Java - Problems with understanding inheritance

Question

I am facing some problems about inheritance in Java. I can't understand why the following two programs have those outputs! Could anyone help me? :)

1)

public class A {
    int foo() {
        return 1;
    }
}

public class B extends A {
    int foo() {
        return 2;
    }
}

public class C extends B {
    int bar(A a) {
        return a.foo();
    }
}

C x = new C();  
System.out.println(x.bar(x));

// OUTPUT:2

2)

public class A {
    int e=1;
}

public class B extends A {
    int e=2;
}

public class C extends B {
    int bar(A a){
        return a.e;
    }
}

C x= new C();
System.out.println(x.bar(x));

// OUTPUT:1

Jake Fairbairn · Accepted Answer · 2016-11-17 14:43:08Z

1

In both cases, you're passing in an object of type C into the print function. The bar function asks for an object of type A, but it's still acceptable for you to pass in an object of type C since it is a subclass of A. So first of all, it's important to keep in mind that a.foo() and a.e are being called on a C object.

So what is happening in both cases is that it's searching for the lowest attribute or method in the list. Here is a very simplified version of what Java is doing in part 1:

Hey, you've passed in an object of type C to the bar method! Now let's call its foo method.
Whoops! C doesn't have a foo method! Let's take the next step up to the B class to see if it has a foo method.
Yay! B has a foo method, so let's call it. No need to work our way up to the A class because we've already found what we need in B.

It's all about understanding that the parameter was downcast from A to C. The exact same sort of logic is used in part 2. It notices that an object of type C was passed in, so it gets the e attribute from object B since its the lowest class in the hierarchy that contains that attribute.

Hopefully that answers your question!

answered Nov 17, 2016 at 14:43

Jake Fairbairn

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6 Comments

Michel_T. Over a year ago

You wrong about example 2 - there output is "1". The class B has its own member e, which is hidden for outside. If B wouldn't have the member e but set a value to it, we'd get an output "2", but it's not so we read a member's value of class A

JJJ Over a year ago

Hm, with your logic the answer in the second program would be 2, wouldn't it?

Michel_T. Over a year ago

@Johanna Is question for me?

JJJ Over a year ago

No, the question is for Jake Fairbaim, but if you can explain it to me better, I don't have a problem :p

Michel_T. Over a year ago

@Johanna I already did. Jake Fairbairn is right about example 1 - there's virtual method works, but in second one a member e from B doesn't overrides member from A. Class it's like a structrure: {members; [table of virtual funcions]}. When an app is executed, and you call a method, JVM looks for it in a table and calls function of a real object, but it can't resolve something like that for a member, so it just gives you e of A.

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