Arrays in Java

What Is an Array?

An array is a fixed-size container that stores many values of the same type at consecutive memory positions. Each slot is reached by an index starting at 0.

Creating Arrays

int[] numbers = new int[5];      // 5 slots, all zero
int[] known = {10, 20, 30, 40}; // initialised directly
String[] names = new String[]{"A", "B", "C"};

Reading and Writing

int[] scores = new int[3];
scores[0] = 90;
scores[1] = 85;
scores[2] = 78;
System.out.println(scores[1]);                // 85
System.out.println(scores.length);            // 3 (number of slots)

The ArrayIndexOutOfBounds Error

int[] a = {1, 2, 3};
System.out.println(a[5]); // runtime exception: index 5 is invalid

Java checks indexes at runtime, so accessing a slot outside the array throws ArrayIndexOutOfBoundsException instead of corrupting memory.

Two-Dimensional Arrays

int[][] grid = {
    {1, 2, 3},
    {4, 5, 6}
};
System.out.println(grid[1][2]); // 6

Looping Over an Array

int[] nums = {3, 7, 2};
for (int value : nums) {
    System.out.println(value);
}

Copying Arrays

int[] original = {1, 2, 3};
int[] copy = java.util.Arrays.copyOf(original, original.length);
// modify copy without touching original

Key Points

• Indexes start at 0; length gives the size.
• Array size is fixed once created.
• Invalid indexes throw ArrayIndexOutOfBoundsException at runtime.