📘LESSON 4

3.1 Lesson Outcomes

After completing this lesson, learners will be able to:

• Define arrays in Java.
• Declare and initialize arrays correctly.
• Access and modify array elements.
• Use loops to process array data.
• Explain the importance of arrays in Java programming.

3.2 Overview

Arrays are data structures used to store multiple values of the same data type in a single variable. Instead of creating many individual variables, arrays allow developers to organize and process collections of related data efficiently.

This lesson introduces learners to Java arrays, including array declaration, initialization, indexing, and iteration. Learners will also explore how arrays simplify data management in software applications.

Arrays are commonly used in:

• banking systems,
• inventory systems,
• student management systems,
• games,
• and enterprise applications.

Understanding arrays is essential because arrays form the foundation for advanced data structures and collections in Java programming.

KT0401 — Introduction to Arrays

An array is a collection of elements of the same data type stored in contiguous memory locations.

Each value inside an array is called an element.

Every element in an array has an index position.

Array indexing starts at 0.

Example of an Array

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

Explanation

Importance of Arrays

Arrays help developers:

• store multiple values efficiently,
• simplify repetitive operations,
• improve data organization,
• and process grouped information.

KT0402 — Declaring Arrays

Array declaration defines the array type and variable name.

Syntax

dataType[] arrayName;

Example

int[] marks;

This creates an integer array variable called marks.

Different Ways to Declare Arrays

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

KT0403 — Initializing Arrays

Array initialization assigns values to an array.

Arrays may be initialized during declaration or later in the program.

Array Initialization During Declaration

int[] numbers = {1, 2, 3, 4, 5};

Array Initialization Using new Keyword

int[] numbers = new int[5];

This creates an array with five elements.

Assigning Values Individually

numbers[0] = 10;
numbers[1] = 20;
numbers[2] = 30;

KT0404 — Accessing Array Elements

Array elements are accessed using indexes.

Indexes begin at 0.

Example

public class Main {

    public static void main(String[] args) {

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

        System.out.println(numbers[0]);
        System.out.println(numbers[1]);
        System.out.println(numbers[2]);
    }
}

Output

10
20
30

Modifying Array Elements

Array values can be updated using indexes.

Example

public class Main {

    public static void main(String[] args) {

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

        numbers[1] = 50;

        System.out.println(numbers[1]);
    }
}

Output

50

KT0405 — Looping Through Arrays

Loops are commonly used to process array elements.

The for loop is frequently used with arrays.

Example Using a for Loop

public class Main {

    public static void main(String[] args) {

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

        for(int i = 0; i < numbers.length; i++) {

            System.out.println(numbers[i]);
        }
    }
}

Output

10
20
30
40

Enhanced for Loop

Java also supports the enhanced for loop.

Example

public class Main {

    public static void main(String[] args) {

        int[] numbers = {5, 10, 15};

        for(int number : numbers) {

            System.out.println(number);
        }
    }
}

Output

5
10
15

KT0406 — Array Length

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

Example

public class Main {

    public static void main(String[] args) {

        int[] marks = {70, 80, 90};

        System.out.println(marks.length);
    }
}

Output

3

Importance of Array Length

The length property helps:

• control loops,
• avoid errors,
• and process arrays dynamically.

KT0407 — Common Array Errors

One common array error is:

• ArrayIndexOutOfBoundsException

This occurs when accessing an invalid index.

Example of an Error

int[] numbers = {1, 2, 3};

System.out.println(numbers[5]);

This causes an error because index 5 does not exist.

Preventing Array Errors

Developers should:

• check array length,
• use proper loop conditions,
• and avoid invalid indexes.

3.5 Key Notes / Summary

• Arrays store multiple values of the same data type.
• Array indexes start at 0.
• Arrays can be declared and initialized in different ways.
• Array elements are accessed using indexes.
• Loops are commonly used to process arrays.
• The length property returns the number of elements.
• Invalid indexes can cause array errors.