Effects of Type Erasure and Bridge Methods

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Effects of Type Erasure

Sometimes type erasure causes a situation that you may not have anticipated. The following example shows how this can occur. The example (described in Bridge Methods) shows how a compiler sometimes creates a synthetic method, called a bridge method, as part of the type erasure process.

Given the following two classes:

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public class Node<T> {

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

public void setData(Integer data) {
System.out.println("MyNode.setData");
super.setData(data);
}
}

Consider the following code:

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MyNode mn = new MyNode(5);
Node n = mn; // A raw type - compiler throws an unchecked warning
n.setData("Hello");
Integer x = mn.data; // Causes a ClassCastException to be thrown.

After type erasure, this code becomes:

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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.

Here is what happens as the code is executed:

  • n.setData(“Hello”); causes the method setData(Object) to be executed on the object of class MyNode. (The MyNode class inherited setData(Object) from Node.)
  • In the body of setData(Object), the data field of the object referenced by n is assigned to a String.
  • The data field of that same object, referenced via mn, can be accessed and is expected to be an integer (since mn is a MyNode which is a Node.
  • Trying to assign a String to an Integer causes a ClassCastException from a cast inserted at the assignment by a Java compiler.

Bridge Methods

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 bridge method, 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.

After type erasure, the Node and MyNode classes become:

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public class Node {

public 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("MyNode.setData");
super.setData(data);
}
}

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

To solve this problem and preserve the polymorphism of generic types after type erasure, a Java compiler generates a bridge method to ensure that subtyping works as expected. For the MyNode class, the compiler generates the following bridge method for setData:

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class MyNode extends Node {

// Bridge method generated by the compiler
//
public void setData(Object data) {
setData((Integer) data);
}

public void setData(Integer data) {
System.out.println("MyNode.setData");
super.setData(data);
}

// ...
}

As you can see, the bridge method, which has the same method signature as the Node class’s setData method after type erasure, delegates to the original setData method.