Day-75 Understanding Arrays in Java

What is an Array in Java?

An array is a data structure used to store multiple values of the same type in contiguous memory locations.

Java arrays:

• Are fixed in size
• Store elements of the same data type
• Are objects (even if they store primitive data)

Syntax of an Array

// When values are known at the time of declaration
datatype[] arrayName = { value1, value2, value3 };

// When size is known but values are not yet assigned
datatype[] arrayName = new datatype[size];

Examples:

String[] cars = { "Volvo", "BMW", "Ford" };   // Known values

int[] num = new int[5];   // If we don't know the values

Accessing & Changing Elements

You can access an array element using its index (starting from 0):

System.out.println(cars[0]);  // Output: Volvo

You can change an element like this:

cars[0] = "Opel";

Finding Array Length

Use .length to get the number of elements in an array:

System.out.println(cars.length);  // Output: 3

Why Use Arrays?

• To store multiple values in a single variable
• For efficient data management
• Stored in contiguous memory locations
• Useful when working with fixed-size collections

Common Errors

int[] num = new int[5];
num[5] = 10;  // ArrayIndexOutOfBoundsException

This error occurs because array indices start from 0, so the valid range is 0 to length - 1.

Arrays are Objects in Java

In Java, every array is an object. That’s why we can also create arrays using the new keyword:

String[] cars = new String[3];

When you print an array directly, you’ll see something like:

System.out.println(cars);
// Output: [Ljava.lang.String;@16bd3586

Let’s break this down:

• [Ljava.lang.String; ➝ Means it’s an array of String
• @ ➝ Separator
• 16bd3586 ➝ Hexadecimal hashcode (reference string, varies each time)

Array of Objects

Arrays can also store objects:

Home[] obj = new Home[2];

This creates an array of Home objects.

Can Arrays Store Negative Values?

Yes, if the data type supports it, you can store negative numbers:

int[] nums = { -10, -20, -30 };

Supported types that allow negative values:

• int
• float
• double
• byte
• short
• long