Java Loops

1. Loop Overview & Types

Loops execute a block of code repeatedly until a specified condition is met. They eliminate code duplication and handle repetitive tasks efficiently.

For Loop

Known number of iterations

for (init; condition; increment)

While Loop

Condition checked before execution

while (condition)

Do-While Loop

Execute at least once

do { } while (condition)

Enhanced For

Iterate through collections

for (type item : collection)

Loop Execution Flow

Start → Initialize → Check Condition → Execute Body → Update/Increment → Repeat

2. For Loops

Traditional For Loop

// Print numbers 1 to 5
for (int i = 1; i 5; i++) {
    System.out.println("Number: " + i);
}

// Countdown from 10 to 1
for (int i = 10; i >= 1; i--) {
    System.out.println(i);
}

Enhanced For Loop (For-Each)

// Iterate through array
int[] numbers = {10, 20, 30, 40, 50};
for (int num : numbers) {
    System.out.println(num);
}

// Iterate through String array
String[] fruits = {"Apple", "Banana", "Orange"};
for (String fruit : fruits) {
    System.out.println("Fruit: " + fruit);
}

For Loop Components

• Initialization: int i = 0 - Executed once at the start
• Condition: i < 10 - Checked before each iteration
• Update: i++ - Executed after each iteration

Common For Loop Patterns

for (int i = 0; i < n; i++)

for (int i = n; i > 0; i—)

for (int i = 0; i < n; i += 2)

for (int i = 0; i < arr.length; i++)

Nested For Loops

// Print multiplication table
for (int i = 1; i 5; i++) {
    for (int j = 1; j 5; j++) {
        System.out.print((i * j) + "\t");
    }
    System.out.println(); // New line after each row
}

// Pattern printing
for (int i = 1; i 5; i++) {
    for (int j = 1; j "* ");
    }
    System.out.println();
}

3. While Loops

While loops continue executing as long as the condition remains true. The condition is checked before each iteration.

while (condition) {
    // Code to execute repeatedly
    // Don't forget to update condition variable!
}

Basic While Loop Examples

// Count from 1 to 5
int count = 1;
while (count 5) {
    System.out.println("Count: " + count);
    count++; // Important: update condition variable
}

// Sum of numbers until user enters 0
Scanner scanner = new Scanner(System.in);
int sum = 0;
int number = 1; // Initialize to non-zero

while (number != 0) {
    System.out.print("Enter a number (0 to stop): ");
    number = scanner.nextInt();
    sum += number;
}
System.out.println("Total sum: " + sum);

Input Validation

// Validate user input
Scanner input = new Scanner(System.in);
int age = -1; // Invalid initial value

while (age 0 || age > 150) {
    System.out.print("Enter your age (0-150): ");
    age = input.nextInt();

    if (age 0 || age > 150) {
        System.out.println("Invalid age! Please try again.");
    }
}
System.out.println("Valid age entered: " + age);

Infinite Loop Warning

Always ensure the loop condition will eventually become false. Forgetting to update the condition variable is a common cause of infinite loops.

4. Do-While Loops

Do-while loops execute the body at least once, then check the condition. Perfect for scenarios where you need guaranteed execution.

While Loop

Check Condition → Execute (if true)

May not execute at all

Do-While Loop

Execute Body → Check Condition

Always executes at least once

do {
    // Code to execute at least once
    // Update condition variable
} while (condition);

Menu System Example

Scanner input = new Scanner(System.in);
int choice;

do {
    System.out.println("=== MENU ===");
    System.out.println("1. Option 1");
    System.out.println("2. Option 2");
    System.out.println("3. Option 3");
    System.out.println("0. Exit");
    System.out.print("Enter your choice: ");

    choice = input.nextInt();

    switch (choice) {
        case 1: System.out.println("Option 1 selected"); break;
        case 2: System.out.println("Option 2 selected"); break;
        case 3: System.out.println("Option 3 selected"); break;
        case 0: System.out.println("Goodbye!"); break;
        default: System.out.println("Invalid choice!");
    }
} while (choice != 0);

When to Use Do-While

• Menu systems that must show at least once
• Input validation with guaranteed prompt
• Game loops that run until player quits
• Operations that must execute at least once

5. Loop Control Statements

Control statements modify the normal flow of loop execution: break exits the loop, continue skips to the next iteration.

break Statement

Exits the loop immediately

Loop → Condition → break → Exit

continue Statement

Skips rest of current iteration

Loop → continue → Next Iteration

Break Statement Examples

// Find first even number
int[] numbers = {1, 3, 8, 5, 9};
for (int num : numbers) {
    if (num % 2 == 0) {
        System.out.println("First even number: " + num);
        break; // Exit loop immediately
    }
}

// Search for value
int target = 25;
boolean found = false;
for (int i = 0; i if (numbers[i] == target) {
        found = true;
        break; // Stop searching
    }
}

Continue Statement Examples

// Print only positive numbers
int[] numbers = {-2, 5, -1, 8, 0, 3};
for (int num : numbers) {
    if (num 0) {
        continue; // Skip negative and zero
    }
    System.out.println("Positive: " + num);
}

// Skip multiples of 3
for (int i = 1; i 10; i++) {
    if (i % 3 == 0) {
        continue; // Skip 3, 6, 9
    }
    System.out.print(i + " "); // Prints: 1 2 4 5 7 8 10
}

Labeled Loops (Advanced)

outerLoop: for (int i = 1; i 3; i++) {
    for (int j = 1; j 3; j++) {
        if (i * j == 4) {
            System.out.println("Found 4 at " + i + "," + j);
            break outerLoop; // Break from outer loop
        }
        System.out.println(i + " x " + j + " = " + (i * j));
    }
}

Label Guidelines

Use labels sparingly and only when absolutely necessary. They can make code harder to read and maintain. Consider restructuring your logic instead.

6. Practical Examples

Sum Calculator

Scanner input = new Scanner(System.in);
System.out.print("Enter a number: ");
int n = input.nextInt();

int sum = 0;
for (int i = 1; i "Sum of 1 to " + n + " = " + sum);

Factorial Calculator

int number = 5;
int factorial = 1;
int i = 1;

while (i "! = " + factorial); // 5! = 120

Number Guessing Game

Scanner input = new Scanner(System.in);
int secretNumber = (int)(Math.random() * 100) + 1;
int guess;
int attempts = 0;

do {
    System.out.print("Guess the number (1-100): ");
    guess = input.nextInt();
    attempts++;

    if (guess "Too low!");
    } else if (guess > secretNumber) {
        System.out.println("Too high!");
    } else {
        System.out.println("Correct! You won in " + attempts + " attempts.");
    }
} while (guess != secretNumber);

7. Best Practices

Do’s

• Choose the right loop type for your scenario
• Always update the loop condition variable
• Use meaningful variable names
• Keep loop bodies simple and focused
• Use enhanced for-loops for simple iteration
• Initialize variables before the loop

Don’ts

• Don’t create infinite loops accidentally
• Don’t modify loop variables inside enhanced for-loops
• Don’t use magic numbers in loop conditions
• Don’t nest loops too deeply (max 2-3 levels)
• Don’t ignore loop efficiency for large datasets
• Don’t use break/continue excessively

Loop Selection Guide

• Known iterations count → For Loop
• Iterate through collection → Enhanced For Loop
• Unknown iterations, condition-based → While Loop
• Must execute at least once → Do-While Loop

Performance Considerations

• Cache Array Length: Store arr.length in a variable for long loops
• Minimize Nested Loops: O(n) becomes O(n²) with nesting
• Early Exit: Use break statements to exit loops early when result is found
• Move Conditions Outside: Check invariant conditions before the loop

Key Takeaways

• Loops eliminate code repetition and handle repetitive tasks efficiently
• Choose the right loop type: for (counting), while (condition), do-while (at least once)
• Always ensure loop termination to avoid infinite loops
• Use break and continue judiciously to control loop flow