Java Arrays – Basic Concepts

High School Computer Science Notes

What is an Array?

An array is a data structure used to store multiple values of the same type in a single variable.

Instead of creating many variables:

int score1 = 90;
int score2 = 85;
int score3 = 78;
int score4 = 92;

You can store them in one array:

int[] scores = {90, 85, 78, 92};

Now all values are stored together in one structure.

Why Use Arrays?

Arrays are useful when you need to:

• Store many related values
• Process data using loops
• Organize data efficiently
• Avoid creating many separate variables

Examples of array data:

• Test scores
• Temperatures
• Game player stats
• Lists of names

Declaring an Array

Declaring an array tells Java:

• The type of data the array will hold
• The name of the array

Syntax

dataType[] arrayName;

Example

int[] numbers;

This tells Java:

• The array will store integers
• The array name is numbers

Creating (Initializing) an Array with Size

To actually create the array in memory, we use the new keyword.

Syntax

arrayName = new dataType[size];

Example

int[] numbers;
numbers = new int[5];

This creates an array that can store 5 integers.

Declaring and Creating in One Line

Most programmers combine the steps:

int[] numbers = new int[5];

This creates an array with 5 elements.

Array Index

Each position in an array has an index. An index is the location of the value inside the array.

If an array size is 5, the last index is 4.

Assigning Values to an Array

You assign values using the index number.

int[] numbers = new int[5];

numbers[0] = 10;
numbers[1] = 20;
numbers[2] = 30;
numbers[3] = 40;
numbers[4] = 50;

Accessing Values in an Array

You can retrieve values using the index.

System.out.println(numbers[0]);
System.out.println(numbers[3]);

Output:

10
40

Initializing an Array with Values

You can create and fill an array at the same time.

int[] numbers = {10, 20, 30, 40, 50};

Java automatically determines the array size.

This is equivalent to:

int[] numbers = new int[5];
numbers[0] = 10;
numbers[1] = 20;
numbers[2] = 30;
numbers[3] = 40;
numbers[4] = 50;

Example Program

public class ArrayBasicsExample {

    public static void main(String[] args) {

        // Declare and initialize an array
        int[] scores = {85, 90, 78, 92, 88};

        // Access elements using indexes
        System.out.println("First score: " + scores[0]);
        System.out.println("Second score: " + scores[1]);
        System.out.println("Third score: " + scores[2]);

        // Modify a value
        scores[2] = 80;

        System.out.println("Updated third score: " + scores[2]);
    }
}

Output:

First score: 85
Second score: 90
Third score: 78
Updated third score: 80

Common Beginner Mistakes

1. Index Out of Bounds

int[] numbers = new int[5];
numbers[5] = 100;   // ERROR

Valid indexes are:

5 does not exist.

Error:

ArrayIndexOutOfBoundsException

2. Forgetting to Create the Array

Incorrect:

int[] numbers;
numbers[0] = 10;

Correct:

int[] numbers = new int[5];
numbers[0] = 10;

Key Terms

Term	Definition
Array	A structure that stores multiple values of the same type
Index	The position of an element in the array
Declare	Define the array variable
Initialize	Create the array or assign values
Element	A value stored inside the array

Quick Summary

• Arrays store multiple values
• All elements must be the same data type
• Array indexing starts at 0
• Use square brackets [ ] to access elements

int[] numbers = {10, 20, 30};

System.out.println(numbers[0]); // 10
System.out.println(numbers[1]); // 20
System.out.println(numbers[2]); // 30