Unit 2 · Java Fundamentals · 8 min

Arrays

An array stores many values of the same type under one name, reached by position (index) starting at 0.

Why this matters

When you need 50 student marks, you do not create 50 variables. Arrays plus loops solve that in a few lines.

Real-life analogy

An array is a row of lockers with numbers painted on them. The row has one name, and each locker has an index starting from 0.

Syntax

int[] marks = new int[5];
int[] marks = {80, 75, 90};

Example

Java

public class Marks {
    public static void main(String[] args) {
        int[] marks = {80, 75, 90, 65, 88};
        int total = 0;

        for (int i = 0; i < marks.length; i++) {
            System.out.println("Subject " + (i + 1) + ": " + marks[i]);
            total += marks[i];
        }

        System.out.println("Total = " + total);
        System.out.println("Average = " + (total / (double) marks.length));
    }
}

Output

Subject 1: 80
Subject 2: 75
Subject 3: 90
Subject 4: 65
Subject 5: 88
Total = 398
Average = 79.6

Step by step

1`{80, 75, 90, 65, 88}` creates an array of 5 ints already filled.
2`marks.length` is 5 — note it is a field, not a method, so no brackets.
3Indexes run from 0 to length-1, so marks[0] is 80 and marks[4] is 88.
4`(double) marks.length` casts to double so the average keeps its decimals.

Key points

Array indexes start at 0
Array size is fixed once created
array.length is a field; String uses length() as a method
Going past the last index throws ArrayIndexOutOfBoundsException

Common mistakes

Using marks[5] on an array of size 5
Writing marks.length() for an array
Expecting an array to grow automatically — use ArrayList for that

Try it yourself

Find the largest number in an array.
Read 5 numbers from the user into an array and print them in reverse.

Open the practice notebook