Java String Immutability & SCP behavior

In Java, String is a non - primitive data type.

String is a sequence of characters / an array of characters.

It belongs to the java.lang package and represents sequence of characters.

String name = "Anushka";

String is an object. It is immutable. Stored in special memory called "String Pool".

Why String is Immutable ?

Once created, a String object cannot be changed.

String s = new String("Hello");

s.concat("World");

What happens internally ?

• "Hello" remains unchanged.


• " World" is checked in SCP (created if not present).


• A new object "Hello World" is created in heap.

What is String Constant Pool (SCP) ?

A String Constant Pool is a special memory area inside the Heap where Java stores string literals.

It is used to :

• Avoid duplicate objects.


• Save memory.


• Improve performance.
  

String s1 = "Java";

String s2 = "Java";

Only ONE Object is created in SCP.

Both s1 and s2 point to the same object.

s1 ─┐

    ├──► "Java"  (SCP)

s2 ─┘

What If We Use new ?

String s3 = new String("Java");

Java is stored in SCP.

A new Object is created in heap.

s3 points to heap object.

So now two objects exist :

1. SCP object


2. Heap object

Why SCP exists ?

• Every string literal would create new object.


• Huge memory waste.


• Performance issues.
  SCP makes Java memory efficient.