Java: Loops
Loops allow programs to repeat certain operations. This lesson covers the loop structures in Java.
1. for Loop
The for loop is the most commonly used loop structure, ideal when the number of iterations is known.
(1) Syntax
TEXT
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for (initialization; condition; update) {
// Loop body
}
▶ Example: Print 1 to 10
JAVA
public class ForDemo {
public static void main(String[] args) {
for (int i = 1; i <= 10; i++) {
System.out.println(i);
}
}
}
(2) Execution Flow
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1. Execute initialization (int i = 1)
2. Check condition (i <= 10)
3. If condition is true, execute loop body
4. Execute update (i++)
5. Go back to step 2
▶ Example: Sum of 1 to 100
JAVA
public class SumDemo {
public static void main(String[] args) {
int sum = 0;
for (int i = 1; i <= 100; i++) {
sum += i;
}
System.out.println("Sum of 1 to 100: " + sum); // 5050
}
}
2. while Loop
The while loop is ideal when the number of iterations is unknown.
(1) Syntax
JAVA
while (condition) {
// Loop body
}
▶ Example: Number Guessing Game
TEXT
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import java.util.Scanner;
import java.util.Random;
public class GuessGame {
public static void main(String[] args) {
Random random = new Random();
int target = random.nextInt(100) + 1;
Scanner scanner = new Scanner(System.in);
int guess = 0;
System.out.println("Number Guessing Game (1-100)");
while (guess != target) {
System.out.print("Enter your guess: ");
guess = scanner.nextInt();
if (guess > target) {
System.out.println("Too high!");
} else if (guess < target) {
System.out.println("Too low!");
} else {
System.out.println("Congratulations! You got it!");
}
}
scanner.close();
}
}
3. do-while Loop
The do-while loop executes the loop body at least once.
(1) Syntax
JAVA
do {
// Loop body
} while (condition);
▶ Example: Menu Loop
TEXT
📖 Display only
import java.util.Scanner;
public class MenuDemo {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int choice;
do {
System.out.println("\n=== Main Menu ===");
System.out.println("1. Start Game");
System.out.println("2. Settings");
System.out.println("3. Exit");
System.out.print("Enter your choice: ");
choice = scanner.nextInt();
switch (choice) {
case 1:
System.out.println("Starting game...");
break;
case 2:
System.out.println("Opening settings...");
break;
case 3:
System.out.println("Goodbye!");
break;
default:
System.out.println("Invalid choice");
}
} while (choice != 3);
scanner.close();
}
}
💡 while vs do-while: while checks the condition first and may never execute. do-while executes first and checks after, guaranteeing at least one execution.
4. for-each Loop
The for-each loop is used to iterate over arrays or collections.
(1) Syntax
JAVA
for (type variable : array or collection) {
// Use variable
}
▶ Example: Iterating an Array
JAVA
public class ForEachDemo {
public static void main(String[] args) {
int[] numbers = {1, 2, 3, 4, 5};
// Regular for loop
for (int i = 0; i < numbers.length; i++) {
System.out.println(numbers[i]);
}
// for-each loop
for (int num : numbers) {
System.out.println(num);
}
}
}
💡 When to use: Use for-each when you only need the elements. Use regular for when you need the index.
5. break and continue
(1) break: Exit the Loop
JAVA
public class BreakDemo {
public static void main(String[] args) {
for (int i = 1; i <= 10; i++) {
if (i == 5) {
break; // Exit the loop when i equals 5
}
System.out.println(i);
}
// Output: 1 2 3 4
}
}
(2) continue: Skip Current Iteration
JAVA
public class ContinueDemo {
public static void main(String[] args) {
for (int i = 1; i <= 10; i++) {
if (i % 2 == 0) {
continue; // Skip even numbers
}
System.out.println(i);
}
// Output: 1 3 5 7 9
}
}
6. Nested Loops
Loops can be nested inside each other.
▶ Example: Multiplication Table
JAVA
public class MultiplicationTable {
public static void main(String[] args) {
for (int i = 1; i <= 9; i++) {
for (int j = 1; j <= i; j++) {
System.out.print(j + " × " + i + " = " + (i * j) + "\t");
}
System.out.println();
}
}
}
Output:
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1 × 1 = 1
1 × 2 = 2 2 × 2 = 4
1 × 3 = 3 2 × 3 = 6 3 × 3 = 9
...
▶ Example: Print a Triangle
JAVA
public class TriangleDemo {
public static void main(String[] args) {
int rows = 5;
for (int i = 1; i <= rows; i++) {
// Print spaces
for (int j = 1; j <= rows - i; j++) {
System.out.print(" ");
}
// Print stars
for (int k = 1; k <= 2 * i - 1; k++) {
System.out.print("*");
}
System.out.println();
}
}
}
Output:
TEXT
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*
***
*****
*******
*********
7. Infinite Loops
If the loop condition is always true, an infinite loop is created.
JAVA
// Infinite loop example (don't do this)
while (true) {
System.out.println("Infinite loop");
}
// Correct infinite loop (with exit condition)
while (true) {
// Read user input
String input = scanner.nextLine();
if (input.equals("exit")) {
break; // Exit when user types "exit"
}
}
8. ❓ Frequently Asked Questions
Q How do I choose between for and while?
A Use for when the number of iterations is known. Use while when it's unknown. for is more concise, while is more flexible.
Q What's the difference between break and return?
A break exits the current loop. return exits the entire method.
Q How do I exit multiple nested loops?
A Use labeled break.
outer: for(...) { for(...) { break outer; } }❓ FAQ
Q When should I use while vs for?
A Use for when the iteration count is known; use while when it depends on a condition.
Q What does break and continue do?
A break exits the loop entirely; continue skips the current iteration and proceeds to the next.
Q Can I use labels with break?
A Yes.
break label; exits the labeled outer loop, useful for nested loops.📖 Summary
- for loops are for known iterations, while loops are for unknown iterations
- do-while executes the loop body at least once
- for-each is used to iterate over arrays or collections
- break exits the loop, continue skips the current iteration
- Loops can be nested; avoid infinite loops
📝 Exercises
- Sum: Calculate the sum of all odd numbers from 1 to 100
- Factorial: Calculate 10 factorial (10! = 10 × 9 × ... × 1)
- Multiplication table: Use nested loops to print the complete multiplication table
9. Next Lesson
In the next lesson, we'll learn about Arrays — how to store and operate on a group of data.
