Java generic classes, how to make an instance

Question

I have the following class structure:

public abstract class Generic<T extends SuperClass>

public class SuperGeneric<T extends SuperClass & SomeInterface> 
    extends Generic<T>

Now I want to make an instance of SuperGeneric covering all possible classes. I tried it like this:

Generic<? extends SuperClass & SomeInterface> myGeneric 
    = new SuperGeneric<? extends SuperClass & SomeInterface>();

Now this doesn't seem to work. On Generic it gives the following error: Incorrect number of arguments for type Generic<T>; it cannot be parameterized with arguments <? extends SuperClass, SomeInterface>.

And on the new SuperGeneric I get a similar error: Incorrect number of arguments for type SuperGeneric<T>; it cannot be parameterized with arguments <? extends SuperClass, SomeInterface>.

Any idea how to correctly create instances of this SuperGeneric?

The idea is that I have 2 different classes that satisfy the extends SuperClass & SomeInterface condition but those cannot be generalized by one type.

hage · Accepted Answer · 2012-05-23 12:22:49Z

2

When you want to instantiate a generic class, you need to provide the concrete type. You said there are two classes that fulfill the constraint. Say these are Type1 and Type2.

Then you should be able to do:

Generic<Type1> myGeneric1 = new SuperGeneric<Type1>();

and

Generic<Type2> myGeneric2 = new SuperGeneric<Type2>();

The wildcards are used only for declaration. They mean: You can put any type here (that fulfills the given constraints)

answered May 23, 2012 at 12:22

hage

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2 Comments

Steven Roose Over a year ago

So where are wildcards used then? In method arguments and return-types?

hage Over a year ago

@stevenroose see my last sentence

nsfyn55 · Accepted Answer · 2012-05-23 12:20:14Z

2

When you instantiate you need to provide a type for the compiler to fill in.

answered May 23, 2012 at 12:20

nsfyn55

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5 Comments

Steven Roose Over a year ago

So where are wildcards used then? In method arguments and return-types?

nsfyn55 Over a year ago

The wildcards describe what T can be. But you actually need to provide something for T.

nsfyn55 Over a year ago

You are conflating Class Literals and other Generics features.

nsfyn55 Over a year ago

docs.oracle.com/javase/tutorial/extra/generics/wildcards.html - This is the oracle tutorial

Michael Buen Over a year ago

@nsfyn55 I find Java syntax confusing too. Whereas in C# you put the type constraints on separate clause(in where); in Java you put the type constraint declaration inside the generics declaration (Generic<T extends ClassNameHere>), hence your confusion on how to instantiate it. Having said that, when you instantiate(be it Java or C#), they are very simple, same syntax(i.e. Generic<SomeClass> x = new SuperGeneric<SomeClass>()), see my comparison at anicehumble.com/2012/05/…

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