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

<br /><b>Lesson:</b> Generics

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<div id="PageTitle"><h1>Generic Types</h1></div>

<div id="PageContent">

<p>Let&#39;s update our <code>Box</code> class to use generics. We&#39;ll first create a <i>generic type declaration</i> by changing the code &quot;<code>public class Box</code>&quot; to &quot;<code>public class Box&lt;T&gt;</code>&quot;; this introduces one <i>type variable</i>, named <code>T</code>, that can be used anywhere inside the class. This same technique can be applied to interfaces as well. There&#39;s nothing particularly complex about this concept. In fact, it&#39;s quite similar to what you already know about variables in general. Just think of <code>T</code> as a special kind of variable, whose &quot;value&quot; will be whatever type you pass in; this can be any class type, any interface type, or even another type variable. It just can&#39;t be any of the primitive data types. In this context, we also say that <code>T</code> is a <i>formal type parameter</i> of the <code>Box</code> class.</p>

<div class="codeblock"><pre>

/**

* Generic version of the Box class.

* @param &lt;T&gt; the type of value being boxed

*/

public class Box<b>&lt;T&gt;</b> {

// T stands for "Type"

private <b>T t</b>;

public void add(<b>T t</b>) {

this.<b>t</b> = <b>t</b>;

}

public <b>T</b> get() {

return <b>t</b>;

}

}

</pre></div>

<p>As you can see, we&#39;ve replaced all occurrences of <code>Object</code> with <code>T</code>.</p>

<p>Also note that a generic type may have multiple type parameters, but each parameter must be unique within its declaring class or interface. A declaration of <code>Box&lt;T,T&gt;</code>, for example, would generate an error on the second occurrence of <code>T</code>, but <code>Box&lt;T,U&gt;</code>, however, would be allowed.</p>

<p>To reference this generic class from within your own code, you must perform a <i>generic type invocation</i>, which replaces <code>T</code> with some concrete value, such as <code>Integer</code>:</p>

<div class="codeblock"><pre>

Box&lt;Integer&gt; integerBox;

</pre></div>

<p>You can think of a generic type invocation as being similar to an ordinary method invocation, but instead of passing an argument to a method, you&#39;re passing a <i>type argument</i> &#151; <code>Integer</code> in this case &#151; to the <code>Box</code> class itself.</p>

<p>Like any other variable declaration, this code does not actually create a new <code>Box</code> object. It simply declares that <code>integerBox</code> will hold a reference to a &quot;<code>Box</code> of <code>Integer</code>&quot;, which is how <code>Box&lt;Integer&gt;</code> is read.</p>

<p>An invocation of a generic type is generally known as a <i>parameterized type</i>.</p>

<p>To instantiate this class, use the <code>new</code> keyword, as usual, but place <code>&lt;Integer&gt;</code> between the class name and the parenthesis:</p>

<div class="codeblock"><pre>

integerBox = new Box&lt;Integer&gt;();

</pre></div>

<p>Or, you can put the entire statement on one line, such as:</p>

<div class="codeblock"><pre>

Box&lt;Integer&gt; integerBox = new Box&lt;Integer&gt;();

</pre></div>

<p>Once <code>integerBox</code> is initialized, you&#39;re free to invoke its <code>get</code> method without providing a cast, as in

<a class="SourceLink" target="_blank" href="examples/BoxDemo3.java" onclick="showCode('../../displayCode.html', 'examples/BoxDemo3.java'); return false;"><code>BoxDemo3</code></a>:</p>

<div class="codeblock"><pre>

public class BoxDemo3 {

public static void main(String[] args) {

Box&lt;Integer&gt; integerBox =

new Box&lt;Integer&gt;();

integerBox.add(new Integer(10));

// no cast!

Integer someInteger = integerBox.get();

System.out.println(someInteger);

}

}

</pre></div>

<p>Furthermore, if you try adding an incompatible type to the box, such as <code>String</code>, compilation will fail, alerting you to what previously would have been a runtime bug:</p>

<div class="codeblock"><pre>

BoxDemo3.java:5: add(java.lang.Integer) in Box&lt;java.lang.Integer&gt;

cannot be applied to (java.lang.String)

integerBox.add("10");

^

1 error

</pre></div>

<p>It&#39;s important to understand that type variables are not actually types themselves. In the above examples, you won&#39;t find <code>T.java</code> or <code>T.class</code> anywhere on the file system. Furthermore, <code>T</code> is not a part of the <code>Box</code> class name. In fact during compilation, all generic information will be removed entirely, leaving only <code>Box.class</code> on the file system. We&#39;ll discuss this later in the section on

<a class="TutorialLink" target="_top" href="erasure.html">Type Erasure</a>.

</p>

<h2><a name="diamond" id="diamond">The Diamond</a></h2>

<p>In Java SE 7 and later, you can replace the type arguments required to invoke the constructor of a generic class with an empty set of type parameters (<code>&lt;&gt;</code>) as long as the compiler can determine, or infer, the type arguments from the context. This pair of angle brackets, <code>&lt;&gt;</code>, is informally called the <em>diamond</em>. For example, you can create an instance of <code>Box&lt;Integer&gt;</code> with the following statement:</p>

<div class="codeblock"><pre>

Box&lt;Integer> integerBox = new Box&lt;>();

</pre></div>

<p>For more information about the diamond, see <a href="gentypeinference.html#type-inference-instantiation">Type Inference and Instantiation of Generic Classes</a>.</p>

<!--

<p>Note that using the diamond to infer the actual type parameters of the generic class being instantiated is different than inferring the actual type parameters of generic constructors (which is similar to generic methods and does not require the diamond operator). See <a href="gentypeinference.html#type-inference-constructors">Type Inference and Generic Constructors of Generic and Non-Generic Classes</a> for more information.</p>

-->

<h2>Type Parameter Naming Conventions</h2>

<p>By convention, type parameter names are single, uppercase letters. This stands in sharp contrast to the variable

<a class="TutorialLink" target="_top" href="../nutsandbolts/variables.html#naming">naming</a> conventions that you already know about, and with good reason: Without this convention, it would be difficult to tell the difference between a type variable and an ordinary class or interface name.</p>

<p>The most commonly used type parameter names are:</p>

<ul>

<li>E - Element (used extensively by the Java Collections Framework)</li>

<li>K - Key</li>

<li>N - Number</li>

<li>T - Type</li>

<li>V - Value</li>

<li>S,U,V etc. - 2nd, 3rd, 4th types</li>

</ul>

<p>You&#39;ll see these names used throughout the Java SE API and the rest of this tutorial.</p>

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