Arrays

Declaration and Initialisation

java
// declaration
int[] numbers;
String[] names;

// allocation
numbers = new int[5];          // default values: 0
names   = new String[3];       // default values: null

// declaration + allocation
double[] prices = new double[10];

// array initialiser (size inferred)
int[] primes = {2, 3, 5, 7, 11};
String[] days = new String[]{"Mon", "Tue", "Wed"};

The [] can go after the type or after the variable name — both compile, but after the type is conventional:

java
int[] a;   // preferred
int a[];   // also valid

Accessing Elements

java
int[] arr = {10, 20, 30, 40, 50};

System.out.println(arr[0]);      // 10  (zero-based)
System.out.println(arr[arr.length - 1]);  // 50 (last element)

arr[2] = 99;
// arr = {10, 20, 99, 40, 50}

Accessing an index outside [0, length-1] throws ArrayIndexOutOfBoundsException.

Iterating

java
// traditional for — gives index control
for (int i = 0; i < arr.length; i++) {
    System.out.println(arr[i]);
}

// enhanced for — cleaner when index not needed
for (int n : arr) {
    System.out.println(n);
}

Multidimensional Arrays

java
int[][] matrix = new int[3][4];   // 3 rows, 4 columns
matrix[1][2] = 42;

// initialiser
int[][] grid = {
    {1, 2, 3},
    {4, 5, 6}
};

// jagged array — rows of different lengths
int[][] jagged = new int[3][];
jagged[0] = new int[2];
jagged[1] = new int[4];
jagged[2] = new int[1];

Arrays Utility Class

java
import java.util.Arrays;

int[] a = {5, 3, 1, 4, 2};

Arrays.sort(a);                         // sorts in place: [1,2,3,4,5]
int idx = Arrays.binarySearch(a, 3);    // 2 (array must be sorted first)
int[] copy = Arrays.copyOf(a, 7);       // [1,2,3,4,5,0,0] — pads with 0
int[] part = Arrays.copyOfRange(a, 1, 4); // [2,3,4] — indices 1..3
System.out.println(Arrays.toString(a)); // "[1, 2, 3, 4, 5]"
boolean eq = Arrays.equals(a, copy);    // false (different lengths)
Arrays.fill(a, 0);                      // [0,0,0,0,0]

Arrays.binarySearch() returns a negative value if the element is not found.

Copying Arrays

Arrays as Objects

• Arrays are objects on the heap.
• arr.length is a field (not a method).
• == compares references; use Arrays.equals() for value equality.
• Passing an array to a method passes the reference — the method can mutate the contents.

Key Rules