What's the advantage of a String being Immutable?

Question

Once I studied about the advantage of a string being immutable because of something to improve performace in memory.

Can anybody explain this to me? I can't find it on the Internet.

Answer 1

Immutability (for strings or other types) can have numerous advantages:

• It makes it easier to reason about the code, since you can make assumptions about variables and arguments that you can't otherwise make.
• It simplifies multithreaded programming since reading from a type that cannot change is always safe to do concurrently.
• It allows for a reduction of memory usage by allowing identical values to be combined together and referenced from multiple locations. Both Java and C# perform string interning to reduce the memory cost of literal strings embedded in code.
• It simplifies the design and implementation of certain algorithms (such as those employing backtracking or value-space partitioning) because previously computed state can be reused later.
• Immutability is a foundational principle in many functional programming languages - it allows code to be viewed as a series of transformations from one representation to another, rather than a sequence of mutations.

Immutable strings also help avoid the temptation of using strings as buffers. Many defects in C/C++ programs relate to buffer overrun problems resulting from using naked character arrays to compose or modify string values. Treating strings as a mutable types encourages using types better suited for buffer manipulation (see StringBuilder in .NET or Java).

Answer 2

Consider the alternative. Java has no const qualifier. If String objects were mutable, then any method to which you pass a reference to a string could have the side-effect of modifying the string. Immutable strings eliminate the need for defensive copies, and reduce the risk of program error.