🚀 Day 29 of My Automation Journey – Arrays (Full Guide + Tricky Questions)

Today’s learning was one of the most important milestones in my journey 🔥

👉 Arrays (core concept)
👉 JVM internal behaviour
👉 Memory understanding
👉 Tricky interview questions
👉 Deep dive into public static void main

This is where coding becomes real engineering 🚀

---

🔹 PART A – ARRAYS COMPLETE THEORY

🔹 1. What is an Array?

👉 An array is an object

👉 It is a container that:
✔ Stores multiple values
✔ Same data type
✔ Fixed size

---

🔹 2. What Happens Internally (JVM 🔥)

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

👉 Internally JVM converts:

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

💡 JVM:

• Creates object in Heap memory
• Stores reference in variable

---

🔹 3. Proof – Array is an Object

System.out.println(arr.getClass());

Output:

class [I

---

🔹 4. Array Creation

int[] arr1 = {10, 20, 60, 30};
int[] arr2 = new int[5];

---

🔹 5. Important Points ⚠️

✔ Fixed size
✔ Index starts from 0
✔ Stored in Heap
✔ Default values assigned

---

🔹 6. Default Values

Type	Default
int	0
boolean	false
object	null

---

🔹 7. Initialization & Access

arr[0] = 10;
System.out.println(arr[0]);

---

🔹 8. Looping Through Array

Normal Loop:

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

---

Enhanced Loop:

for(int num : arr) {
    System.out.println(num);
}

---

🔹 9. Multi-Dimensional Array

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

---

🔹 PART B – PROGRAMS

✅ Program 1 – Default Values

int[] arr = new int[3];
System.out.println(arr[0] + " " + arr[1] + " " + arr[2]);

Output:

0 0 0

---

❌ Program 2 – Index Error

int[] arr = new int[3];
System.out.println(arr[3]);

Output:

ArrayIndexOutOfBoundsException

---

❌ Program 3 – Length Modification

int[] arr = {1, 2, 3};
System.out.println(arr.length);
arr.length = 10;

Output:

3
Compilation Error

---

🔥 Program 4 – Reference Behavior

int[] a = {1, 2, 3};
int[] b = a;
b[0] = 100;
System.out.println(a[0]);

Output:

100

---

🔥 Program 5 – Comparing Arrays

int[] a = {1, 2, 3};
int[] b = {1, 2, 3};
System.out.println(a == b);

Output:

false

---

❌ Program 6 – Null Array

int[] arr = null;
System.out.println(arr.length);

Output:

NullPointerException

---

❌ Program 7 – Negative Size

int[] arr = new int[-5];

Output:

NegativeArraySizeException

---

🔹 PART C – PRACTICE PROGRAM

package modul4;

import java.util.Scanner;

public class ArrayPractise {

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        System.out.println("Enter array Length: ");
        int count = sc.nextInt();

        byte[] by = new byte[count];
        short[] sh = new short[count];

        System.out.println("Byte ");
        for (int i = 0; i < count; i++) {
            System.out.println(by[i]);
        }

        System.out.println("Short ");
        for (int i = 0; i < count; i++) {
            System.out.println(sh[i]);
        }
    }
}

🔍 Output Insight

👉 Prints 0 because of default values

---

🔹 PART D – MEMORY & ADVANCED CONCEPTS

Separate Memory

int[] a = new int[3];
int[] b = new int[3];

👉 Stored in different memory

---

Passing Array

void test(int[] arr) {
    arr[0] = 999;
}

👉 Passed by reference

---

🔹 PART E – TRICKY QUESTIONS

❓ Q1

int[] arr = new int[3];
System.out.println(arr[0] + " " + arr[1] + " " + arr[2]);

✔ Output: 0 0 0
👉 Default values

---

❓ Q2

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

❌ Output: Exception
👉 Index out of range

---

❓ Q3

arr.length = 10;

❌ Compilation error
👉 Length is final

---

❓ Q4

int[] b = a;

✔ Same reference

---

❓ Q5

System.out.println(a == b);

❌ false
👉 Different objects

---

❓ Q6

int[] arr = null;
System.out.println(arr.length);

❌ NullPointerException

---

❓ Q7

new int[-5];

❌ NegativeArraySizeException

---

🔹 PART F – public static void main (DETAILED 🔥)

public static void main(String[] args)

---

🔍 Deep Breakdown

🔹 public

👉 JVM must access it

---

🔹 static

👉 No object required

👉 JVM calls directly:

ClassName.main()

---

🔹 void

👉 No return value

---

🔹 main

👉 Entry point

---

🔹 String[] args

👉 Command-line input

java Test hello

👉 args = ["hello"]

---

💡 Important Insight

👉 Without main() → program won’t run

---

🔚 FINAL TAKEAWAYS

✔ Array = Object
✔ JVM handles memory
✔ Default values exist
✔ Reference behavior matters
✔ Exceptions are important
✔ main() is entry point

---

💡 GOLDEN INSIGHT

👉 Arrays are the foundation of all data structures 🚀

---

Stay tuned 🚀

🤖 A Small Note
I used ChatGPT to help structure and refine this blog while keeping the concepts aligned with my learning.