In computer science, interning is re-using objects of equal value on-demand instead of creating new objects. This
creational pattern is frequently used for numbers and strings in different programming languages. In many object-oriented languages such as
Python, even
primitive types such as integer numbers are objects. To avoid the overhead of constructing a large number of integer objects, these objects get reused through interning.
For interning to work the interned objects must be immutable, since state is shared between multiple variables.
String interning is a common application of interning, where many strings with identical values are needed in the same program.
History
Lisp
A lisp is a speech impairment in which a person misarticulates sibilants (, , , , , , , ). These misarticulations often result in unclear speech.
Types
* A frontal lisp occurs when the tongue is placed anterior to the target. Interdental lispi ...
introduced the notion of interned strings for its
symbols
A symbol is a mark, sign, or word that indicates, signifies, or is understood as representing an idea, object, or relationship. Symbols allow people to go beyond what is known or seen by creating linkages between otherwise very different co ...
. The LISP 1.5 Programmers Manual describes a function called
intern which either evaluates to an existing symbol of the supplied name, or if none exists, creates a new symbol of that name. This idea of interned symbols persists in more recent dialects of Lisp, such as
Clojure
Clojure (, like ''closure'') is a dynamic and functional dialect of the Lisp programming language on the Java platform. Like other Lisp dialects, Clojure treats code as data and has a Lisp macro system. The current development process is ...
in special forms such a
(def symbol) which perform symbol creation and interning
In the
object-oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of " objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...
paradigm interning is an important mechanism in the
flyweight pattern, where an interning method is called to store the intrinsic state of an object such that this can be shared among different objects which share different extrinsic state, avoiding needless duplication.
Interning continues to be an important technique for managing memory use in programming language implementations; for example, the Java Language Specification requires that identical string literals (that is, literals that contain the same sequence of code points) must refer to the same instance of class String, because string literals are "interned" so as to share unique instances. In the Python programming language small integers are interned, though the details of exactly which are dependent on language version.
Motivation
Interning saves memory and can thus improve performance and memory footprint of a program.
[{{Cite book, last=Oaks, first=Scott, url=https://www.worldcat.org/oclc/878059649, title=Java performance : the definitive guide, date=2014, publisher=O'Reilly Media, isbn=1-4493-6354-7, location=Sebastopol, CA, oclc=878059649] The downside is time required to search for existing values of objects which are to be interned.
See also
*
Flyweight pattern
*
Hash consing
References
External links
Design Patterns - University of Washington
Software optimization
String (computer science)