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Immutable interface

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In <a href="Object-oriented_programming" title="Object-oriented programming">object-oriented programming</a>, "immutable interface" is a <a href="Design_pattern_(computer_science)" class="mw-redirect" title="Design pattern (computer science)">pattern</a> for designing an <a href="Immutable_object" title="Immutable object">immutable object</a>.<a href="#cite_note-1">[1]</a> The immutable interface pattern involves defining a type which does not provide any <a href="Method_(computer_science)" class="mw-redirect" title="Method (computer science)">methods</a> which mutate state. Objects which are referenced by that type are not seen to have any mutable state, and appear immutable.

<div class="mw-heading mw-heading2"><h2 id="Example">Example</h2></div>

Consider a Java class which represents a two-dimensional point.

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>public class Point2D {

private int x;

private int y;

public Point2D(int x, int y) { this.x = x; this.y = y; }

public int getX() { return this.x; }

public int getY() { return this.y; }

public void setX(int newX) { this.x = newX; }

public void setY(int newY) { this.y = newY; }

}

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The class Point2D is mutable: its state can be changed after construction, by invoking either of the setter methods (<code>setX()</code> or <code>setY()</code>).

An immutable interface for Point2D could be defined as:

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>public interface ImmutablePoint2D {

public int getX();

public int getY();

}

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By making Point2D implement ImmutablePoint2D, client code could now reference a type which does not have mutating methods, and thus appears immutable. This is demonstrated in the following example:

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>ImmutablePoint2D point = new Point2D(0,0); // a concrete instance of Point2D is referenced by the immutable interface

int x = point.getX(); // valid method call

int y = point.setX(42); // compile error: the method setX() does not exist on type ImmutablePoint2D

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By referencing only the immutable interface, it is not valid to call a method which mutates the state of the concrete object.

<div class="mw-heading mw-heading2"><h2 id="Advantages">Advantages</h2></div>

<ul><li>Clearly communicates the immutable intent of the type.</li>

<li>Unlike types implementing the Immutable Wrapper pattern, does not need to "cancel out" mutating methods by issuing a "<a href="NOP_(code)" title="NOP (code)">No Operation</a>" instruction, or throwing a runtime exception when a mutating method is invoked.</li></ul>

<div class="mw-heading mw-heading2"><h2 id="Disadvantages">Disadvantages</h2></div>

<ul><li>It is possible for instances referenced by the immutable interface type to be <a href="Type_conversion" title="Type conversion">cast</a> to their concrete, mutable type, and have their state mutated. For example:<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>public void mutate(ImmutablePoint2D point) {

((Point2D)point).setX(42); // this call is legal, since the type has

// been converted to the mutable Point2D class

}

</pre></div></li>

<li>Concrete classes have to explicitly declare they implement the immutable interface. This may not be possible if the concrete class "belongs to" third-party code, for instance, if it is contained within a library.</li>

<li>The object is not really immutable and hence not suitable for use in data structures relying on immutability like hash maps. And the object could be modified concurrently from the "mutable side".</li>

<li>Some compiler optimizations available for immutable objects might not be available for mutable objects.</li></ul>

<div class="mw-heading mw-heading2"><h2 id="Alternatives">Alternatives</h2></div>

An alternative to the immutable interface pattern is the immutable wrapper pattern.

<a href="Persistent_data_structure" title="Persistent data structure">Persistent data structures</a> are effectively immutable while allowing modified views of themselves.

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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<li id="cite_note-1"><a href="#cite_ref-1">^</a> <a rel="nofollow" class="external text" href="http://c2.com/ppr/wiki/JavaIdioms/ImmutableInterface.html">Immutable Interface</a>, c2.com - <a rel="nofollow" class="external text" href="https://mindprod.com/jgloss/immutable">immutable</a>, mindprod.com - Peter Haggar, <a rel="nofollow" class="external text" href="http://www.informit.com/articles/article.aspx?p=20522">"Practical Java Praxis 65: Use Inheritance or Delegation to Define Immutable Classes"</a>, informIT, January 22, 2001

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