🧠 Java Memory Management

📋 What is Memory Management in Java?

When you run a Java program, Java needs memory to:

Store objects
Run methods
Store variables
Handle class information

👉 Key Point: Java automatically manages memory using the JVM (Java Virtual Machine) and Garbage Collector (like a cleaner).

🏠 Real-Life Example

Imagine your computer is a house:

Heap = Big cupboard shared by everyone — stores all objects
Stack = Small desk each person (thread) uses — stores local variables and method info
Method Area = Library — stores blueprints (class info, static data)
PC Register = Pointer showing what you're currently doing
Native Stack = A side tool area if you call C/C++ functions

📦 Java Memory Areas (Explained Simply)

1. Heap Memory (Main Storage)

Stores: All objects (new keyword), instance variables
Shared by: All threads
Cleaned by: Garbage Collector

🔧 Example:

String name = new String("Deepak");

👉 Result: "Deepak" is stored in Heap

2. Stack Memory (Per Thread)

Stores: Local variables, method calls
Each thread gets: Its own stack
Cleared when: Method ends

🔧 Example:

int a = 10;

👉 Result: 'a' is a local variable, stored in Stack

3. Method Area / MetaSpace (Blueprints)

Stores: Class definitions, static variables, method names
Loaded: Once per class

🔧 Example:

static int collegeCode = 123;

👉 Result: 'collegeCode' is stored in the Method Area

4. PC Register

Keeps track of which line in the method is being executed
Each thread has one

5. Native Method Stack

Only used when you call native (C/C++) code from Java using JNI

👨‍💻 Full Example Code + Memory Map

public class Student {
    int age = 20;                 // Stored in Heap
    static String school = "ABC"; // Stored in Method Area

    public static void main(String[] args) {
        int roll = 101;            // Stored in Stack
        Student s1 = new Student(); // Object in Heap, ref in Stack
    }
}

🧠 Memory Map:

Memory Area	What It Stores
Heap	Object s1, instance variable age
Stack	roll, reference to s1
Method Area	Class info, school

🧹 What is Garbage Collection?

It automatically removes unused objects from the Heap to free memory
No need to use free() like in C/C++

🔧 Example:

Student s1 = new Student();
s1 = null; // Now the object is not used → eligible for GC

You can suggest GC like this:

System.gc(); // Suggests garbage collection (not guaranteed)

✅ Simple Interview Questions and Answers

❓ Question	✅ Simple Answer
What is memory management?	Java automatically allocates and frees memory using JVM.
What is the Heap?	Stores objects. Shared by all. Cleaned by garbage collector.
What is Stack memory?	Stores method calls and local variables. One per thread.
What is garbage collection?	JVM removes unused objects from memory automatically.
Where are static variables stored?	In the Method Area.
What is stored in Stack vs Heap?	Stack = local vars, refs. Heap = objects.
Can you force GC in Java?	You can suggest it using System.gc(), but it's not guaranteed.

📝 Final Summary

Part	Description	Example
Heap	Stores objects	new Student()
Stack	Stores method calls and variables	int a = 5;
Method Area	Stores class data, static vars	static int x;
Garbage Collection	Removes unused objects	obj = null;

🎯 Key Points for Interview

Java uses automatic memory management (no manual free())
JVM memory has Heap, Stack, and Method Area
Objects → Heap, Variables → Stack, Class Info → Method Area
Garbage Collector frees memory automatically