Arrays in depth

Arrays are the lowest-level container

A Java array is a fixed-size, homogeneous, first-class object that knows its own length (a.length, a field, not a method — and not length()). Once created its size never changes. Arrays are the fastest random-access container and the only one that holds primitives without boxing, but the trade-off is that you cannot grow or shrink them — that is exactly the gap the Collections framework fills.

### Arrays of objects hold references

When you write Event[] events = new Event[3]; you get an array of three slots, each initialised to null — no Event objects exist yet. The array reserves space for references, not for the objects themselves. You must fill the slots:

Event[] events = new Event[3];   // {null, null, null}
events[0] = new LightOn(200);     // now slot 0 refers to a real Event

Contrast with int[] nums = new int[3];, which is {0, 0, 0} — primitive arrays are zero-filled, object arrays are null-filled. Reading events[1].action() before assigning slot 1 throws NullPointerException; reading nums[5] throws ArrayIndexOutOfBoundsException.

### Multidimensional arrays are arrays of arrays

int[][] grid = new int[2][3]; is really a length-2 array whose elements are each length-3 int[] arrays. Because each row is a separate object, rows can have different lengths (a jagged array): int[][] j = new int[2][]; j[0] = new int[]{1,2}; j[1] = new int[]{9};.

### The Arrays utility class

java.util.Arrays provides the static helpers you reach for constantly:

• Arrays.sort(a) — in-place sort (natural order, or supply a Comparator for object arrays).
• Arrays.fill(a, value) — set every element.
• Arrays.copyOf(a, newLen) — a resized copy (how ArrayList grows internally).
• Arrays.asList(x, y, z) — a fixed-size List view backed by the array.
• Arrays.toString(a) / Arrays.deepToString(a) — readable text (plain a.toString() prints a useless hash like [LEvent;@1b6d3586).

Worked example. Sort an array of String by length using a Comparator. Arrays.sort takes the array and a comparator; the comparator's compare(a, b) returns negative/zero/positive, and Integer.compare gives that safely without overflow:

String[] words = { "sort", "a", "list", "by", "length" };
Arrays.sort(words, (a, b) -> Integer.compare(a.length(), b.length()));
System.out.println(Arrays.toString(words));
// [a, by, sort, list, length]   (stable: equal lengths keep input order)

### Varargs are arrays in disguise

A varargs parameter T... args is received by the method body as a T[]. Arrays.asList, String.format, and printf all use varargs. You may pass zero, one, or many arguments, or an existing array directly.