Java Arrays

An array is a container that holds a fixed number of values of the same data type. Instead of creating separate variables for each value, an array groups related values together under a single name. Arrays are one of the most fundamental data structures in Java.

Think of an array like a row of numbered boxes — each box holds one value, and each box is identified by its position number (called the index).

Key Characteristics of Arrays

• All elements must be of the same data type.
• The size (length) of an array is fixed once created — it cannot grow or shrink.
• Array indexing starts at 0 (the first element is at index 0).
• Arrays are stored in contiguous memory locations.

Declaring an Array

dataType[] arrayName;

Example

int[] scores;
String[] names;
double[] prices;

Creating (Allocating) an Array

After declaring, the array must be created using the new keyword and specifying its size:

scores = new int[5];   // array to hold 5 integers

Or declare and create in one step:

int[] scores = new int[5];

By default, numeric arrays are initialized to 0, boolean arrays to false, and object arrays to null.

Initializing an Array with Values

Values can be assigned directly during declaration using curly braces:

int[] scores = {85, 90, 78, 95, 88};

Java automatically determines the array size based on the number of values provided.

Accessing Array Elements

Array elements are accessed using the array name and the index in square brackets.

int[] scores = {85, 90, 78, 95, 88};

System.out.println(scores[0]);   // 85 – first element
System.out.println(scores[2]);   // 78 – third element
System.out.println(scores[4]);   // 88 – fifth (last) element

Modifying Array Elements

int[] scores = {85, 90, 78, 95, 88};
scores[2] = 100;   // changes the third element from 78 to 100
System.out.println(scores[2]);   // 100

Array Length

The length property returns the total number of elements in an array.

int[] scores = {85, 90, 78, 95, 88};
System.out.println("Length: " + scores.length);   // 5

Iterating Over an Array

Using a for Loop

int[] scores = {85, 90, 78, 95, 88};

for (int i = 0; i < scores.length; i++) {
    System.out.println("Score at index " + i + ": " + scores[i]);
}

Output:

Score at index 0: 85
Score at index 1: 90
Score at index 2: 78
Score at index 3: 95
Score at index 4: 88

Using a for-each Loop

int[] scores = {85, 90, 78, 95, 88};

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

Example – Finding Maximum Value

int[] numbers = {42, 78, 15, 93, 56};
int max = numbers[0];

for (int i = 1; i < numbers.length; i++) {
    if (numbers[i] > max) {
        max = numbers[i];
    }
}

System.out.println("Maximum: " + max);   // 93

Example – Calculating Average

int[] marks = {70, 85, 90, 60, 75};
int total = 0;

for (int mark : marks) {
    total += mark;
}

double average = (double) total / marks.length;
System.out.printf("Average: %.2f%n", average);   // Average: 76.00

Multidimensional Arrays

A two-dimensional array is an array of arrays — similar to a table with rows and columns. It is useful for storing data in grid format, like a matrix or a spreadsheet.

Declaration and Initialization

int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

Accessing Elements

System.out.println(matrix[0][0]);   // 1 – row 0, col 0
System.out.println(matrix[1][2]);   // 6 – row 1, col 2
System.out.println(matrix[2][1]);   // 8 – row 2, col 1

Printing a 2D Array

for (int row = 0; row < matrix.length; row++) {
    for (int col = 0; col < matrix[row].length; col++) {
        System.out.print(matrix[row][col] + "\t");
    }
    System.out.println();
}

Output:

1	2	3	
4	5	6	
7	8	9	

Arrays Class – Utility Methods

The java.util.Arrays class provides helpful methods for working with arrays.

import java.util.Arrays;

int[] nums = {5, 2, 8, 1, 9};

// Sort in ascending order
Arrays.sort(nums);
System.out.println(Arrays.toString(nums));   // [1, 2, 5, 8, 9]

// Fill all elements with a value
Arrays.fill(nums, 0);
System.out.println(Arrays.toString(nums));   // [0, 0, 0, 0, 0]

Common Array Mistakes

Summary

• An array stores multiple values of the same type under one name.
• Array indexing starts at 0. The last index is length - 1.
• Arrays have a fixed size once created.
• Use arrayName.length to get the size of an array.
• Use for or for-each loops to iterate through array elements.
• Two-dimensional arrays represent grid/matrix-style data.
• The Arrays class provides utility methods like sort(), fill(), and toString().