Marker interface pattern: Difference between revisions

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{{Short description|Design pattern in computer science}}

{{Refimprove|date=June 2013}}

The '''markupmarker 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.

The '''markup 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'''<ref name="EffectiveJava">

{{cite book

| last = Bloch

| first = Joshua

| title = Effective Java (Second edition)

| page = [https://archive.org/details/effectivejava00bloc_0/page/179 179]

| page = 179

| chapter = Item 37: Use marker interfaces to define types

| year = 2008

| isbn = 978-0-321-35668-0

| publisher = Addison-Wesley

| 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.

| chapter-url-access = registration

| chapter-url = https://archive.org/details/effectivejava00bloc_0/page/179

| edition = Second

| publisher = Addison-Wesley}}</ref> (also called '''tagging interface''') which is an empty interface,<ref>{{Cite web |date=2017-03-06 |title=Marker interface in Java |url=https://www.geeksforgeeks.org/marker-interface-java/ |access-date=2022-05-01 |website=GeeksforGeeks |language=en-us}}</ref> 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.

== CritiqueExample ==

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>.

An example of the application of marker interfaces from the [[Java (programming language)|Java programming language]] is the {{Javadoc:SE|java/io|Serializable}} interface:<syntaxhighlight lang="java">

package java.io;

public interface Serializable {

== Critique ==

}

An example of the application of marker interfaces from the [[Java (programming language)|Java programming language]] is the {{Javadoc:SE|java</io|Serializable}} interface. syntaxhighlight>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>writeObjectwriteObject0(Object,boolean)</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>.

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 <code>NotSerializableException</code> (per <code>ObjectOutputStream</code> docs)

==Critique==

Another solution is for the language to support [[metadata]] directly:

A majorOne problem with marker interfaces is that, since an interface defines a contract for implementing classes, and that contract is inherited by all subclasses., Thisa meansmarker thatcannot you cannotbe "unimplementunimplemented" a marker. In the example given, if you create aany subclass that you do not wantintended tofor serializeserialization (perhaps because it depends on transient state), you must resort to explicitly throwingthrow <code>NotSerializableException</code> exceptions (per <code>ObjectOutputStream</code> docs).

Another solution is for the language to support [[metadata]] directly:

* Both the [[.NET Framework|.NET framework]] and [[Java (software platform)|Java]] (as of Java 5 (1.5)) provide support for such metadata. In .NET, they are called ''"custom attributes"'', in Java they are called ''"[[Java annotation|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 (computer science)|reflection]].

* In [[Python (programming language)|Python]], the term "marker interface" is common in [[Zope]] and [[Plone (software)|Plone]]. Interfaces are declared as metadata and subclasses can use <code>implementsOnly</code> to declare they do not implement everything from their super classes.

== See also ==

* [[Design marker]]s for an expansion of this pattern.

* Joshua Bloch, "Effective Java (Second edition)," Item 37: Use marker interfaces to define types, page 179.

==References==

{{Reflist}}

{{reflist|colwidth = 30em}}

== Further reading ==

''Effective Java''<ref>{{Cite book |last=Bloch |first=Joshua |title=Effective Java |date=2018 |isbn=978-0-13-468599-1 |edition=Third |___location=Boston |oclc=1018432176}}</ref> by [[Joshua Bloch]].

{{Design Patterns patterns}}

[[Category:Software design patterns]]

[[Category:Java (programming language)]]