CODEWITHSUNDEEP
javaoverriding
overexchange
overexchange

Reputation: 1

How 'toString' get resolved here?

Below interface I implicitly has the public methods of class Object as abstract members. One of them is toString()

interface I{ 
   void test(int i);
   //implicitly has abstract members matching every public method of 'class Object'
   // one of them is toString()
}

Below class W inherits class Object public methods, one of them is toString,

class W implements I{
    public static void main(String[] args){
       I w = new W();
       w.toString();   //How toString() method is resolved here?
    }
}

As both, super interface(I) and super class(Object) of class W have toString() method,

How does javac resolve w.toString() at compile time? I learnt that invokevirtual instruction is used here.

How does jvm resolve w.toString() at runtime?

Upvotes: 1

Views: 122

Answers (4)

Sotirios Delimanolis
Sotirios Delimanolis

Reputation: 280174

Because the method is invoked on an expression of type I, an interface type, the bytecode will contain an invokeinterface instruction for the corresponding method.

At runtime, the method invoked will be determined as such

Let C be the class of objectref. The actual method to be invoked is selected by the following lookup procedure:

  • If C contains a declaration for an instance method with the same name and descriptor as the resolved method, then it is the method to be invoked.
  • Otherwise, if C has a superclass, a search for a declaration of an instance method with the same name and descriptor as the resolved method is performed, starting with the direct superclass of C and continuing with the direct superclass of that class, and so forth, until a match is found or no further superclasses exist. If a match is found, then it is the method to be invoked.
  • Otherwise, if there is exactly one maximally-specific method (§5.4.3.3) in the superinterfaces of C that matches the resolved method's name and descriptor and is not abstract, then it is the method to be invoked.

In your case, objectref is a reference to an object of type W. W does not contain a declaration for an instance method with the same name and descriptor as the resolved method. We therefore check the superclass of W, Object. Object does have such a method. That method is therefore invoked.

Upvotes: 3

Ankush soni
Ankush soni

Reputation: 1459

Interface doesn't have any super class not even the Object class e.g.

public class Main {
  public static void main(String[] argv) throws Exception {

    Class cls = java.lang.Cloneable.class;
    Class sup = cls.getSuperclass(); // will return null
  }
}

Hence interface never override a method of an Object class.

When an interface has no direct SuperInterface, it will create abstract public method for all those public methods present in the Object class.

Upvotes: -2

Aditya Singh
Aditya Singh

Reputation: 2453

Interfaces don't override methods from any class. When you use the toString() method, the default toString() is executed from the Object class because your class W extends from Object. For more clarification, override the toString() method in your class W. 

@Override
public String toString() {

    return "Yo, baby! M here!";
}

Then the output will be: 

Yo, baby! M here!

Upvotes: 0

Davide Lorenzo MARINO
Davide Lorenzo MARINO

Reputation: 26946

An interface doesn't override a method.

Also your class doesn't override the method toString(). You have to rewrite the method toString() to overerride it.

So simply it is called the implementation of toString() of the superclass Object.

Upvotes: 1

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