🧠 What is Stack, Heap Memory, and String Pool in Java?

If you’ve started learning Java, you’ve probably heard people say things like:

“That variable is stored on the stack,” or “The object lives in the heap.”

And then there’s something called the String Pool — sounds confusing, right?
Don’t worry 😄 — this post will make everything super simple to understand.

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🧩 Let’s start with: What happens when you run a Java program?

When your Java code runs, the JVM (Java Virtual Machine) divides memory into different sections.
The two most important ones are:

1. Stack Memory

2. Heap Memory

Let’s see what each one does 👇

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🥞 1. Stack Memory — for short-term data

Think of Stack Memory as a temporary notebook 📒 that the JVM uses to store small, short-lived data.

✳️ Used for:

• Primitive variables (int, char, boolean, etc.)

• References to objects (not the objects themselves)

• Method calls and local variables

Every time a method is called, the JVM creates a stack frame for it.
When the method finishes, its frame is removed automatically.

🔹 Example:

void demo() {
    int a = 10;
    int b = 20;
}

🧠 Memory view:

Stack:
a → 10
b → 20

When demo() ends, both a and b are removed from the stack.

✅ Key idea: Stack memory is fast but temporary.

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🧱 2. Heap Memory — for long-term objects

Now imagine the Heap as a big warehouse 🏗️ where Java stores objects created using the new keyword.

✳️ Used for:

• Objects and arrays

• Instance variables

• String objects (created using new)

🔹 Example:

String name = new String("John");

🧠 Memory view:

Stack:
name → 0x101 (reference to heap)
Heap:
0x101 → "John"

Even if the method that created name ends, the object "John" stays in the heap until the Garbage Collector removes it (when no references point to it).

✅ Key idea: Heap memory is shared and used for all objects in your program.

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💬 Stack vs Heap — quick comparison

Feature	Stack Memory	Heap Memory
Stores	Primitives, references	Objects, arrays
Lifetime	Short (method-level)	Long (until GC cleans up)
Managed by	JVM automatically	Garbage Collector
Access speed	Very fast	Slower
Thread safety	Each thread has its own	Shared by all threads

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💎 3. String Pool — a special part of the Heap

Strings are special in Java because they are immutable (cannot be changed once created).
So Java uses a special memory area called the String Pool to reuse strings instead of creating duplicates.

🔹 Example:

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

🧠 Memory view:

Heap (String Pool):
"Hello" ←─ s1, s2

➡️ Both s1 and s2 point to the same string — no new object created.

But if you use new String("Hello"):

String s3 = new String("Hello");

Then a new object is created in the heap (outside the pool).

🧠 Memory view:

Heap:
String Pool: "Hello" ←─ s1, s2
Outside Pool: "Hello" ←─ s3

You can make it reuse the pooled version using .intern():

String s4 = new String("Hello").intern();
System.out.println(s1 == s4); // true

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🚀 Summary

Memory Area	Stores	Lifetime	Managed By	Example
Stack	Primitives, references	Temporary (method-level)	JVM	int x = 10;
Heap	Objects, arrays	Long (until GC runs)	Garbage Collector	new Student()
String Pool	String literals	Long	JVM	"Hello"

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🧠 Quick Recap:

• 🥞 Stack → short-term, stores primitive values and object references.

• 🧱 Heap → long-term, stores actual objects and arrays.

• 💎 String Pool → special memory inside the heap that reuses string literals to save space.

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💬 Final Thought

Understanding how Java memory works helps you write efficient and bug-free programs.
Next time you create an object or a string, you’ll know exactly where it lives in memory! 🚀