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The marker interface pattern is a design pattern in
computer science Computer science is the study of computation, information, and automation. Computer science spans Theoretical computer science, theoretical disciplines (such as algorithms, theory of computation, and information theory) to Applied science, ...
, used with languages that provide run-time type information about objects. It provides a means to associate metadata with a class where the language does not have explicit support for such metadata. To use this pattern, a
class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...
implements a marker interface (also called tagging interface) which is an empty interface, and methods that interact with instances of that class test for the existence of the interface. Whereas a typical interface specifies functionality (in the form of method declarations) that an implementing class must support, a marker interface need not do so. The mere presence of such an interface indicates specific behavior on the part of the implementing class. Hybrid interfaces, which both act as markers and specify required methods, are possible but may prove confusing if improperly used.


Example

An example of the application of marker interfaces from the
Java programming language Java is a high-level, general-purpose, memory-safe, object-oriented programming language. It is intended to let programmers ''write once, run anywhere'' ( WORA), meaning that compiled Java code can run on all platforms that support Jav ...
is the interface: package java.io; public interface Serializable A class implements this interface to indicate that its non- transient data members can be written to an . The ObjectOutputStream private method writeObject0(Object,boolean) contains a series of instanceof tests to determine writeability, one of which looks for the Serializable interface. If any of these tests fails, the method throws a NotSerializableException.


Critique

One problem with marker interfaces is that, since an interface defines a contract for implementing classes, and that contract is inherited by all subclasses, a marker cannot be "unimplemented". In the example given, any subclass not intended for serialization (perhaps it depends on transient state), must explicitly throw NotSerializableException exceptions (per ObjectOutputStream docs). Another solution is for the language to support
metadata Metadata (or metainformation) is "data that provides information about other data", but not the content of the data itself, such as the text of a message or the image itself. There are many distinct types of metadata, including: * Descriptive ...
directly: * Both the .NET Framework and
Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...
(as of Java 5 (1.5)) provide support for such metadata. In .NET, they are called ''"custom attributes"'', in Java they are called ''" annotations"''. Despite the different name, they are conceptually the same thing. They can be defined on classes, member variables, methods, and method parameters and may be accessed using reflection. * In Python, the term "marker interface" is common in
Zope Zope is a family of free and open-source software, free and open-source World Wide Web, web application servers written in Python (programming language), Python, and their associated online community. Zope stands for "Z Object Publishing Environm ...
and
Plone Plone is a free software, free and open source software, open source content management system (CMS) built on top of the Zope application server. Plone is positioned as an enterprise CMS and is commonly used for intranets and as part of the web ...
. Interfaces are declared as metadata and subclasses can use implementsOnly to declare they do not implement everything from their super classes.


See also

* Design markers for an expansion of this pattern.


References


Further reading

''Effective Java'' by Joshua Bloch. {{Design Patterns patterns Software design patterns Java (programming language)