In java, how to call the constructor of the instance creator of the current class ?

Question

I have a class say 'A', an object 'a' of which creates an object 'b' of class'B'. Now 'b' runs a method which wants to create a new instance of b's parent(please dont confuse it with inheritance) - the class which created 'b', which is 'A' as in here.

Is this anyhow possible in Java ? Or maybe some design suggestion may work.

Thanks !

Edit : As per suggestions, I am adding my design requirement too. I have an "ExitController" class, which works differently as per the caller function - like if it is invoked by a UserX, it terminates program, if it is invoked by a UserY it logs him off, etc. Typically I should be placing an ExitController method in my User interface, but there are some constraints due to which I cant. Also, passing the parent object/classname is seeming very expensive compared to the use it offers.

Answer 1

Basically, you've to pass the parent itself or the parent type into B's constructor. The below example illustrates the last case.

class A {
    B b;

    A() { 
        b = new B(getClass());
    }
}

class B {
    Class<?> parentClass;

    B(Class<?> parentClass) {
        this.parentClass = parentClass;
    }

    void createParent() throws Exception {
        Object o = parentClass.newInstance();
    }
}

With generics you could make it more type safe. But since the functional requirement is not entirely clear, I've left that away.

---

Update as per your update, creating instances is pointless. You could just check the class type and handle accordingly. Instead of createParent() in above example, do:

void exit() {
    if (parentClass == UserX.class) {
        // Terminate program.
    } else if (parentClass == UserY.class) {
        // Logoff user.
    }
}

This is however plain ugly. Are you really not allowed to add another method to the User interface? Otherwise the following would be a better design (called the Strategy pattern):

interface User {
    void exit();
}

class ExitController {
    void exit(User user) {
        user.exit();
    }
}

class UserX implements User {
    public void exit() {
        // Terminate program.
    }
}

class UserY implements User {
    public void exit() {
        // Logoff user.
    }
}

---

As per your last statement:

Also, passing the parent object/classname is seeming very expensive compared to the use it offers

This makes no sense. Have you measured it? Show us the results.

Answer 2

If I understand your question correctly, I think that class B would have to store a reference to either object a or A.class. That would allow b to construct a new instance of A. The simplest way to to do this would be for B's constructor to take a class argument.