Polymorphism - Ambiguous error

Question

I asked a similiar question yesterday, but another issue has arisen.

class Cat {
    public void isClawedBy(Cat c, Kitten k){
        System.out.println("Clawed by a cat");
    }
}

class Kitten extends Cat{
    public void isClawedBy(Kitten k, Cat c){
        System.out.println("Clawed by a Kit");
    }
}


Cat g = new Cat();
Cat s = new Kitten();
Kitten t = new Kitten();

g.isClawedBy(s,t);
s.isClawedBy(t,s);
t.isClawedBy(t,t);

The issue i am am confused about is around t.isClawedBy(t,t);. I understand that s.isClawedBy(t,s); will throw an error, because s is of static type cat.

But t.isClawedBy(t,t); is throwing a "The method isClawedBy(Kitten, Cat) is ambiguous for the type Kitten" error. If i change the code to t.isClawedBy(s,t); or t.isClawedBy(t,s); it works, but unsure as to why it doesnt work for (t,t).

Thanks in advance

Answer 1

In Java method calls are resolved dynamically. When you call a method the JVM tries to find a method that matches the signature i.e. method name, parameter types and return types. It does this by looking in the method table of the class used, which will contain the method signatures of super types also.

When checking if a signature in the method table is suitable it will take into account the supertypes of the parameter (and return) types. In the case of t.isClawedBy(t,t) we have two methods that can match as the method defined in Kitten matches and that defined in Cat matches - note that these are different methods as they have different parameter types.

As two different methods match the method call is ambiguous.

For isClawed(s,t) and isClawed(t,s) there is no ambiguity as s is a Kitten and cannot be a Cat.

Answer 2

It is ambiguous because Kittens are Cats.

Thus, given:

Cat    isClawedBy(Cat c, Kitten k)
Kitten isClawedBy(Kitten k, Cat c)

For a Kitten, both methods are available. A Kitten is a Cat, so a call to Cat.isClawedBy(Cat, Kitten) with two Kitten arguments fits the signature. Likewise, a call to Kitten.isClawedBy(Kitten, Cat) with two Kittens also matches the signature.

There is no way for the compiler to tell which method was intended.

Answer 3

It is because you didn't override that method.

For t.isClawedBy(t,t);there are two possible methods to execute.The Cat's isclawed method and the Kitten's isclawed method.

To override a method paremeters must be the same.

Answer 4

kitten extends cat, so kitten gets the atributs and methods of cat, so kitten class knows 2 methods

1. public void isClawedBy(Cat c, Kitten k)
2. public void isClawedBy(Kitten k, Cat c)

But it dosent know a metoth public void isClawedBy(Kitten k, Kitten k) so it dosent know what to do when u call isClawedBy(t,t);