🧠 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

Mnemonic to Remember: JVM is like a "Janitor Vacuuming Messes" – it cleans up automatically so you don't have to worry about forgetting to free memory.

🏠 Real-Life Example

Imagine your computer is a house:

• Heap = Big cupboard shared by everyone — stores all objects (think of it as a party hall where objects come and go).
• Stack = Small desk each person (thread) uses — stores local variables and method info (like a stack of plates, last in first out).
• 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.

Easy to Remember: Heap for "Heavy Objects", Stack for "Short-term Stuff".

📦 Java Memory Areas (Explained Simply)

1. Heap Memory (Main Storage)

• Stores:
  • All objects created using new keyword
  • Instance variables
  • Runtime constant pool (part of Method Area)
  • String Pool (special memory area inside Heap)
• Shared by: All threads
• Cleaned by: Garbage Collector (GC)
• Divided into:
  • Young Generation: Newly created objects (Eden + Survivor spaces) – most objects die young, like butterflies.
  • Old Generation: Long-lived objects – survivors promoted here.
  • Metaspace (Java 8+): Class metadata, method info – replaced PermGen.

Mnemonic: Heap = "Huge Elderly Area for Persistent Stuff" (Young for new, Old for survivors).

🔧 Example 1:

String name = new String("Deepak");

👉 Result: One "Deepak" in String Pool + another new object in Heap.

🔧 Example 2 (String Pool):

String s1 = "Hello";  
String s2 = "Hello";

👉 Result: Both s1 and s2 point to the same object in String Pool, no duplicate created.

🔧 Example 3 (intern() method):

String s1 = new String("World");  
String s2 = s1.intern();  
String s3 = "World";

👉 Result: s2 and s3 refer to the same object in String Pool, but s1 is a separate object in Heap.

📌 Important Notes on Heap & String Pool:

• Heap is the biggest memory area in JVM, used for almost all object storage.
• Memory management in Heap is automatic (handled by Garbage Collector).
• String Pool optimizes memory by reusing immutable String objects.
• Too many Strings without pooling can cause OutOfMemoryError.

🔑 About intern()

• intern() is a method of String class.
• It checks whether the String already exists in the String Pool.
• If it exists → returns reference from the pool.
• If it doesn’t exist → adds the String to the pool and returns that reference.

String x = new String("Java");
String y = x.intern();
String z = "Java";

System.out.println(x == y); // false
System.out.println(y == z); // true
                        

👉 Why use intern()?

• Helps save memory by avoiding duplicate String objects.
• Improves performance when many identical Strings are used (e.g., large text processing, XML/JSON parsing).
• But: Excessive interning may increase GC pressure (trade-off).

2. Stack Memory (Per Thread)

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

Mnemonic: Stack = "Swift Temporary Area for Calls and Keywords" (LIFO like a stack of books).

🔧 Example:

int a = 10;

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

static int collegeCode = 123;

👨‍💻 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:

🧹 What is Garbage Collection?

• It automatically removes unused objects from the Heap to free memory
• No need to use free() like in C/C++
• Generational GC: Divides heap into Young (short-lived) and Old (long-lived) for efficiency.
• Stop-the-World: Pauses app during GC – minimize with tuning.

Mnemonic: GC = "Garbage Cleaner" – like a robot vacuum that runs when the room (heap) is messy.

🔧 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

🤔 Interactive Self-Quiz for Interviews

Test your understanding with these expandable questions. Click to reveal answers.

💼 Real Interview Questions on Java Memory Management

These are commonly asked questions in interviews, compiled from sources like Baeldung, Medium, JavaCodeGeeks, and in28minutes.

🎯 Key Points for Interview