Do-While Loop in Java

Sometimes you need code to run before you know whether it should repeat. A login prompt, a menu, a dice roll — all of these must happen at least once before the user decides to continue or stop. That is exactly what the do-while loop is for.

What is an exit-controlled loop?

Every loop has one question: *should I run again?*

• A while loop checks the question before running the body. If the answer is "no" from the start, the body never runs.
• A do-while loop checks the question after running the body. So the body always runs at least once, no matter what.

This "check after" behaviour is called exit-controlled, because the exit decision happens at the end, not the beginning.

Syntax

Two things students forget:

1. The semicolon after while (condition) — it is required. Without it, Java gives a compile error.
2. The body still needs a variable that changes, otherwise the condition never becomes false and you get an infinite loop.

Your first do-while

Run this example to see its output.

Line by line:

• int count = 1; — counter starts at 1.
• do { — Java enters the body immediately without checking anything.
• System.out.println(...) — prints the current count.
• count++ — increases count by 1 each time.
• } while (count <= 5); — after each body run, Java checks: is count still ≤ 5? If yes, repeat. If no, stop.

When count reaches 6, the condition 6 <= 5 is false, so the loop stops. Total prints: 5.

do-while vs while — the key difference

Run this example to see its output.

value is 10, which is not less than 5. The while loop skips entirely — zero prints. The do-while loop runs the body once first, then checks the condition, finds it false, and stops — one print.

When to prefer do-while: any situation where the action must happen before the user or program can decide whether to repeat — menus, retry prompts, input validation.

Common mistake: forgetting the semicolon

Java sees } while (true) without a semicolon and does not know the statement is complete. Always end do-while with a semicolon.

Common mistake: infinite do-while

If the variable that the condition tests is never updated inside the body, the condition stays true forever.

Real use: menu-driven program

Menu programs are the classic do-while use case. The menu must appear before the user types anything.

Run this example to see its output.

Line by line:

• int choice; — declared outside the loop so the condition choice != 0 can see it.
• do { — menu prints before Java knows choice's value. This is the whole reason to use do-while here.
• scanner.nextInt() — reads what the user typed as an integer.
• } while (choice != 0); — if the user entered 0, exit. Otherwise, show the menu again.

Note: this program reads from keyboard (stdin). Run it in your own terminal, not a browser-based output window.

Practice task

Task: Write a number guessing game. Set a secret number (e.g., 42). Use a do-while loop to keep asking the user to guess until they get it right. Print "Too low", "Too high", or "Correct!" each round.

Hint: You need a variable for the user's guess declared before the loop. The condition should repeat while the guess is not equal to the secret number.

Solution:

Run this example to see its output.

What to observe: Try entering values less than 42, greater than 42, and finally 42 itself. Notice the loop always runs at least once — even before you type anything.