Loops — for, while, do-while, for-each, break, continue, and Labeled Loops

In this tutorial, you will learn about Loops. We cover key concepts, practical examples, and best practices to help you master this topic.

Loops in Java execute a block of code repeatedly, with for, while, do-while, and for-each constructs providing different iteration strategies. Without loops, processing a list of a thousand items would require a thousand lines of code — loops let you write the logic once and let the machine handle repetition.

What You'll Learn

• When to use each loop type
• Infinite loops and how to avoid them
• break, continue, and labeled loops for fine-grained control
• The enhanced for-each loop for collections and arrays

Why It Matters

Choosing the wrong loop pattern leads to off-by-one errors, infinite loops, or unreadable code. The enhanced for-each loop, for example, is cleaner but cannot modify the collection during iteration — using it incorrectly causes ConcurrentModificationException.

Real-World Use

Loops Process data in every layer: a web server loops through request headers, a batch job loops through database records, and a game engine loops through the update cycle 60 times per second.

The for Loop

The classic for loop has three parts: initialization, condition, and update.

for (int i = 0; i < 5; i++) {
    System.out.println("Iteration: " + i);
}

Output:

Iteration: 0
Iteration: 1
Iteration: 2
Iteration: 3
Iteration: 4

Execution order:

1. int i = 0 runs once before the loop
2. i < 5 is evaluated before each iteration — if false, the loop exits
3. The body executes
4. i++ runs after the body
5. Repeat from step 2

Multiple Variables

for (int i = 0, j = 10; i < j; i++, j--) {
    System.out.println("i=" + i + ", j=" + j);
}

Off-by-One Errors

// Common bug: using <= instead of <
for (int i = 0; i <= 5; i++) {  // runs 6 times (0-5)
    // ...
}

The while Loop

Use while when the number of iterations is not known in advance:

Scanner scanner = new Scanner(System.in);
String input = "";
while (!input.equals("quit")) {
    System.out.print("Enter a command: ");
    input = scanner.nextLine();
    System.out.println("You entered: " + input);
}

The condition is checked before each iteration. If it is initially false, the body never executes.

Infinite Loops

// Bug: condition never becomes false
int x = 0;
while (x < 10) {
    // forgot to increment x
}

Always ensure the loop variable is updated toward the termination condition.

The do-while Loop

do-while guarantees the body executes at least once:

Scanner scanner = new Scanner(System.in);
int number;
do {
    System.out.print("Enter a positive number: ");
    number = scanner.nextInt();
} while (number <= 0);

This is useful for menu-driven programs where you want to show the menu at least once before checking for exit.

The Enhanced for-each Loop

The for-each loop iterates over arrays and collections without an explicit index:

String[] fruits = {"Apple", "Banana", "Cherry"};
for (String fruit : fruits) {
    System.out.println(fruit);
}

Output:

Apple
Banana
Cherry

With Collections

List<Integer> numbers = List.of(10, 20, 30);
for (int n : numbers) {
    System.out.println(n * 2);
}

Limitation: Cannot Modify the Collection

The for-each loop uses an internal Iterator. If you modify the collection during iteration (adding or removing elements), you get a ConcurrentModificationException:

List<String> list = new ArrayList<>(List.of("a", "b", "c"));
for (String s : list) {
    if (s.equals("b")) {
        list.remove(s); // throws ConcurrentModificationException
    }
}

Use an explicit Iterator or removeIf() for structural modification.

break and continue

break

break exits the loop immediately:

for (int i = 0; i < 10; i++) {
    if (i == 5) {
        break; // loop stops when i is 5
    }
    System.out.print(i + " ");
}
// Output: 0 1 2 3 4

continue

continue skips the rest of the current iteration and proceeds to the next:

for (int i = 0; i < 10; i++) {
    if (i % 2 == 0) {
        continue; // skip even numbers
    }
    System.out.print(i + " ");
}
// Output: 1 3 5 7 9

Labeled Loops

Labels allow break and continue to target an outer loop from within nested loops:

outer:
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
        if (i == 1 && j == 1) {
            break outer; // exits both loops
        }
        System.out.println("i=" + i + ", j=" + j);
    }
}

Output:

i=0, j=0
i=0, j=1
i=0, j=2
i=1, j=0

Without the label, break would only exit the inner loop. Labels make the intent explicit, though some developers prefer extracting the nested loop into a method with a return statement.

Loop Comparison

Common Mistakes

1. Off-by-one errors with <= instead of <. for (int i = 0; i <= array.length; i++) causes ArrayIndexOutOfBoundsException.
2. Modifying a collection during for-each iteration. Use Iterator.remove() or collect removals in a separate list.
3. Using continue in a while loop without updating the loop variable. This creates an infinite loop because the increment is skipped.
4. Forgetting to advance the Scanner in a while loop. while (scanner.hasNext()) without calling next() loops forever.
5. Using == for string comparison in loop conditions. while (!input.equals("quit")) is correct; while (input != "quit") compares references.

Practice Questions

1. What is the output of this code?

int i = 0;
while (i < 3) {
    System.out.print(i + " ");
    i++;
}

0 1 2

2. How does break differ from continue?
break exits the loop entirely. continue skips the current iteration and moves to the next.

3. Why does modifying a List during for-each iteration throw an exception?
The for-each loop uses an internal iterator that checks for structural modification via a modCount field. Any change increments the count, and the iterator throws ConcurrentModificationException when it detects the change.

4. What is the difference between while and do-while?
while checks the condition before executing the body (zero or more iterations). do-while checks after (one or more iterations).

5. What does break outer do in nested loops?
It exits the loop labeled outer, which can be the outer of multiple nested loops. Without the label, break only exits the innermost loop.

Challenge Question:
Write a program that prints a multiplication table (1-12) using nested loops. Use the \t tab character for alignment. Then use a labeled continue to skip printing any row or column where the product is a multiple of 5.

FAQ

Mini Project

Write a program PrimeFinder.java that:

1. Uses nested loops to find and print all prime numbers between 2 and 100
2. The outer loop iterates through candidate numbers
3. The inner loop checks divisibility by numbers from 2 to sqrt(candidate)
4. Uses labeled continue or break to skip non-primes
5. Prints the result in the format: Prime numbers: 2, 3, 5, 7, 11, ...

Then extend it to accept an upper limit from command-line arguments. Finally, rewrite the inner loop as a separate method isPrime(int n).

What's Next

Now that you can repeat operations, you need data structures to hold collections of values. Lesson 8 introduces arrays — the most fundamental data structure in Java — including declaration, initialization, multi-dimensional arrays, and the Arrays utility class for sorting, searching, and filling.