Java String Immutability & SCP behavior

Source: Dev.to

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 the String Constant Pool (SCP) and created if not present.
• A new object "Hello World" is created on the 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 the SCP; both s1 and s2 reference the same object.

s1 ─┐
    ├──► "Java"  (SCP)
s2 ─┘

What If We Use new?

String s3 = new String("Java");

• "Java" is stored in the SCP.
• A new String object is created on the heap.
• s3 points to the heap object.

Thus, two objects exist:

1. SCP object ("Java" literal)
2. Heap object (created by new)

Why SCP Exists?

• Without the SCP, every string literal would create a new object, leading to huge memory waste.
• The SCP makes Java memory usage more efficient and improves performance.