Java Base Object Multi Type Variable

Question

I'm sure this is incredibly common with as OOP centered as Java is. In java is there a way to make a base type variable that accepts all inherited subtypes? Like if I have;

class Mammal {...}
class Dog extends Mammal {...}
class Cat extends Mammal {...}

class ABC {
    private Mammal x;

    ABC() {
        this.x = new Dog();
        -or-
        this.x = new Cat();
    }
}

I need the variable to be able to accept any extended version too, but not in specific one extended kind.

There are some ways that I know, but don't want to use. I could make an attribute for each subtype, then only have the one attribute actually used. Make an array and shove it in there.

Any other ideas or a way to get a "base class" type variable?

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Ok since I know using polymorphic duck typing isn't a great idea in Java, but since I don't think I can avoid it. Is the only way to use subclass methods dynamically to re assign a casted version of the varible ie, I get an error with this;

Mammal x;
x = new Dog();
System.out.println(x.getClass());
x.breath();
if (x instanceof Dog) {
  x.bark();
} else if (x instanceof Cat) {
  x.meow();
}

Saying symbol not found, however this works;

Mammal x;
x = new Dog();
System.out.println(x.getClass());
x.breath();
if (x instanceof Dog) {
  Dog d = (Dog) x;
  d.bark();
} else if (x instanceof Cat) {
  Cat c = (Cat) x;
  c.meow();
}

That last one the only way to do it?

Answer 1

If you have the following:

class Mammal {...}
class Dog extends Mammal {...}
class Cat extends Mammal {...}

Then Dog is a subtype of Mammal. Cat is also a subtype of Mammal. This type polymorphism does in fact allow you to do the following:

Mammal x;
x = new Dog(); // fine!
x = new Cat(); // also fine!

If in fact later there's the following:

class Platypus extends Mammal {...} // yes it's true!

Then you can also do:

x = new Platypus(); // fine!

This polymorphic subtyping relationship is one of the basic tenets of object-oriented programming.

See also

• Java Tutorials/Object-Oriented Programming Concepts
• Wikipedia/Polymorphism in object-oriented programming

  Subtype polymorphism, almost universally called just polymorphism in the context of object-oriented programming, is the ability of one type, A, to appear as and be used like another type, B

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On instanceof type comparison operator

Suppose we have the following:

class Mammal { void lactate(); }
class Dog extends Mammal { void bark(); }
class Cat extends Mammal { void meow(); }

Then you can use instanceof type comparison operator (§15.20.2) to do something like this:

Mammal x = ...;

if (x instanceof Dog) {
   Dog d = (Dog) x;
   d.bark();
} else if (x instanceof Cat) {
   Cat c = (Cat) x;
   c.meow();
}
if (x != null) {
   x.lactate();
}

There are also ways to do this without if-else; this is just given as a simple example.

Note that with appropriate design, you may be able to avoid some of these kinds of subtype differentiation logic. If Mammal has a makeSomeNoise() method, for example, you can simply call x.makeSomeNoise().

Related questions

• When should I use the Visitor Design Pattern? - sometimes used to simulate double dispatch

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On reflection

If you must deal with new types not known at compile-time, then you can resort to reflection. Note that for general applications, there are almost always much better alternatives than reflection.

See also

• Java Technical Articles/Advanced Language Topics/Reflection
• Effective Java 2nd Edition, Item 53: Prefer interfaces to reflection