Arrays: Store and Process Multiple Values

Introduction

An array holds a fixed number of values of the same type under one name. Arrays are essential for scores, lists of IDs, and tables of data. This chapter covers one-dimensional and multi-dimensional arrays, length, traversal, and how arrays differ from collections you will meet in Collections Overview.

Prerequisites

• Variables, Loops
• Basic if / else from Conditionals

What Is an Array

An array is a container with a fixed size and indexed slots (starting at 0).

Metaphor: a row of labeled lockers—each locker has a number (index) and holds one value of the declared type.

int[] scores = {90, 85, 88};
System.out.println(scores[0]);  // 90
System.out.println(scores.length);  // 3

1) Declaring and Creating Arrays

Declaration syntax

int[] numbers;           // preferred style
// int numbers[];        // legal but uncommon

Create with size (default values)

int[] counts = new int[5];   // five zeros
String[] names = new String[3];  // three null references

Create with initial values

int[] primes = {2, 3, 5, 7, 11};
String[] cities = {"Beijing", "Shanghai", "Guangzhou"};

Once created, length is fixed. You cannot grow an array in place—use ArrayList later when you need dynamic size.

2) Accessing and Updating Elements

int[] scores = {90, 85, 88};
 
scores[1] = 92;              // change second element
System.out.println(scores[2]);  // 88
 
// scores[5] = 100;          // ArrayIndexOutOfBoundsException

3) Traversing Arrays

Indexed for loop

int[] scores = {90, 85, 88, 92};
 
for (int i = 0; i < scores.length; i++) {
    System.out.println("Index " + i + ": " + scores[i]);
}

Enhanced for (for-each)

for (int score : scores) {
    System.out.println(score);
}

Use indexed loops when you need the index; use for-each when you only need each value.

4) Common Array Operations with java.util.Arrays

Import helpers:

import java.util.Arrays;

int[] scores = {88, 95, 78, 92};
System.out.println(Arrays.toString(scores));  // [88, 95, 78, 92]
 
Arrays.sort(scores);
System.out.println(Arrays.toString(scores));  // [78, 88, 92, 95]

Arrays.sort on int[] sorts ascending. Descending ranking patterns appear in Ranking Game.

5) Multi-Dimensional Arrays

Java supports arrays of arrays—useful for grids and tables.

int[][] grid = {
        {1, 2, 3},
        {4, 5, 6}
};
 
System.out.println(grid.length);      // 2 rows
System.out.println(grid[0].length);   // 3 columns
System.out.println(grid[1][2]);       // 6
 
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();
}

Ragged arrays (rows of different lengths) are allowed but less common in beginner exercises.

6) Arrays vs Collections (Preview)

From chapter 34 onward, you will use lists and maps heavily. Arrays remain important for performance, algorithms, and interop with older APIs.

Real Mini Example: Class Average

public class ClassAverage {
    public static void main(String[] args) {
        int[] scores = {90, 85, 88, 92, 76};
 
        int sum = 0;
        for (int score : scores) {
            sum += score;
        }
 
        double average = (double) sum / scores.length;
        System.out.printf("Average: %.2f%n", average);
        System.out.println("Highest: " + max(scores));
    }
 
    static int max(int[] values) {
        int best = values[0];
        for (int i = 1; i < values.length; i++) {
            if (values[i] > best) {
                best = values[i];
            }
        }
        return best;
    }
}

Common Beginner Mistakes

Mistake 1: Assuming Length Changes

int[] data = new int[3];
// data.add(4);  // arrays have no add method

Mistake 2: length vs length()

Arrays use length (field). String uses length() (method).

Mistake 3: Printing an Array Directly

int[] nums = {1, 2, 3};
System.out.println(nums);  // something like [I@1b6d3586
System.out.println(Arrays.toString(nums));  // [1, 2, 3]

Mistake 4: Forgetting Default Values

new int[n] fills with 0; new boolean[n] with false; object arrays with null.

Mini Practice

1. Create an array of five integers and print the sum
2. Find the minimum value in an array
3. Print a 3×3 multiplication table using a 2D array

What’s Next

Sort scores in Ranking Game, then build Guess the Number with Random.

FAQ

Can arrays store different types?

No. Each array holds one element type (int[], String[], etc.). Use objects or collections for mixed shapes later.

Why do indices start at 0?

Historical and mathematical convention in C-family languages—including Java.

Should I use arrays or ArrayList?

Use arrays when size is fixed and simple. Use ArrayList when you need to add/remove elements dynamically.

Does Arrays.sort change the original array?

Yes. It sorts in place. Copy first if you need the original order: int[] copy = Arrays.copyOf(scores, scores.length);

Can I sort descending with Arrays.sort(int[])?

Not directly for primitives. Techniques for ranking appear in the next chapter.