Java generic collections and inheritance

Question

Basically I need to have an abstract class, that contains a field of a list that details it can hold subclasses of a certain class, and then create a concrete class that stores a specific subclass of that certain class.

Better explained in code I am sure:

public class A {
}

public class B extends A {
}

public abstract class AbsClass {
    protected List<? extends A> list;
}

public class ConClass extends AbsClass {
    list = new ArrayList<B>();
}

With the above code i get the compiler error

The method add(capture#3-of ? extends A) in the type List < capture#3-of ? extends A> is not applicable for the arguments (B)

the line where the list is instantiated.

How can I get this to work?

Regarding your compile error: you're leaving out details. This error has to do with trying to invoke the method list.add, not with your assignment. Your code won't compile for other reasons; namely the assignment occurs outside of a constructor or initializer block. — Mark Peters
– Mark Peters, Commented Mar 29, 2011 at 20:18
@mlaw no, it doesn't. Nowhere is <A> declared! We're left to guess where it is declared. — corsiKa
– corsiKa, Commented Mar 29, 2011 at 20:28
@glow: A is a class, not a type parameter. A bit misleading since it's one character but it's a common fake class name, and it's included in the question. — Mark Peters
– Mark Peters, Commented Mar 29, 2011 at 20:31

Mark Peters · Accepted Answer · 2011-03-29 20:13:14Z

7

Can you just type AbsClass?

public abstract class AbsClass<T extends A> {
    protected List<T> list;
}

public class ConClass extends AbsClass<B> {
    list = new ArrayList<B>();
}

Of course at that point it's not necessary to even move the instantiation to the subclass:

public abstract class AbsClass<T extends A> {
    protected List<T> list = new ArrayList<T>();
}

public class ConClass extends AbsClass<B> {
    //... other stuff ...
}

answered Mar 29, 2011 at 20:13

Mark Peters

81.3k17 gold badges164 silver badges196 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Java generic collections and inheritance

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related