        while (c >= 3);
    }
}

Hello GFG!

Explanation:

• The do-while loop executes the do block at least once, even though the condition c >= 3 is false.
• Since there is only one statement inside the loop, it runs once and then terminates when the condition fails.

Related Articles:

• for loop
• while loop