Chapter 20: Arrays in Java

This chapter is the big-picture chapter for arrays: what they are, how they relate to variables you already know, and the rules every array shares. One-dimensional patterns get extra depth in Chapter 21; two-dimensional grids are Chapter 22.

1. Topic title

Arrays in Java: fixed-size, indexed, single-variable name for many slots

2. What it means

An array is an object that holds a fixed number of slots of the same type, accessed by index starting at 0.

int[] scores;          // reference variable, no array yet
scores = new int[4]; // array object of four ints, heap allocated

int[] scores reads “scores is a reference that can point to an int array.” Until you assign new int[...] or an initializer, the reference may be null.

Length is fixed at creation time. You cannot push like some dynamic lists—ArrayList comes later in the collections part.

3. Why it is used

When you have many values of the same kind—temperature readings, pixel rows, student IDs for one class—it is clumsy to use score1, score2, … score40. One array keeps them under one name and lets loops walk them.

4. Real-world example

A weekly sales total: seven numbers, one per day. double[] dayTotals = new double[7]; — index 0 might mean Monday in your convention; document that in a comment.

5. Java code example

public class ArraysIntroDemo {
    public static void main(String[] args) {
        int[] a = new int[3];
        System.out.println("default ints: " + a[0] + ", " + a[1] + ", " + a[2]);
 
        double[] temps = new double[2];
        System.out.println("default doubles: " + temps[0]);
 
        boolean[] flags = new boolean[2];
        System.out.println("default boolean: " + flags[0]);
 
        String[] names = new String[2];
        System.out.println("default String slot: " + names[0]);
 
        int[] primes = {2, 3, 5, 7};
        System.out.println("length of primes: " + primes.length);
 
        primes[0] = 1;
        System.out.println("after change, first = " + primes[0]);
    }
}

Output notes: numeric primitives default to 0, boolean to false, reference slots like String to null. primes.length is 4 (no parentheses—length is a field, not a method).

6. Explanation of code

• new int[3] allocates three int cells, all zero.
• String[] array: each cell is a reference defaulting to null, not empty string.
• {2, 3, 5, 7} is an array initializer—size inferred, no new int[] needed (Java fills it in).
• Valid indices for primes are 0 through primes.length - 1.

7. Common mistakes

ArrayIndexOutOfBoundsException — index length or negative index.

Thinking length can grow — it cannot for a plain array; you need a new array or a collection class.

Using length() on arrays — that is for String; arrays use length.

Comparing arrays with == — compares references, not contents. Use Arrays.equals (later) or compare element by element.

8. Best practices

Comment index meanings when they stand for days, seats, or channels.

Prefer enhanced for when you only read values (Chapter 15); use index when you need position.

Do not leave String[] slots null if your code assumes text—initialize to "" or check null.

9. Small practice task

Create char[] letters = new char[5];. Assign 'H', 'e', 'l', 'l', 'o' at indices 0–4 in code (not a string literal trick). Print each char on one line using a for index loop. Print letters.length.

Beginner tip

Say “length is the count, last index is length minus one” until it is muscle memory.