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<b>Trail:</b> Learning the Java Language

<br /><b>Lesson:</b> Interfaces and Inheritance

<br /><b>Section:</b> Interfaces

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<div class="linkLESSON"><a href="index.html">Interfaces and Inheritance</a></div>

<div class="linkAHEAD"><a href="createinterface.html">Interfaces</a></div>

<div class="linkBHEAD"><a href="interfaceDef.html">Defining an Interface</a></div>

<div class="linkBHEAD"><a href="usinginterface.html">Implementing an Interface</a></div>

<div class="nolinkBHEAD">Using an Interface as a Type</div>

<div class="linkBHEAD"><a href="nogrow.html">Evolving Interfaces</a></div>

<div class="linkBHEAD"><a href="defaultmethods.html">Default Methods</a></div>

<div class="linkBHEAD"><a href="summary-interface.html">Summary of Interfaces</a></div>

<div class="linkQUESTIONS"><a href="QandE/interfaces-questions.html">Questions and Exercises</a></div>

<div class="linkAHEAD"><a href="subclasses.html">Inheritance</a></div>

<div class="linkBHEAD"><a href="multipleinheritance.html">Multiple Inheritance of State, Implementation, and Type</a></div>

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<div class="linkBHEAD"><a href="polymorphism.html">Polymorphism</a></div>

<div class="linkBHEAD"><a href="hidevariables.html">Hiding Fields</a></div>

<div class="linkBHEAD"><a href="super.html">Using the Keyword super</a></div>

<div class="linkBHEAD"><a href="objectclass.html">Object as a Superclass</a></div>

<div class="linkBHEAD"><a href="final.html">Writing Final Classes and Methods</a></div>

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<div class="linkBHEAD"><a href="summaryinherit.html">Summary of Inheritance</a></div>

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<div id="PageTitle"><h1>Using an Interface as a Type</h1></div>

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<p>When you define a new interface, you are defining a new reference data type. You can use interface names anywhere you can use any other data type name. If you define a reference variable whose type is an interface, any object you assign to it <i>must</i> be an instance of a class that implements the interface.</p>

<p>As an example, here is a method for finding the largest object in a pair of objects, for <i>any</i> objects that are instantiated from a class that implements <code>Relatable</code>:</p>

<div class="codeblock"><pre>

public Object findLargest(Object object1, Object object2) {

Relatable obj1 = (Relatable)object1;

Relatable obj2 = (Relatable)object2;

if ((obj1).isLargerThan(obj2) &gt; 0)

return object1;

else

return object2;

}

</pre></div>

<p>By casting <code>object1</code> to a <code>Relatable</code> type, it can invoke the <code>isLargerThan</code> method.</p>

<p>If you make a point of implementing <code>Relatable</code> in a wide variety of classes, the objects instantiated from <i>any</i> of those classes can be compared with the <code>findLargest()</code> method&#151;provided that both objects are of the same class. Similarly, they can all be compared with the following methods:</p>

<div class="codeblock"><pre>

public Object findSmallest(Object object1, Object object2) {

Relatable obj1 = (Relatable)object1;

Relatable obj2 = (Relatable)object2;

if ((obj1).isLargerThan(obj2) &lt; 0)

return object1;

else

return object2;

}

public boolean isEqual(Object object1, Object object2) {

Relatable obj1 = (Relatable)object1;

Relatable obj2 = (Relatable)object2;

if ( (obj1).isLargerThan(obj2) == 0)

return true;

else

return false;

}

</pre></div>

<p>These methods work for any &quot;relatable&quot; objects, no matter what their class inheritance is. When they implement <code>Relatable</code>, they can be of both their own class (or superclass) type and a <code>Relatable</code> type. This gives them some of the advantages of multiple inheritance, where they can have behavior from both a superclass and an interface.</p>

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