Array Introduction in Java One Page.md

Arrays Introduction in Java — ONE PAGE

1. What is an Array?

An array in Java is an object used to store multiple values of the same data type under a single variable name.

Instead of:

int a = 10; int b = 20; int c = 30; int d = 40;

we can use:

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

Simple definition

An array is a fixed-size collection of elements of the same data type, stored and accessed using an index.

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2. Why Do We Need Arrays?

Suppose we need to store marks of 5 students.

Without an array:

int m1 = 80; int m2 = 75; int m3 = 90; int m4 = 65; int m5 = 88;

With an array:

int[] marks = {80, 75, 90, 65, 88};

So arrays make it easier to:

Store multiple values

Process values using loops

Access values using indexes

Reduce repetitive code

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3. Important Properties of Arrays

Property Explanation

Same data type Elements normally have the same component type Fixed size Length is fixed after creation Index-based Elements are accessed using indexes Zero-based First index is 0 Object An array is an object in Java length Gives the number of elements

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4. Array Index

Consider:

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

Memory representation:

Value: 10 20 30 40 Index: 0 1 2 3

Therefore:

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

Output:

10

And:

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

Output:

40

Golden Rule

Array index starts from 0 and ends at length - 1.

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5. Declaration of an Array

Two common forms:

int[] numbers;

or:

int numbers[];

Both are valid Java.

Recommended style

int[] numbers;

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6. Creating an Array

Use new:

numbers = new int[5];

This creates an array capable of storing 5 integers.

Combined:

int[] numbers = new int[5];

Indexes are:

0 1 2 3 4

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7. Declaration + Initialization

You can directly provide values:

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

This creates an array of length 4.

You can also write:

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

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8. Accessing Elements

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

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

Output:

10 20 30

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9. Changing an Element

Arrays are mutable.

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

numbers[1] = 50;

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

Output:

50

Array becomes:

10 50 30

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10. Finding Array Length

Use:

numbers.length

Example:

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

System.out.println(numbers.length);

Output:

4

⚠️ Notice:

Array → length String → length()

For arrays:

numbers.length

not:

numbers.length()

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11. Array + for Loop

Arrays are commonly processed using loops.

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

for (int i = 0; i < numbers.length; i++) { System.out.println(numbers[i]); }

Output:

10 20 30 40

Flow:

i = 0 → numbers[0] i = 1 → numbers[1] i = 2 → numbers[2] i = 3 → numbers[3]

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12. Enhanced for Loop

You can also use:

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

for (int n : numbers) { System.out.println(n); }

Output:

10 20 30 40

Read:

For each element n in numbers.

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13. Default Values

When an array is created with new, Java automatically assigns default values.

Example:

int[] a = new int[3];

Initially:

0 0 0

For common types:

Type Default

int 0 float 0.0f double 0.0 char '\u0000' boolean false Reference types null

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14. ArrayIndexOutOfBoundsException

Consider:

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

Valid indexes:

0, 1, 2

This is invalid:

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

It causes:

ArrayIndexOutOfBoundsException

Because the last valid index is:

length - 1

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15. Simple Complete Program

class ArrayDemo {

public static void main(String[] args) {

    int[] marks = {80, 75, 90, 65, 88};

    System.out.println("Number of students: " + marks.length);

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

}

Output:

Number of students: 5 80 75 90 65 88

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🧠 ONE-PAGE REVISION

ARRAY ↓ Multiple values ↓ Same data type ↓ Fixed size ↓ Index-based ↓ Index starts at 0

Main syntax

int[] a;

int[] a = new int[5];

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

Access

a[0] a[1] a[2]

Modify

a[1] = 50;

Length

a.length

Loop

for (int i = 0; i < a.length; i++) { System.out.println(a[i]); }

Remember the formula

First index = 0

Last index = length - 1

Number of elements = length

Array size is fixed after creation.