Java Generics Nested Type Paramaters

Question

I have a class defined like so:
public class AddRecordsToolbar<D extends IDataSource<T>, T extends Serializable> extends AbstractToolbar<D, T>

which my IDE IntelliJ IDEA declares as legal. It looks and feels wrong to me. I want to declare it like this:
public class AddRecordsToolbar<D extends IDataSource<T extends Serializable>, T> extends AbstractToolbar<D, T>
however that syntax is illegal thanks to something to do with Javas type erasure.

D extends IDataSource<T> is required by the superclass.
My Class is using Serializable to do a deep copy. Hence the T extends Serializable.

So now on to the Question: If I specify T extends Serializable as the second type parameter for my class will it still enforce T extends Serializable for D as well?

Damian Leszczyński - Vash · Accepted Answer · 2012-02-02 21:35:33Z

3

Answering to your question, yes its do.

The order of generic parameter it only in your mind.

If we would rephrase that implementation to:

public class AddRecordsToolbar<T extends Serializable, D extends IDataSource<T>> extends AbstractToolbar<D, T>

you will be not so surprised, and looks that the way it should be.

I will try to find the explanation for this in Java Language Specification (when it will work) but for now that the way it is.

edited Feb 2, 2012 at 21:35

answered Feb 2, 2012 at 21:27

Damian Leszczyński - Vash

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Comments

jtoberon · Accepted Answer · 2012-02-02 21:44:33Z

1

I believe that this is addressed in section 8.1.2 of the java language spec:

The scope of a class' type parameter is the entire declaration of the class including the type parameter section itself. Therefore, type parameters can appear as parts of their own bounds, or as bounds of other type parameters declared in the same section.

answered Feb 2, 2012 at 21:44

jtoberon

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PaulMurrayCbr · Accepted Answer · 2012-02-03 02:21:42Z

0

Yes. The "T" in the second parameter is the same T as in the first. Nothing world work, otherwise.

This means that

class IFoo extends IDataSource<String>{};
AddRecordsToolbar<IFoo, Integer> x;

Is illegal. Integer and String are both serialisable, but the declaration of AddRecordsToolbar says that the data source has to be a source of the data type in your second parameter. And that second parameter says that it has to be serializable.

answered Feb 3, 2012 at 2:21

PaulMurrayCbr

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