The marker interface pattern is a

design pattern A design pattern is the re-usable form of a solution to a design problem. The idea was introduced by the architect Christopher Alexander and has been adapted for various other disciplines, particularly software engineering. The " Gang of Four" b ...

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, 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 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 differentl ...

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 Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * ''Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Inte ...

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, class-based, object-oriented programming language that is designed to have as few implementation dependencies as possible. It is a general-purpose programming language intended to let programmers ''write once, run anywh ...

is the interface: package java.io; public interface Serializable A class implements this interface to indicate that its non-

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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

A major problem with marker interfaces is that an interface defines a contract for implementing classes, and that contract is inherited by all subclasses. This means that you cannot "unimplement" a marker. In the example given, if you create a subclass that you do not want to serialize (perhaps because it depends on transient state), you must resort to explicitly throwing NotSerializableException (per ObjectOutputStream docs) Another solution is for the language to support

metadata Metadata is "data that provides information about other data", but not the content of the data, such as the text of a message or the image itself. There are many distinct types of metadata, including: * Descriptive metadata – the descriptive ...

directly: * Both the

.NET Framework The .NET Framework (pronounced as "''dot net"'') is a proprietary software framework developed by Microsoft that runs primarily on Microsoft Windows. It was the predominant implementation of the Common Language Infrastructure (CLI) until bein ...

and

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(as of Java 5 (1.5)) provide support for such metadata. In .NET, they are called ''"custom attributes"'', in Java they are called ''"

annotations An annotation is extra information associated with a particular point in a document or other piece of information. It can be a note that includes a comment or explanation. Annotations are sometimes presented in the margin of book pages. For anno ...

"''. 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

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. * In

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, the term "marker interface" is common in

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and

Plone Plone is a free and 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 presence of large organizations. Hi ...

. Interfaces are declared as metadata and subclasses can use implementsOnly to declare they do not implement everything from their super classes.

Example

Critique

See also

References

Further reading