Loops

Introduction

In the previous lesson, you learned how Java makes decisions using if-else and switch statements.

However, many real-world applications require executing the same block of code repeatedly.

Imagine:

• Printing invoices for 100 customers
• Reading thousands of records from a database
• Processing files line by line
• Validating multiple user inputs
• Iterating through collections

Writing the same code repeatedly is inefficient and difficult to maintain.

Java provides loops to automate repetitive tasks.

A loop executes a block of code multiple times until a specified condition becomes false.

In this lesson, you'll learn:

• Why loops are needed
• Types of loops
• for loop
• while loop
• do-while loop
• Enhanced for loop (For-Each)
• Nested loops
• Infinite loops
• Loop control statements (break & continue)
• Real-world examples
• Best practices
• Common mistakes
• Hands-on exercises
• Professional interview questions

What is a Loop?

A loop repeatedly executes a block of code while a condition remains true.

Instead of writing:

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

Use a loop:

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

Output:

Welcome
Welcome
Welcome
Welcome
Welcome

Types of Loops in Java

Java provides four looping mechanisms:

1. for Loop
2. while Loop
3. do-while Loop
4. Enhanced for Loop (For-Each)

Each serves different use cases.

The for Loop

The for loop is ideal when the number of iterations is known.

Syntax:

for(initialization; condition; update) {
    // Code
}

Example:

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

Output:

1
2
3
4
5

How a for Loop Works

Execution order:

1. Initialization executes once.
2. Condition is checked.
3. Loop body executes.
4. Update expression executes.
5. Repeat until the condition becomes false.

Flow:

Initialization
      ↓
 Condition
      ↓
 True
      ↓
 Execute Body
      ↓
 Update
      ↓
 Condition Again

The while Loop

Use a while loop when the number of iterations is unknown.

Syntax:

while(condition) {
    // Code
}

Example:

int count = 1;

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

Output:

1
2
3
4
5

The do-while Loop

A do-while loop executes the code at least once, even if the condition is false.

Syntax:

do {
    // Code
} while(condition);

Example:

int number = 1;

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

Output:

1
2
3
4
5

Difference Between while and do-while

Enhanced for Loop (For-Each)

The enhanced for loop is used to iterate through arrays and collections.

Example:

String[] fruits = {"Apple", "Banana", "Orange"};

for (String fruit : fruits) {
    System.out.println(fruit);
}

Output:

Apple
Banana
Orange

Advantages:

• Cleaner syntax
• No index management
• Less error-prone

Nested Loops

A loop inside another loop is called a nested loop.

Example:

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        System.out.print("* ");
    }
    System.out.println();
}

Output:

* * *
* * *
* * *

Nested loops are commonly used for matrices, tables, and patterns.

Infinite Loops

A loop that never terminates is called an infinite loop.

Example:

while (true) {
    System.out.println("Running...");
}

Use infinite loops carefully in servers, event listeners, or applications waiting for events.

Using break

The break statement immediately exits the loop.

Example:

for (int i = 1; i <= 10; i++) {
    if (i == 6) {
        break;
    }
    System.out.println(i);
}

Output:

1
2
3
4
5

Using continue

The continue statement skips the current iteration and proceeds to the next one.

Example:

for (int i = 1; i <= 5; i++) {
    if (i == 3) {
        continue;
    }
    System.out.println(i);
}

Output:

1
2
4
5

Real-World Example: ATM PIN Validation

int attempts = 0;

while (attempts < 3) {
    System.out.println("Enter PIN");
    attempts++;
}

Real-World Example: Processing Orders

String[] orders = {"Laptop", "Mobile", "Keyboard"};

for (String order : orders) {
    System.out.println("Processing: " + order);
}

Best Practices

• Use for when the iteration count is known.
• Use while when the iteration count depends on a condition.
• Use do-while when the loop must execute at least once.
• Prefer the enhanced for loop for arrays and collections when you don't need the index.
• Keep loop bodies small and readable.
• Avoid unnecessary nested loops for better performance.

Common Mistakes

Forgetting to Update the Loop Variable

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

This creates an infinite loop.

Wrong Loop Condition

Using < instead of <= (or vice versa) may result in missing or extra iterations.

Modifying Collections Incorrectly

Changing a collection while using an enhanced for loop may throw a ConcurrentModificationException.

Deeply Nested Loops

Excessive nesting reduces readability and may affect performance.

Hands-on Exercise

Create a Java program that:

1. Prints numbers from 1 to 100 using a for loop.
2. Prints even numbers using a while loop.
3. Demonstrates a do-while loop with user input.
4. Uses an enhanced for loop to print an array of employee names.
5. Uses nested loops to print a multiplication table.
6. Demonstrates the use of break and continue.
7. Creates a simple menu that repeats until the user selects "Exit".

Summary

Loops are one of the most fundamental programming constructs in Java. They eliminate repetitive code, improve readability, and make programs scalable. Choosing the right type of loop depends on whether the number of iterations is known, unknown, or guaranteed to execute at least once.

Key Takeaways

• Loops automate repetitive tasks.
• for loops are best for fixed iterations.
• while loops are ideal for condition-based repetition.
• do-while loops execute at least once.
• Enhanced for loops simplify array and collection traversal.
• break exits a loop immediately.
• continue skips the current iteration.
• Nested loops are useful for matrices and pattern generation.

Professional Interview Questions