Arrays

Arrays are the low-level building block under most collections, and appear in main(String[] args), varargs and performance-sensitive code.

An array is a fixed-size, ordered block of elements of one type. Create one with a size or with initial values:

int[] scores = new int[5];            // five zeros
String[] names = {"Ana", "Bo", "Cy"};

Indexes start at 0, and the last valid index is length - 1. Note that length is a field, not a method: names.length. Accessing an index outside the range throws ArrayIndexOutOfBoundsException — Java always checks bounds.

New arrays are filled with default values: 0 for numbers, false for booleans, null for references.

An array variable holds a reference. Assigning int[] b = a; makes both names point at the same array, so changing b[0] changes a[0]. To copy, use Arrays.copyOf(a, a.length) or a.clone().

The java.util.Arrays class has the helpers you'll use constantly:

Arrays.toString(arr) — printing an array directly shows something like [I@1b6d3586, which is useless.
Arrays.sort(arr) — sorts in place.
Arrays.fill(arr, value) and Arrays.equals(a, b) — the latter compares contents, unlike ==.

Multi-dimensional arrays are arrays of arrays: int[][] grid = new int[3][4]; and grid[row][col].

Because arrays can't grow, application code usually prefers ArrayList, which you'll meet in the collections module. Reach for arrays for fixed-size data, primitives in hot loops, and APIs that require them.

Example

import java.util.Arrays;

public class ArraysDemo {
    public static void main(String[] args) {
        int[] nums = {5, 3, 9, 1};
        int[] alias = nums;              // same array, not a copy
        int[] copy = Arrays.copyOf(nums, nums.length);

        Arrays.sort(nums);
        System.out.println(Arrays.toString(alias)); // [1, 3, 5, 9]
        System.out.println(Arrays.toString(copy));  // [5, 3, 9, 1]

        int max = Integer.MIN_VALUE;
        for (int n : copy) max = Math.max(max, n);
        System.out.println(max + " of " + copy.length); // 9 of 4

        int[][] grid = new int[2][3];
        grid[1][2] = 7;
        System.out.println(Arrays.deepToString(grid)); // [[0, 0, 0], [0, 0, 7]]
    }
}

Remember

Indexes run from 0 to length - 1; length is a field
Array variables are references — assignment shares, copyOf copies
Use Arrays.toString, sort, equals instead of hand-written loops

Watch out for

Printing an array with println(arr) and getting [I@6d06d69c instead of its contents.

Check yourself

After `int[] a = {1, 2}; int[] b = a; b[0] = 9;` what is `a[0]`?

Official docs: docs.oracle.com/javase/tutorial/java/nutsandbolts/arrays.html

Example

Remember

Watch out for

Check yourself

Builds on

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