Chapter 17: do-while Loop in Java

This chapter is only about the do-while loop: run the body first, then check whether to repeat. The body runs at least once. while (Chapter 16) checks before the first run and can skip entirely.

1. Topic title

do { ... } while (condition); — guaranteed first pass, then same rules as while

2. What it means

do {
    // runs at least once
} while (condition);

Note the semicolon after while (condition) — required syntax.

Flow:

1. Execute the body.
2. Evaluate condition; if true, go to step 1; if false, exit.

Compare to while: step 2 happens before the body in while, so the body might never run.

3. Why it is used

Whenever the program must show something or read input once before it makes sense to ask “again?”—menus, “press 0 to exit” consoles, validating “try again until valid” where you want to read before checking.

4. Real-world example

A text menu: print options, read user choice, act, loop until user picks Quit. The menu should appear once even if the default choice is “continue”—do-while matches that story.

5. Java code example

public class DoWhileDemo {
    public static void main(String[] args) {
        int attempt = 0;
 
        do {
            attempt++;
            System.out.println("Guess attempt #" + attempt + " (simulated wrong guess)");
        } while (attempt < 3);
 
        int x = 10;
        do {
            System.out.println("x = " + x);
            x--;
        } while (x > 7);
 
        int counter = 0;
        do {
            counter++;
        } while (counter < 1);
 
        System.out.println("counter after odd do-while = " + counter);
    }
}

Trace second loop: prints 10, 9, 8 — then x is 7, condition x > 7 is false, stop.

Third mini-example shows a degenerate pattern on purpose: body runs, then condition fails—still one iteration. Useful to see that “at least once” is literal.

6. Explanation of code

• First do-while: always logs at least one “simulated guess” before checking attempt < 3.
• Second: demonstrates numeric countdown with post-body check.
• Third: counter becomes 1 inside; counter < 1 is false → loop ends after one pass.

7. Common mistakes

Forgetting the semicolon after while (...); — compile error.

Using do-while when while is clearer — if “zero times” is valid, do-while is the wrong tool.

Wrong condition polarity — do { ... } while (quit == false) vs !quit; pick readable names.

Infinite do-while — same as while; body must move state.

8. Best practices

Default to while unless you truly need the first pass unconditionally.

Keep bodies short; long menu switch inside do-while is common in student code—still try to extract methods when it grows.

Match user mental model: “show menu, then decide repeat” → do-while.

9. Small practice task

Write a do-while that prints Entering zone once, then keeps adding step to position starting from 0 while position < 15, printing position each time. Use step = 5. Predict output before you run.

Beginner tip

If you can rewrite as while without changing behavior, you did not need do-while. That is a valid sanity check.