Confusion in Java Generics

Question

I'm beginner in Java, I'm trying to compile some small program, can somebody explain me, what is my problem, thanks in advance:

public abstract class SumFunction<Y,X> {
public abstract Y op (Y y, X x);
}

public class sumStringToInt  extends SumFunction{
        public int op(int num, String s){
            return s.length() + num;
        }
}

errors

Multiple markers at this line
    - The type sumStringToInt must implement the inherited abstract method SumFunction.op(Object, 
     Object)
    - SumFunction is a raw type. References to generic type SumFunction<Y,X> should be 
     parameterized

edited

is it possible in Java inherit without instantiaion of Base class?, thanks in advance

Answer 1

You are indicating that SumFunction<Y,X> is taking two arguments.

Thus your class should look like this:

public class sumStringToInt extends SumFunction<Integer,String> {
    public Integer op(Integer num, String s){
        return s.length() + num;
    }
}

Try that...

Answer 2

Should be

public class sumStringToInt extends SumFunction<Integer,String>{
        public Integer op(Integer num, String s){
            return s.length() + num;
        }
}

Answer 3

You haven't specified the type parameters when extending SumFucntion. sumStringToInt needs to extend SumFunction<Integer,String>.

Also, you can't use primitives in generics. So, use Integer instead of int.

Answer 4

public class sumStringToInt extends SumFunction<Integer, String>
{
    @Override
    public Integer op(int num, String s)
    {
        return s.length() + num;
    }
}

Answer 5

You're missing the type parameters for SumFunction. Because of that the compiler assumes you want to use the raw type which means Y and X use Object as their type.

public class sumStringToInt  extends SumFunction{
        public int op(int num, String s){
            return s.length() + num;
        }
}

If you pass the types you want to use for SumFunction, this should compile. Like so:

public class sumStringToInt extends SumFunction<Integer, String> {
        public int op(int num, String s){
            return s.length() + num;
        }
}

You may have noticed that we use Integer instead of int as the type. This is because you cannot use primitive types with generics. Luckily, there are wrapper Objects for each primitive type, in this case Integer for int.