Why are Strings immutable in Java/C#? What are the benefits?

Difficulty: 5/10

string immutability, memory management, thread safety

String Immutability

Immutable strings cannot be changed after they are created. In Java this is a fundamental property of String. In modern .NET, System.String is also immutable. If a modification is requested, a new string is created rather than modifying the original object.

1
String interning and pooling become safe because shared strings cannot be changed by one consumer.
2
Immutability makes strings easier to reason about in concurrent programs.
3
Hash codes can be safely cached because the contents cannot change.
4
Strings used as map keys remain stable after insertion.
5
Immutability reduces certain classes of accidental state-sharing bugs.
6
Repeated concatenation can still be inefficient, so StringBuilder or equivalent builders should be used for many modifications.

Scenario Questions

0-2 years experience

2-5 years experience

1We noticed a memory leak in a web service after processing many user‑generated strings. The code was using `String` concatenation heavily. Walk me through why immutability could be causing the issue and how you would fix it.

Follow-up Questions