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, PHP (since 5.4), Lua,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.

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.

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