Does Java optimize immutable objects?

immutability, java, optimization

Solution

Wrt. Strings, the word you're looking for is interning.

Java won't do this for your own immutable objects. It does have cached versions of boxed primitives, though. See this article on wrapper class caching for more info.

Problem

Java strings are immutable, and instantiating multiple Strings with the same values returns the same object pointer. (Is there a term for this? "pooling" seems to fit, but that already refers to doing caching to save time by doing fewer instantiations.) Does Java also do this (the thing without a term) with other (user-defined) classes that are immutable? Can Java even detect that a class is immutable, or is this something unique to the string class?

Original source