Why are Strings immutable in Java/C#? What are the benefits?
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.
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String interning and pooling become safe because shared strings cannot be changed by one consumer.
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Immutability makes strings easier to reason about in concurrent programs.
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Hash codes can be safely cached because the contents cannot change.
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Strings used as map keys remain stable after insertion.
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Immutability reduces certain classes of accidental state-sharing bugs.
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Repeated concatenation can still be inefficient, so StringBuilder or equivalent builders should be used for many modifications.
Follow-up Questions
- How does string interning work?
- Why is StringBuilder more efficient for repeated concatenation?