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Thread: type erasure

  1. #1
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    Default type erasure

    Note: This may not be the appropriate subforum, but I found no way to select one for my thread. I apologize in advance.

    I am quite familiar with C and C++, but new to Java. With some trivial modifications for brevity, I find the following example of type erasure in Oracle's The Java Tutorials perplexing:

    Given the following two classes:

         public class Node<T> {
     
              public T data;
     
              public Node(T data) { this.data = data; }
     
              public void setData(T data) { this.data = data; }
         }
     
         public Class MyNode extends Node<Integer> {
     
              public MyNode(Integer data) { super(data); }
     
              public void setData(Integer data) { super.setData(data); }
         }
    }

    Consider the following code:

         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, the code becomes:

         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.

    End of example

    Type erasure merely adds the two type casts to the original code. The author writes that "trying to assign a String to an Integer (my note: in the final line of the type erasure code) causes a ClassCastException from a cast inserted at the assignment by a Java compiler."

    To me, though, the two type casts are superfluous. (MyNode) type casts a MyNode object to a MyNode. How does this contribute anything? And the ClassCastException should be thrown at run-time even without the (String) type cast.

    What am I missing?

    Thanks in advance.

  2. #2
    Super Moderator Norm's Avatar
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    Default Re: type erasure

    What happens when you fix statement with the warning?
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: type erasure

    This is just an example of how the compiler catches misuses of generic types. I just don't see why the two compiler casts in the type erasure code are needed at all here--especially why the misuse wouldn't be caught at runtime without the (String) cast in the erasure code, which the author of the example said was needed for this purpose.

  4. #4
    Super Moderator Norm's Avatar
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    Default Re: type erasure

    Sorry, I'm not into why the compiler writers did what they did. Perhaps some of the helpers on this forum could explain it better:
    http://www.coderanch.com/forums
    If you don't understand my answer, don't ignore it, ask a question.

  5. #5
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    Default Re: type erasure

    This is just an example of how the compiler catches misuses of generic types. I just don't see why the two compiler casts in the type erasure code are needed at all here--especially why the misuse wouldn't be caught at runtime without the (String) cast in the erasure code, which the author of the example said was needed for this purpose.

    --- Update ---

    I should add that generics are not the issue here, but rather, type casting. I am quite willing to accept that the (MyNode) cast is just an artifact of the compiler and, in fact, is superfluous. However, the writer's comment regarding the (String) cast bothers me in that some exception should be thrown without the need for the cast. The cast, though, does make it a ClassCastException, so maybe that's the point of the cast.

    I think I may have answered my own question. The compiler probably casts every expression in the type erasure code like this as a matter of course. Its(MyNode) cast is superfluous, and its (String) cast results in a ClassCastException. A different runtime exception would result without the cast.

    Thanks, Norm, for making me think this through!
    Last edited by Steve; July 16th, 2019 at 12:51 PM.

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