JAVA

Loops

Many programs need to perform the same task multiple times. A social media application may need to process thousands of posts, notifications, or comments. An e-commerce platform may need to display hundreds of products in a catalog. A banking application may need to process multiple transactions. Writing the same code repeatedly would be inefficient and difficult to maintain.

To solve this problem, Java provides loops. A loop allows a block of code to execute repeatedly until a specified condition becomes false.

In this chapter, you will learn how to use loops in Java to repeat tasks efficiently using while, do-while, and for loops.

What is a Loop?

A loop is a control structure that repeatedly executes a block of code as long as a condition remains true.

Without loops:

System.out.println("Hello");
System.out.println("Hello");
System.out.println("Hello");
System.out.println("Hello");
System.out.println("Hello");

With a loop:

for(int i = 1; i <= 5; i++) {
    System.out.println("Hello");
}

Both programs produce the same output, but the loop version is shorter, cleaner, and easier to maintain.

The while Loop

A while loop executes a block of code as long as its condition remains true.

Syntax

while(condition) {
    // code to repeat
}

Example

int count = 1;

while(count <= 5) {
    System.out.println(count);
    count++;
}

Output

1
2
3
4
5

Explanation

• count starts with the value 1.
• Java checks whether count <= 5.
• If the condition is true, the loop executes.
• After each iteration, count++ increases the value by one.
• The loop stops when the condition becomes false.

The do-while Loop

A do-while loop is similar to a while loop, but the code block executes at least once before the condition is checked.

Syntax

do {
    // code to repeat
}
while(condition);

Example

int count = 1;

do {
    System.out.println(count);
    count++;
}
while(count <= 5);

Output

1
2
3
4
5

Explanation

The code inside the loop executes first, then Java checks the condition. Because of this, a do-while loop always runs at least once.

The for Loop

The for loop is commonly used when the number of iterations is known in advance.

Syntax

for(initialization; condition; update) {
    // code to repeat
}

Example

for(int i = 1; i <= 5; i++) {
    System.out.println(i);
}

Output

1
2
3
4
5

Explanation

for(int i = 1; i <= 5; i++)

• int i = 1 → initialization (runs once)
• i <= 5 → condition (checked before every iteration)
• i++ → update (runs after every iteration)

The loop continues until the condition becomes false.

Printing Even Numbers

Example

for(int i = 2; i <= 10; i += 2) {
    System.out.println(i);
}

Output

2
4
6
8
10

Explanation

i += 2 increases the value by 2 after each iteration, producing only even numbers.

Nested Loops

A loop can be placed inside another loop.

This is known as a nested loop.

Example

for(int i = 1; i <= 3; i++) {

    for(int j = 1; j <= 2; j++) {
        System.out.println("i = " + i + ", j = " + j);
    }

}

Output

i = 1, j = 1
i = 1, j = 2
i = 2, j = 1
i = 2, j = 2
i = 3, j = 1
i = 3, j = 2

Nested loops are commonly used when working with patterns, tables, and multidimensional arrays.

The break Statement

The break statement immediately terminates a loop.

Example

for(int i = 1; i <= 10; i++) {

    if(i == 5) {
        break;
    }

    System.out.println(i);
}

Output

1
2
3
4

When i becomes 5, the loop stops completely.

The continue Statement

The continue statement skips the current iteration and moves to the next iteration.

Example

for(int i = 1; i <= 5; i++) {

    if(i == 3) {
        continue;
    }

    System.out.println(i);
}

Output

1
2
4
5

When i becomes 3, that iteration is skipped.

Complete Program

Example

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);

        System.out.print("Enter a number: ");
        int number = sc.nextInt();

        for(int i = 1; i <= number; i++) {
            System.out.println(i);
        }
    }
}

Input

5

Output

1
2
3
4
5

Explanation

• The user enters a number.
• The loop starts from 1.
• It continues until the entered number is reached.
• Each value is displayed using System.out.println().

Common Mistakes

Forgetting to Update the Loop Variable

Wrong

int i = 1;

while(i <= 5) {
    System.out.println(i);
}

Problem

The value of i never changes, causing an infinite loop.

Correct

int i = 1;

while(i <= 5) {
    System.out.println(i);
    i++;
}

Using the Wrong Condition

Wrong

for(int i = 1; i >= 5; i++)

The condition is false from the beginning, so the loop never executes.

Correct

for(int i = 1; i <= 5; i++)

Key Takeaways

• Loops allow code to execute repeatedly.
• while loops check the condition before execution.
• do-while loops execute at least once.
• for loops are commonly used when the number of iterations is known.
• Nested loops are loops inside other loops.
• break terminates a loop immediately.
• continue skips the current iteration.
• Loops help reduce code duplication and make programs more efficient.