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{{Refimprove|date=June 2013}}The marker interface pattern is a design pattern in computer 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 implements a marker interfaceBOOK

Example

An example of the application of marker interfaces from the Java programming language is the {{Javadoc:SE|java/io|Serializable}} 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 {{Javadoc:SE|java/io|ObjectOutputStream}}. 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 directly:

• Both the .NET Framework and Java (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 and Plone. 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

{{Reflist}}

Further reading

Effective JavaBOOK, Bloch, Joshua,weblink Effective Java, 2018, 978-0-13-468599-1, Third, Boston, 1018432176, by Joshua Bloch.{{Design Patterns patterns}}