Loops

Imagine you need to print "Hello" 1,000 times, or add up every number in a list, or check every word in a document for spelling errors. You can't write 1,000 lines of code for that! Loops let you repeat a block of code as many times as you need — they're one of the most powerful concepts in all of programming.

Learn Loops in our free Java course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Java course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn

• ✅ for loops — when you know exactly how many times to repeat
• ✅ while loops — when you don't know when to stop
• ✅ do-while loops — guaranteed to run at least once
• ✅ Enhanced for-each loops — the easiest way to iterate over arrays
• ✅ break and continue — controlling loop flow
• ✅ Nested loops — loops inside loops (multiplication tables, patterns)
• ✅ How to avoid the dreaded infinite loop

💡 Real-World Analogy

A loop is like a washing machine cycle. You set the conditions ("wash for 30 minutes" or "until the clothes are clean"), and the machine repeats the same actions (agitate, rinse) until the condition is met. A for loop is like a timed cycle ("run 10 times"). A while loop is like a sensor-based cycle ("keep going until clean").

1️⃣ The For Loop — "Repeat Exactly N Times"

The for loop is the most common loop in Java. Use it when you know how many times you want to repeat. It packs three things into one line: where to start, when to stop, and how to count.

• int i = 0 — create the counter (runs once)
• i < 5 → true → run the body → print "i = 0"
• i++ → i becomes 1
• i < 5 → true → run the body → print "i = 1"
• ...continues until i reaches 5
• 5 < 5 → false → loop ends

2️⃣ The While Loop — "Keep Going Until..."

The while loop checks its condition before each iteration. If the condition is false from the start, the body never runs at all. Use it when you don't know in advance how many iterations you need.

Common use case: Reading user input until they type "quit":

⚠️ Critical: The condition variable ( power , input ) must change inside the loop. If it never changes, the condition never becomes false, and you get an infinite loop — your program freezes!

3️⃣ Do-While — "Do It At Least Once"

The do-while loop is like a while loop, but it checks the condition after running the body. This means the body always runs at least once , even if the condition is false. It's perfect for menu systems and input validation.

🔑 while vs do-while: If the user might not need to see the prompt at all, use while . If you always want to show it at least once (menus, prompts), use do-while .

4️⃣ Enhanced For-Each — "For Every Item In..."

When you want to go through every item in an array or collection, the for-each loop is the cleanest option. No counter variable, no index, no chance of an off-by-one error.

• When you need the index (e.g., "item 3 of 10")
• When you need to modify the array while looping
• When you need to loop backwards

In these cases, use a regular for loop instead.

5️⃣ Break & Continue — Loop Flow Control

Sometimes you need to exit a loop early or skip certain iterations. Java gives you two tools:

break — "Stop the loop NOW"

continue — "Skip this one"

💡 Use sparingly: Too many break and continue statements make code hard to follow. Often you can restructure your condition instead. But for search operations, break is perfectly natural.

6️⃣ Nested Loops — Loops Inside Loops

You can put a loop inside another loop. The inner loop runs completely for each iteration of the outer loop. If the outer loop runs 3 times and the inner loop runs 4 times, the body runs 3 × 4 = 12 times total.

Common uses: 2D grids (game boards, images), generating tables, comparing every pair of items, building patterns.

7️⃣ The Infinite Loop — And How to Avoid It

An infinite loop is a loop that never stops. Your program freezes, your CPU spins at 100%, and nothing else can happen. It's the most common loop bug:

🛟 If your program is stuck in an infinite loop:

• In terminal: Press Ctrl + C to force-stop
• In IDE: Click the red "Stop" button
• To prevent: Always verify your loop variable changes in the right direction

8️⃣ Which Loop Should I Use?

Question

Answer

Use

Do you know how many times?

Yes

for

No

while

Must it run at least once?

do-while

Iterating an array/list?

Yes, all items

for-each

Common Mistakes

• ❌ Infinite loop — forgetting to update the counter:
• ❌ Off-by-one error: Using <= when you meant < , or starting at 1 instead of 0. Arrays start at index 0!
• ❌ Modifying a collection during for-each: You cannot add/remove items from an ArrayList while iterating with for-each. Use an Iterator or collect changes separately.
• ❌ Wrong loop variable in nested loops: Using i in both outer and inner loop causes shadowing bugs. Use different names like i and j .

Pro Tips

💡 Prefer for-each when possible — it's cleaner and prevents index errors. Only use a regular for loop when you need the index.

💡 Use meaningful variable names — for (String student : students) is much clearer than for (String s : arr) .

💡 Extract loop bodies into methods — if the loop body is more than 5-10 lines, pull it into a separate method for readability.

💡 Consider Stream API later — once you learn Streams (Lesson 27), you'll often replace loops with .filter() , .map() , and .collect() .

📋 Quick Reference

Loop

Syntax

Best For

for (init; cond; update)

Known iteration count

while (condition)

Unknown count, check first

do { } while (cond);

Must run at least once

for (T item : collection)

Iterate all items in array/list

break

break;

Exit loop immediately

continue

continue;

Skip to next iteration

🎉 Lesson Complete!

You've mastered all four types of Java loops, plus break and continue for flow control! You can now automate repetitive tasks, process arrays, build patterns, and even solve FizzBuzz — a classic coding interview question.

Next up: Methods — write reusable code blocks with parameters and return values to organize your programs into clean, testable pieces.

Practice quiz

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