Output........?

[Daniel Pearl]

class Mammal{
void eat(Mammal m){
System.out.println("Mammal eats food");
}
}
class Cattle extends Mammal{
void eat(Cattle c){
System.out.println("Cattle eats hay");
}
}

class Horse extends Cattle{
void eat(Horse h){
System.out.println("Horse eats hay");
}
}
public class Test{
public static void main(String[] args){
Mammal h = new Horse();
Cattle c = new Horse();
c.eat(h);
}
}

[Sanjeev Singh]

What confusion you have?

[Gunjan Kumar]

Hi

I have tried this code and got output "Mammal eats food"

But my confusion is
1.c's type is cattle and is referring to instance of Horse.So which eat method should get called and why? what is the funda behind this.

[Sanjeev Singh]

You should be very clear with the concept of inheritance before solving the problem.You can think of class Cattle contains two overloaded version of eat method one takes Mammal object and other takes Cattle object.Since h is an instance of Mammal so the eat method which takes Mammal as an argument will get invoked at runtime.

[Richard Boren]

Sanjeev Singh
...takes Mammal as an argument will get invoked at runtime.

Sanjeev, shouldn�t it be �� takes Mammal as an argument passed at compile time?�

Richard - "With his K&B book open to page 109."

[Raghav Aggarwala]

Originally posted by Sanjeev Singh:
Since h is an instance of Mammal so the eat method which takes Mammal as an argument will get invoked at runtime.

HI Sanjeev,

I tried inserting System.out.println(h.getClass()); after c.eat(h) and it prints class horse.

Since eat method in Horse is an instance method and h is an instance of Horse, shouldn't c.eat(h) should print "Horse eats Hay"?

[Meena R. Krishnan]

Which type's method gets invoked at runtime is different from which type is being passed as the method argument.

Eventhough the horse class's method will be called at runtime, since the argument type is Mammal, it uses the method that takes Mammal argument which is present only in the base class. The following is a slightly modified version of the above code. I added another method that takes no argument. Hope this helps.

class Mammal{
void eat(Mammal m){
System.out.println("Mammal eats food");
}

void eat2(){
System.out.println("Mammal eats2 food");
}
}
 
class Cattle extends Mammal{
void eat(Cattle c){
System.out.println("Cattle eats hay");
}
void eat2(){
System.out.println("Cattle eats2 hay");
}
}

class Horse extends Cattle{
void eat(Horse h){
System.out.println("Horse eats hay");
}
void eat2(){
System.out.println("Horse eats2 hay");
}
}
public class Test{
public static void main(String[] args){
Mammal h = new Horse();
h.eat2(); //1. prints 'Horse eats2 hay'
h.eat(h); //2.prints 'Mammal eats food'
Cattle c = new Horse();
c.eat2();//3.prints 'Horse eats2 hay'
c.eat(c); //4.prints 'Cattle eats hay'
c.eat(h);//5.prints 'Mammal eats food'
}
}

In lines 1 and 3, since there is no method argument and horse class does have a method with no argument, Horse class' method gets called.

One other way to test this is to add a method eat(Mammal m) to the Horse class and see that getting called.