Java Loops

Introduction to Loops

Loops in Java are used to execute a block of code repeatedly based on a condition. Java provides several types of loops, each suited for different scenarios.

1. For Loop

The for loop is used when you know exactly how many times you want to execute a block of code.

Basic Syntax

for (initialization; condition; increment/decrement) {
    // code block to be executed
}

Examples of For Loop

Simple Counter

// Counting from 1 to 5
for (int i = 1; i <= 5; i++) {
    System.out.println("Count is: " + i);
}

Counting Backwards

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

Iterating Over Array

int[] numbers = {1, 2, 3, 4, 5};
for (int i = 0; i < numbers.length; i++) {
    System.out.println("Element at index " + i + " is: " + numbers[i]);
}

Using Different Step Values

// Counting by twos
for (int i = 0; i <= 10; i += 2) {
    System.out.println("Even number: " + i);
}

2. Enhanced For Loop (For-Each)

The enhanced for loop is specifically designed to iterate over arrays and collections.

Basic Syntax

for (dataType item : array/collection) {
    // code block to be executed
}

Examples of Enhanced For Loop

Iterating Over Array

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

Iterating Over ArrayList

import java.util.ArrayList;

ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(1);
numbers.add(2);
numbers.add(3);

for (Integer num : numbers) {
    System.out.println("Number: " + num);
}

3. While Loop

The while loop executes a block of code as long as a condition is true.

Basic Syntax

while (condition) {
    // code block to be executed
}

Examples of While Loop

Basic Counter

int count = 1;
while (count <= 5) {
    System.out.println("Count is: " + count);
    count++;
}

Input Validation

import java.util.Scanner;

Scanner scanner = new Scanner(System.in);
int number = 0;

while (number <= 0) {
    System.out.println("Enter a positive number: ");
    number = scanner.nextInt();
}
System.out.println("You entered: " + number);

4. Do-While Loop

The do-while loop executes a block of code at least once before checking the condition.

Basic Syntax

do {
    // code block to be executed
} while (condition);

Examples of Do-While Loop

Basic Counter

int count = 1;
do {
    System.out.println("Count is: " + count);
    count++;
} while (count <= 5);

Menu System

import java.util.Scanner;

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

do {
    System.out.println("\nMenu:");
    System.out.println("1. Option One");
    System.out.println("2. Option Two");
    System.out.println("3. Exit");
    System.out.print("Enter your choice: ");

    choice = scanner.nextInt();

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

5. Nested Loops

Loops can be nested inside other loops to create more complex iterations.

Examples of Nested Loops

Multiplication Table

for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= 5; j++) {
        System.out.printf("%4d", i * j);
    }
    System.out.println();
}

Pattern Printing

// Printing a triangle pattern
for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= i; j++) {
        System.out.print("* ");
    }
    System.out.println();
}

6. Loop Control Statements

Break Statement

Used to exit a loop prematurely.

for (int i = 1; i <= 10; i++) {
    if (i == 5) {
        break; // Exit loop when i equals 5
    }
    System.out.println("Number: " + i);
}

Continue Statement

Used to skip the current iteration and continue with the next one.

for (int i = 1; i <= 5; i++) {
    if (i == 3) {
        continue; // Skip printing when i equals 3
    }
    System.out.println("Number: " + i);
}

7. Common Use Cases and Best Practices

1. Iterating Over Collections

List<String> items = Arrays.asList("Item1", "Item2", "Item3");
for (String item : items) {
    System.out.println(item);
}

2. File Processing

BufferedReader reader = new BufferedReader(new FileReader("file.txt"));
String line;
while ((line = reader.readLine()) != null) {
    System.out.println(line);
}

3. Input Validation

Scanner scanner = new Scanner(System.in);
int input;
do {
    System.out.print("Enter a number between 1 and 10: ");
    input = scanner.nextInt();
} while (input < 1 || input > 10);

Best Practices

1. Choose the appropriate loop type for your needs
2. Always use braces {} even for single-line loop bodies
3. Be careful with loop conditions to avoid infinite loops
4. Initialize counter variables properly
5. Use meaningful variable names for loop counters
6. Consider using enhanced for loop when possible
7. Be cautious with nested loops as they can impact performance

Remember to always include a way to exit your loops and to test your loop conditions thoroughly to prevent infinite loops.