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The '''marker interface pattern''' is a [[design pattern (computer science)|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 (computer science)|class]] implements a '''marker interface'''
{{cite book
| last = Bloch
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| year = 2008
| isbn = 978-0-321-35668-0
| publisher = Addison-Wesley}}</ref> (also called '''tagging interface'''), and methods that interact with instances of that class test for the existence of the interface. Whereas a typical [[interface (computer science)|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.
An example of the application of marker interfaces from the [[Java (programming language)|Java programming language]] is the {{Javadoc:SE|java/io|Serializable}} interface. A class implements this interface to indicate that its non-[[Transient (computer programming)|transient]] data members can be written to an {{Javadoc:SE|java/io|ObjectOutputStream}}. The <code>ObjectOutputStream</code> private method <code>writeObject()</code> contains a series of <code>instanceof</code> tests to determine writeability, one of which looks for the <code>Serializable</code> interface. If any of these tests fails, the method throws a <code>NotSerializableException</code>.
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