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

<br /><b>Lesson:</b> Generics (Updated)

<br /><b>Section:</b> Type Erasure

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<div id="PageTitle"><h1>Effects of Type Erasure and Bridge Methods</h1></div>

<div id="PageContent">

<p>

Sometimes type erasure causes a situation that you may not have anticipated. The following example shows how this can occur. The example (described in

<a class="TutorialLink" target="_top" href="#bridgeMethods">Bridge Methods</a>) shows how a compiler sometimes creates a synthetic method, called a bridge method, as part of the type erasure process.</p>

<p>

Given the following two classes:</p>

<div class="codeblock"><pre>

public class Node&lt;T&gt; {

private T data;

public Node(T data) { this.data = data; }

public void setData(T data) {

System.out.println("Node.setData");

this.data = data;

}

}

public class MyNode extends Node&lt;Integer&gt; {

public MyNode(Integer data) { super(data); }

public void setData(Integer data) {

System.out.println("MyNode.setData");

super.setData(data);

}

}

</pre></div>

<p>

Consider the following code:</p>

<div class="codeblock"><pre>

MyNode mn = new MyNode(5);

Node n = mn; // A raw type - compiler throws an unchecked warning

n.setData("Hello"); // Causes a ClassCastException to be thrown.

Integer x = mn.data;

</pre></div>

<p>

After type erasure, this code becomes:</p>

<div class="codeblock"><pre>

MyNode mn = new MyNode(5);

Node n = (MyNode)mn; // A raw type - compiler throws an unchecked warning

n.setData("Hello");

Integer x = (String)mn.data; // Causes a ClassCastException to be thrown.

</pre></div>

<p>

Here is what happens as the code is executed:</p>

<ul>

<li><tt>n.setData("Hello");</tt> causes the method <tt>setData(Object)</tt> to be executed on the object of class <tt>MyNode</tt>. (The <tt>MyNode</tt> class inherited <tt>setData(Object)</tt> from <tt>Node</tt>.)</li>

<li>In the body of <tt>setData(Object)</tt>, the data field of the object referenced by <tt>n</tt> is assigned to a <tt>String</tt>.</li>

<li>The data field of that same object, referenced via <tt>mn</tt>, can be accessed and is expected to be an integer (since <tt>mn</tt> is a <tt>MyNode</tt> which is a <tt>Node&lt;Integer&gt;</tt>.</li>

<li>Trying to assign a <tt>String</tt> to an <tt>Integer</tt> causes a <tt>ClassCastException</tt> from a cast inserted at the assignment by a Java compiler.</li>

</ul>

<h2><a name="bridgeMethods" id="bridgeMethods">Bridge Methods</a></h2>

<p>

When compiling a class or interface that extends a parameterized class or implements a parameterized interface, the compiler may need to create a synthetic method, called a <em>bridge method</em>, as part of the type erasure process. You normally don't need to worry about bridge methods, but you might be puzzled if one appears in a stack trace.</p>

<p>

After type erasure, the <tt>Node</tt> and <tt>MyNode</tt> classes become:</p>

<div class="codeblock"><pre>

public class Node {

private Object data;

public Node(Object data) { this.data = data; }

public void setData(Object data) {

System.out.println("Node.setData");

this.data = data;

}

}

public class MyNode extends Node {

public MyNode(Integer data) { super(data); }

public void setData(Integer data) {

System.out.println(Integer data);

super.setData(data);

}

}

</pre></div>

<p>

After type erasure, the method signatures do not not match. The <tt>Node</tt> method becomes <tt>setData(Object)</tt> and the <tt>MyNode</tt> method becomes <tt>setData(Integer)</tt>. Therefore, the <tt>MyNode</tt> <tt>setData</tt> method does not override the <tt>Node</tt> <tt>setData</tt> method.</p>

<p>

To solve this problem and preserve the

<a class="TutorialLink" target="_top" href="../IandI/polymorphism.html">polymorphism</a> of generic types after type erasure, a Java compiler generates a bridge method to ensure that subtyping works as expected. For the <tt>MyNode</tt> class, the compiler generates the following bridge method for <tt>setData</tt>:</p>

<div class="codeblock"><pre>

class MyNode extends Node {

<strong>// Bridge method generated by the compiler

//

public void setData(Object data) {

setData((Integer) data);

}</strong>

public void setData(Integer data) {

System.out.println("MyNode.setData");

super.setData(data);

}

// ...

}

</pre></div>

<p>

As you can see, the bridge method, which has the same method signature as the <tt>Node</tt> class's <tt>setData</tt> method after type erasure, delegates to the original <tt>setData</tt> method.</p>

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