Polymorphic Method Call

[Mike Cutter]

I am taking a Java class and we are covering polymorphism.

I have to utilize the given code in my assignment:

abstract class A {
    abstract void a1();
    void a2(){}
}

class B extends A {
    void a1() {}
    void a2() {}
}

class C extends B { void c1() {} }

A x = new B();
C y = new C();
A z = new C();

x.a2();

I put all this into one big public class. When I complied it, I get the following error:

<identifier> expected
x.a2();

How do I get this polymorphic method call x.a2(); to compile without errors?

Thanks,

Mike

[Jesper de Jong]

What does the exact code that you are trying to compile look like?

Note that you cannot put statements like x.a2(); at class level - statements have to be inside a method. So, something like this is not going to work:

public class MyClass {
    // ...

    x.a2();   // statement directly at the class level does not work
}

You need to put the statement inside a method, for example the main() method:

public class MyClass {
    // ...

    public static void main(String[] args) {
        // ...

        x.a2();   // statement inside a method works OK
    }
}

[Mike Cutter]

I put the x.a2(); in the main() method.

I now get this error:

non-static variable this cannot be referenced from a static context

[Rob Spoor]

x is a non-static field. To access it from any static method you need an instance of C. So either create a new C and use it's x field, or make x static.

[Mike Cutter]

I tried the following code:

public class MyClass {
    abstract class A {
        abstract void a1();
        void a2() {}
    }

    class B extends A {
        void a1() {}
        void a2() {}
    }

    class C extends B { void c1() {} }

    A x = new B();
    C y = new C();
    A z = new C();

    public static void main(String[] args) {
        x.a2();
        z.a2();
        z.a1();
        x.a1();
    }
}

I get the following compilation errors:

non-static variable x cannot be referenced from a static context
x.a2();
non-static variable z cannot be referenced from a static context
z.a2();
non-static variable z cannot be referenced from a static context
z.a1();
non-static variable x cannot be referenced from a static context
x.a1();

I'll admit I am pretty lost on why the syntax is wrong and the logic is not making sense.

[Rob Spoor]

x, y and z are all declared inside the class and are therefore non-static instance fields. Notice the word "instance" - you need an instance of class MyClass to access them.

There are three ways to solve this:
1) create an instance of MyClass (let's call it "my"), then use that to access them ("my.x.a2()").
2) make them static; simply put the static keyword in front of their declarations.
3) move them inside the main method.