Passing generic types as generic method parameters in Java

Question

First let me apologize for the terrible title, but I had no idea how to summarize this in a single sentence.

public class GenericFun {
    public class TypedStream<I extends OutputStream> {
        I input;

        public I getInput() { return input; }
        public void setInput(I input) { this.input = input; }
    }

    public abstract class GarbageWriter<I extends OutputStream> {
        public void writeGarbage(I output) throws Exception {
            output.write("Garbage".getBytes());
        }
    }

    public class GarbageWriterExecutor<I extends OutputStream> extends GarbageWriter<I> {
        public void writeTrash(TypedStream stream) throws Exception{
            this.writeGarbage(stream.getInput());       // Error
            this.writeGarbage((I)stream.getInput());    // OK
        }
    }
}

In the above code (the OutputStream is just an example) in class GarbageWriterExecutor class in the method first line causes compilation error, while the second one don't. I have two questions regarding this.

Why stream.getInput() causes error, even though TypedStream.I is known to extend OutputStream?
How can I solve this issue without the ugly casting?

Jonathan Drapeau · Accepted Answer · 2013-10-02 14:13:35Z

4

Because your method

public void writeTrash(TypedStream stream)

should also make sure TypedStream type is defined, like this :

 public void writeTrash(TypedStream<I> stream)

Edit : Thomas answer actually explain why

TypedStream stream will disable generic type checking and thus the compiler only knows that getInput() will return an object, hence the error.

answered Oct 2, 2013 at 14:13

Jonathan Drapeau

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Comments

Thomas · Accepted Answer · 2013-10-02 14:13:36Z

TypedStream stream will disable generic type checking and thus the compiler only knows that getInput() will return an object, hence the error.

Try writeTrash(TypedStream<I> stream) instead.

Maybe you event want to use writeTrash(TypedStream<? extends I> stream) in order to be able to pass any TypedStream which is parameterized for I or subclasses of I.

Yet another alternative would be

public class GarbageWriterExecutor extends GarbageWriter<OutputStream> {
  public void writeTrash(TypedStream<?> stream) throws Exception{
    this.writeGarbage(stream.getInput());     
  }
}

or

public class GarbageWriterExecutor extends GarbageWriter<OutputStream> {
  public void writeTrash(TypedStream<? extends OutputStream> stream) throws Exception{
    this.writeGarbage(stream.getInput());     
  }
}

sp00m · Accepted Answer · 2013-10-02 14:14:15Z

1

Simply use:

public class GarbageWriterExecutor<I extends OutputStream> extends GarbageWriter<I> {
    public void writeTrash(TypedStream<I> stream) throws Exception {
        this.writeGarbage(stream.getInput());
    }
}

I.e. parameterize your TypedStream parameter with I.

answered Oct 2, 2013 at 14:14

sp00m

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Comments

Rohit V Jain · Accepted Answer · 2013-10-02 14:30:05Z

0

1.Resolve this by using this code.

 public void writeTrash(TypedStream<I> stream) throws Exception{
            this.writeGarbage(stream.getInput());       
            this.writeGarbage(stream.getInput());   
        }
2.  In Generic class class name is followed by a type parameter section. If you are not    doing this then you have to do casting.

answered Oct 2, 2013 at 14:30

Rohit V Jain

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