Intermediate

Collections & Data Structures

Learn to organize and manipulate data efficiently with Java's powerful collection framework.

Topics

Arrays

Arrays are fixed-size containers that hold elements of the same type. They are the foundation of data structures in Java.

Declaring and Initializing Arrays

// Method 1: Declare then initialize
int[] numbers = new int[5];  // Array of 5 integers
numbers[0] = 10;
numbers[1] = 20;
numbers[2] = 30;
numbers[3] = 40;
numbers[4] = 50;

// Method 2: Declare and initialize together
int[] scores = {95, 87, 92, 78, 88};

// String array
String[] names = {"Alice", "Bob", "Charlie"};

// Access elements
System.out.println(names[0]);  // Alice
System.out.println(scores[2]); // 92

// Array length
System.out.println("Length: " + names.length);  // 3

Looping Through Arrays

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

// Using for loop
for (int i = 0; i < fruits.length; i++) {
    System.out.println(fruits[i]);
}

// Using for-each loop (enhanced for loop)
for (String fruit : fruits) {
    System.out.println(fruit);
}

// Using Arrays.toString() for quick printing
System.out.println(Arrays.toString(fruits));
// Output: [Apple, Banana, Orange, Mango]

Multi-Dimensional Arrays

// 2D Array (matrix)
int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

// Access elements
System.out.println(matrix[0][0]);  // 1
System.out.println(matrix[1][2]);  // 6

// Loop through 2D array
for (int i = 0; i < matrix.length; i++) {
    for (int j = 0; j < matrix[i].length; j++) {
        System.out.print(matrix[i][j] + " ");
    }
    System.out.println();
}

Common Array Operations

import java.util.Arrays;

int[] numbers = {5, 2, 8, 1, 9};

// Sort array
Arrays.sort(numbers);
System.out.println(Arrays.toString(numbers));  // [1, 2, 5, 8, 9]

// Binary search (array must be sorted)
int index = Arrays.binarySearch(numbers, 5);
System.out.println("Index of 5: " + index);  // 2

// Fill array with a value
int[] filled = new int[5];
Arrays.fill(filled, 10);
System.out.println(Arrays.toString(filled));  // [10, 10, 10, 10, 10]

// Copy array
int[] copy = Arrays.copyOf(numbers, numbers.length);

// Compare arrays
boolean areEqual = Arrays.equals(numbers, copy);
System.out.println("Arrays equal: " + areEqual);  // true