Arrays

What's an Array?

An array is like a row of mailboxes in an apartment building. Each mailbox has a number (starting at 0), and each one holds exactly one item. When you create an array, you decide two things: what type goes in the boxes, and how many boxes you need.

Once you set the size, it's locked in — this is what makes them a static array. You can't add more mailboxes later — that's a job for ArrayList (a dynamic array that resizes itself). But arrays are fast, simple, and everywhere in Java.

Creating & Using Arrays

public class Main {
    public static void main(String[] args) {
        // Way 1: Declare size, fill later
        int[] scores = new int[5];
        scores[0] = 95;
        scores[1] = 87;
        scores[2] = 92;
        scores[3] = 78;
        scores[4] = 88;

        // Way 2: Declare and fill at once
        String[] colors = {"Red", "Green", "Blue", "Yellow"};

        // Access by index
        System.out.println("First score: " + scores[0]);
        System.out.println("Last color: " + colors[colors.length - 1]);
        System.out.println("Number of colors: " + colors.length);

        // Loop through with for-each
        System.out.print("All scores: ");
        for (int s : scores) {
            System.out.print(s + " ");
        }
        System.out.println();

        // Modify an element
        colors[1] = "Lime";
        System.out.println("Changed: " + colors[1]);
    }
}

Output

First score: 95
Last color: Yellow
Number of colors: 4
All scores: 95 87 92 78 88 
Changed: Lime

Useful Array Tricks

Java's Arrays utility class (from java.util.Arrays) gives you handy shortcuts:

Arrays.toString() — prints the array nicely instead of a weird memory address.
Arrays.sort() — sorts the array in ascending order in-place.
Arrays.fill() — fills every slot with the same value.
Arrays.copyOf() — creates a copy of the array (optionally with a different size).

Arrays Utility Methods

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] nums = {42, 17, 8, 95, 3, 61};

        // toString — pretty print
        System.out.println("Original: " + Arrays.toString(nums));

        // sort — arranges in order
        Arrays.sort(nums);
        System.out.println("Sorted: " + Arrays.toString(nums));

        // fill — set everything to the same value
        int[] zeros = new int[5];
        Arrays.fill(zeros, 7);
        System.out.println("Filled: " + Arrays.toString(zeros));

        // copyOf — make a copy
        int[] copy = Arrays.copyOf(nums, nums.length);
        copy[0] = 999;
        System.out.println("Copy: " + Arrays.toString(copy));
        System.out.println("Original unchanged: " + Arrays.toString(nums));
    }
}

Output

Original: [42, 17, 8, 95, 3, 61]
Sorted: [3, 8, 17, 42, 61, 95]
Filled: [7, 7, 7, 7, 7]
Copy: [999, 8, 17, 42, 61, 95]
Original unchanged: [3, 8, 17, 42, 61, 95]

Multidimensional Arrays — Grids & Tables

public class Main {
    public static void main(String[] args) {
        // 2D array — like a chessboard or spreadsheet
        int[][] grid = {
            {1, 2, 3},
            {4, 5, 6},
            {7, 8, 9}
        };

        // Access: grid[row][column]
        System.out.println("Center: " + grid[1][1]);  // 5
        System.out.println("Bottom-right: " + grid[2][2]);  // 9

        // Print the whole grid
        System.out.println("Full grid:");
        for (int row = 0; row < grid.length; row++) {
            for (int col = 0; col < grid[row].length; col++) {
                System.out.print(grid[row][col] + " ");
            }
            System.out.println();
        }

        System.out.println("Rows: " + grid.length);
        System.out.println("Cols: " + grid[0].length);
    }
}

Output

Center: 5
Bottom-right: 9
Full grid:
1 2 3 
4 5 6 
7 8 9 
Rows: 3
Cols: 3

What's an Array?

Creating & Using Arrays

Useful Array Tricks

Arrays Utility Methods

Multidimensional Arrays — Grids & Tables

Quick check