Java abstract method with generic argument

Question

I have the following situation: There is abstract Class (abs) and two classes which extend from abs (con1 and con2)

Now I want abs to have an abstract function with an argument where the type of the argument has to be the type of the class itself.

I tried the following in the abs class:

public abstract void foo(abs i);

and the following in con1 and con2:

public abstract void foo(con1 i);
public abstract void foo(con1 i);

but this didn't work. What is the general method to solve problems like that?

"this didnt work" is never enough information. Rather than just describing the problem, you should give a short but complete example demonstrating it, and include the exact error message you got. — Jon Skeet
– Jon Skeet, Commented Mar 19, 2015 at 11:32

Jon Skeet · Accepted Answer · 2015-03-19 11:35:25Z

2

No, basically that doesn't implement the method. With the declarations you've got, I should be able to write:

abs x = new con1();
abs y = new con2();
x.foo(y);

It sounds like you want:

public abstract class Abstract<T extends Abstract<T>>
{
    public abstract void foo(T t);
}

Then:

public final class Concrete1 extends Abstract<Concrete1>
{
    @Override public void foo(Concrete1 x) { ... }
}

Note that this doesn't completely enforce it, as you could still write:

public final class Evil extends Abstract<Concrete1>

... but it at least works when everyone's "playing by the rules".

answered Mar 19, 2015 at 11:35

Jon Skeet

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2 Comments

Daniel Ross Over a year ago

Thanks Jon for the answer. You said you should be able to write abs x = new con1(); abs y = new con2(); x.foo(y); where this should be illegal in my setting. And "public final class Concrete1 extends Abstract<Concrete1>" sounds very redundant to me. But if this the only solution I will take that approach. Thank you :)

Jon Skeet Over a year ago

@DanielRoss: With the way you described the code in your question, yes. That wouldn't be the case with the code I've provided.

Daniel Langdon · Accepted Answer · 2015-03-19 11:33:46Z

0

The method definition should be the same on con1 and con2:

@override
public abstract void foo(abs i)
{ ...

Here the annotation check that you are really overriding the parent here. Google "inheritance and polymorphism"

answered Mar 19, 2015 at 11:33

Daniel Langdon

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