CODEWITHSUNDEEP
javapolymorphismdispatch
user2469515
user2469515

Reputation: 373

Polymorphism - Ambiguous error

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

Upvotes: 2

Views: 194

Answers (4)

Luca8a
Luca8a

Reputation: 1

Kitten extends cat, so kitten gets the attributes 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 doesn't know a method

public void isClawedBy(Kitten k, Kitten k)

so it doesn't know what to do when you call isClawedBy(t, t);

Upvotes: 0

selig
selig

Reputation: 4844

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.

Upvotes: 1

Arthur Dent
Arthur Dent

Reputation: 795

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.

Upvotes: 1

Salih Erikci
Salih Erikci

Reputation: 5087

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.

Upvotes: 0

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