Java Programming Lesson 6 Part-1 | Arrays in Java Explained

LESSON-6: ARRAYS (1D & 2D)

📘 Lesson 6 — Arrays in Java

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✅ What is an Array?

An array is a collection of multiple values stored in one variable.

Example:

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

📌 Arrays store same data type
📌 Index starts from 0
📌 Size is fixed

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✅ 1️⃣ One-Dimensional (1D) Array

✅ Declaration Methods

int arr[] = {10, 20, 30, 40};  // Method 1

int arr2[] = new int[5];      // Method 2
arr2[0] = 5;                   // Assign later

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✅ Loop through Array

public class ArrayExample1D {
    public static void main(String[] args) {
        int marks[] = {85, 90, 75, 88, 92};

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

✔ marks.length = array size

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✅ Enhanced for-loop (foreach loop)

for(int x : marks){
    System.out.println(x);
}

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✅ 2️⃣ Two-Dimensional (2D) Array (Matrix Form)

Arrays inside arrays → rows & columns

int numbers[][] = {
    {1, 2, 3},
    {4, 5, 6}
};

✅ numbers has → 2 rows, each row 3 elements

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✅ Print 2D Array

public class ArrayExample2D {
    public static void main(String[] args) {
        int matrix[][] = {
            {10, 20, 30},
            {40, 50, 60}
        };

        for(int i = 0; i < 2; i++){
            for(int j = 0; j < 3; j++){
                System.out.print(matrix[i][j] + " ");
            }
            System.out.println();
        }
    }
}

✅ Output:

10 20 30
40 50 60

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✅ Input Array from User (Important ✅)

import java.util.Scanner;

public class UserArray {
    public static void main(String[] args) {
        
        Scanner sc = new Scanner(System.in);
        int arr[] = new int[5];

        System.out.println("Enter 5 numbers:");
        for(int i = 0; i < 5; i++){
            arr[i] = sc.nextInt();
        }

        System.out.println("You entered:");
        for(int x : arr){
            System.out.print(x + " ");
        }
    }
}

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✅ Common Array Mistakes

Mistake	Issue
Accessing arr[5] when size is 5	❌ Index out of bounds (0–4 valid)
Mixed data types in same array	❌ Not allowed
Forgetting array size is fixed	❌ Cannot expand

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📝 Exercise for You ✅ (Do it now)

Write 2 programs:

1️⃣ 1D Array → Store 5 integers and print the sum
2️⃣ 2D Array → Print this pattern using array:

1 2 3
4 5 6
7 8 9

Reply with:

✅ Your Code
✅ Output

I will check and give feedback ✅🔥

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🎯 Quick Quiz

1️⃣ Array index always starts from:
A) 0
B) 1
C) -1

2️⃣ Which gets array size?
A) size()
B) count()
C) length

Reply format:
1 → ?, 2 → ?

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✅ Lesson-6 Completed!

Next → What should we learn?

➡ NEXT Lesson-6 Part-2: Strings (String class & methods)