Class must either be declared abstract or implement abstract method toArray

Question

I have a Foo class that extends AbstractList and implements List . This class implements some of the List methods, but some just throw UnsupportedOperationException .

toArray is one of the later and while the compiler doesn't complain about other not really implemented methods, it complains about the toArray with error:

Class must either be declared abstract or implement abstract method toArray(T[]) in List .

public class Foo extends AbstractList implementst List  {
    ...
     public <T> T[] toArray(T[] a) throws UnsupportedOperationException {
        throw new UnsupportedOperationException(error);
    }
}

What is wrong here and why the compiler still thinks the toArray(T[]) method is not implemented?

Answer 1

Since you are using generic method public T[] toArray(T[] a), you should add parameter to class signature and make it extend and implement parameterized class and interface respectively, not the raw ones. Then it will compile:

public class Foo<T> extends AbstractList<T> implements List<T> {

    @Override
    public <E> E[] toArray(E[] a) throws UnsupportedOperationException {
        throw new UnsupportedOperationException("Error!");
    }

    ...
}

Answer 2

This code compiles :

import java.util.AbstractList;
import java.util.List;

public class Foo extends AbstractList implements List  {

    @Override
    public Object[] toArray(Object[] a) {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public Object get(int index) {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public int size() {
        // TODO Auto-generated method stub
        return 0;
    }
}

If you use java.util package.