String Interning

In computer science, string interning is a method of storing only one copy of each distinct string value, which must be immutable. Interning strings makes some string processing tasks more time- or space-efficient at the cost of requiring more time when the string is created or interned. The distinct values are stored in a string intern pool.

The single copy of each string is called its 'intern' and is typically looked up by a method of the string class, for example String.intern in Java. All compile-time constant strings in Java are automatically interned using this method.

String interning is supported by some modern object-oriented programming languages, including Python, Ruby (with its symbols), Java and .NET languages. Lisp, Scheme, and Smalltalk are among the languages with a symbol type that are basically interned strings. The library of the Standard ML of New Jersey contains an atom type that does the same thing. Objective-C's selectors, which are mainly used as method names, are interned strings. Both Lua and JavaScript string values are immutable and interned.

Objects other than strings can be interned. For example, in Java, when primitive values are boxed into a wrapper object, certain values (any boolean, any byte, any char from 0 to 127, and any short or int between -128 and 127) are interned, and any two boxing conversions of one of these values are guaranteed to result in the same object.