Usage
Strings and other concrete objects are typically expressed as immutable objects to improve readability and runtime efficiency in object-oriented programming. In Python, Java and the .NET Framework, strings are immutable objects. Both Java and the .NET Framework have mutable versions of string. In Java these are StringBuffer and StringBuilder (mutable versions of Java String) and in .NET this is StringBuilder (mutable version of .Net String). Python 3 has a mutable string (bytes) variant, named bytearray.
Additionally, all of the primitive wrapper classes in Java are immutable.
Enforcement of the pattern can be checked by using specialized compilers (see for example http://pec.dev.java.net/), and there is a proposal to add immutable types to Java.
Similar patterns are the Immutable Interface and Immutable Wrapper.
In pure functional programming languages it is not possible to create mutable objects, so all objects are immutable.
Read more about this topic: Immutable Object
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