Combining generic type parameters with interfaces in Java

Question

As with many inheritance problems, I find it hard to explain what I want to do. But a quick (but peculiar) example should do the trick:

public interface Shell{


    public double getSize();

}

public class TortoiseShell implements Shell{
     ...
     public double getSize(){...} //implementing interface method
     ...
     public Tortoise getTortoise(){...} //new method
     ...
}

public class ShellViewer<S extends Shell>{

     S shell;

     public ShellViewer(S shell){
         this.shell = shell;
         ...
     }

}

public class TortoiseShellViewer<T extends TortoiseShell> extends ShellViewer{

    public TortoiseShellViewer(T tShell){
         super(tShell); //no problems here...
    }

    private void removeTortoise(){
        Tortoise t = tShell.getTortoise(); //ERROR: compiler can not find method in "Shell"
        ...
    }
}

The compiler does not recognise that I want to use a specific implementation of Shell for getTortoise(). Where have I gone wrong?

Mostly you've gone wrong by assuming that Java generics are the same as C++'s templates. — Paul Tomblin
– Paul Tomblin, Commented Feb 23, 2011 at 18:55
Might want to remove the huge red herring of getTortoise returning a double in one place and a Tortoise elsewhere. — Mark Peters
– Mark Peters, Commented Feb 23, 2011 at 19:04
Another thing that'd be helpful would be to include all relevant information in the code you're showing... the declaration of the field tShell in ShellViewer is important here but missing. — ColinD
– ColinD, Commented Feb 23, 2011 at 19:10

ColinD · Accepted Answer · 2011-02-23 18:59:55Z

4

Based on what you've given here, the problem is that:

public class TortoiseShellViewer<T extends TortoiseShell> extends ShellViewer

does not specify ShellViewer (which is generic) correctly. It should be:

public class TortoiseShellViewer<T extends TortoiseShell> extends ShellViewer<T>

answered Feb 23, 2011 at 18:59

ColinD

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1 Comment

MattLBeck Over a year ago

Perfect! Thank you. My understanding in generics just increased 10-fold.

Kris Pruden · Accepted Answer · 2011-02-23 19:00:08Z

4

You want this:

public class TortoiseShellViewer<T extends ToroiseShell> extends ShellViewer<T>

answered Feb 23, 2011 at 19:00

Kris Pruden

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Comments

Kevin · Accepted Answer · 2011-02-23 19:04:58Z

0

What is tShell in removeTortoise? Is it an instance of type Shell that is in the base class ShellViewer?

answered Feb 23, 2011 at 19:04

Kevin

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1 Comment

MattLBeck Over a year ago

It should be an instance of TortoiseShell. Or, an instance of generic type T, which extends TotoiseShell. Sorry if it's a bit confsuing to read. My actual code was too bogged down in its own application to use here...

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