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📦 Chapter 9: Arrays (Java)

🚀 1. What is an Array?

👉 Simple Idea

Array = collection of same type data stored together

❌ Without Array

int r1 = 1001;
int r2 = 1002;
int r3 = 1003;

✅ With Array

int[] rollNumbers = new int[3];

✔ One variable ✔ Stores multiple values ✔ Clean & scalable

🧠 Concept

  • Array stores values in continuous memory
  • Each value has an index (position)
  • Index starts from 0

📌 From lecture: arrays store elements in contiguous memory locations

🔢 2. Declaration & Initialization

👉 Declaration

int[] arr;

👉 Initialization

arr = new int[3];

👉 Combined

int[] arr = new int[3];

🧠 Meaning

  • int → type
  • [] → array
  • 3 → size

🧩 3. Indexing (Important)

Index Value
0 First
1 Second
2 Third

👉 Example

int[] arr = new int[3];

arr[0] = 1001;
arr[1] = 1002;
arr[2] = 1003;

📤 4. Access / Print Elements

System.out.println(arr[0]); // 1001
System.out.println(arr[1]); // 1002
System.out.println(arr[2]); // 1003

🔁 5. Using Loop with Array (Very Important)

❌ Manual

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

✅ Using loop

for (int i = 0; i < 3; i++) {
    System.out.println(arr[i]);
}

🔥 Best Practice (Dynamic)

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

🧠 Why .length?

  • Avoid hardcoding
  • Works for any size

⚙️ 6. Filling Array using Loop

int[] arr = new int[3];
int x = 1001;

for (int i = 0; i < arr.length; i++) {
    arr[i] = x;
    x++;
}

✔ Efficient ✔ No manual work

⚠️ 7. Common Error

❌ Out of Bound

System.out.println(arr[3]); // Error

🚨 Error Name

ArrayIndexOutOfBoundsException

📌 Happens when index ≥ size

📏 8. Array Length

System.out.println(arr.length);

✔ Returns size of array

🔷 9. Types of Arrays

🟢 (A) 1D Array

int[] arr = new int[3];

👉 Looks like:

[10, 20, 30]

🔵 (B) 2D Array (Very Important)

👉 Concept

Array of Arrays

👉 Example (Marks System)

Sub1 Sub2 Sub3
S1 50 30 90
S2 60 40 80
S3 70 50 70

👉 Code

int[][] marks = new int[3][3];

👉 Indexing

marks[0][0] = 50;
marks[0][1] = 30;
marks[0][2] = 90;

👉 Access

System.out.println(marks[1][2]); // 80

🔁 10. Loop in 2D Array

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

🧠 Concept

  • i → rows
  • j → columns

⚖️ 11. 1D vs 2D Array

Feature 1D 2D
Structure Line Table
Index 1 2
Use Simple list Matrix / grid

🎯 Final Summary

  • Array = collection of same type
  • Index starts from 0
  • Stored in continuous memory
  • Use .length instead of hardcoding
  • Use loops for efficiency
  • 2D array = array of arrays

🧠 Intuition (Must Remember)

  • Array = container with slots
  • Index = address of slot
  • Loop = automatic filling / reading