How to Check if a class extends another

Question

I have seen variants of this question but none of them really address my problem.

Lets say I am building an army with classes. At the top of my Inheritance structure I have an abstract " Unit " class. Then an abstract " Flying ", " Ground " and " Building " class that extends unit. Then a concrete " Helicopter " and " Jet " class that extends Flying. As well as a concrete " Soldier " and " Tank " class that extends Ground. and finally a " HQ " and " Supply " that extends building.

The following is a method under the " Soldier " Class:

public void attack(Unit enemy){
if(enemy.getSuperclass().equals(Flying)){
    System.out.print("Soldiers cant attack flying units");
}
else{
    //Code to attack enemy here
}

I want enemy to be any form of unit. This is because a soldier should be able to attack both buildings and other ground units, However I don't want the Soldiers to be able to attack flying objects.

The obvious problem is that because I declared enemy as a Unit , it doesn't know which subclass it belongs to and therefore is trying to find a SuperClass for Unit which doesn't exist.

I'm sure I could have a getter for every unit which has manually set what type of unit it is... but that is more work and doesn't seem efficient.

Answer 1

Change

if(enemy.getSuperclass().equals(Flying)){

to

if(enemy instanceof Flying){

That will check if enemy is an instance of any of the classes that derive from Flying , rather than checking specifically for the Flying class (which we know enemy won't be, as Flying is abstract).

instanceof is quite handy, but whenever I use it (and I sometimes do), I step back and look at my structure in hopes that I can find some way to refactor to avoid it (perhaps things that can be attacked by soldiers have some common characteristic and could implement an abstract Unit sub-class — GroundBased or something — which you could use for the argument instead of Unit ). You may not be able to in this case, but it's worth double-checking.

Answer 2

Try using instanceof operator.

    if(enemy instanceof Flying){
        System.out.print("Soldiers cant attack flying units");
    }
    else{
        attack(enemy);
    }

Answer 3

You can use instanceof for this. This also works ie for interfaces.

Take also care that the variable may not be null when using instanceof.

class B extends A

   public class test
   {
    public static void main(String[] args)
    {
        A a = new B();
        B b = new B();
        if(a instanceof A)
            System.out.println("B derived from A");
        if(b instanceof A)
            System.out.println("B derived from A");
    }
   }

Answer 4

if (enemy instanceof Flying)
   ...

Answer 5

你试过了吗：

if (enemy instanceof Flying)

Answer 6

I think you are looking for instanceof operator .

The instanceof operator compares an object to a specified type. You can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface.

So you can update you function like following:

if(enemy instanceof Flying){
   System.out.print("Soldiers cant attack flying units");
}
else{
    attack(enemy);
}

Answer 7

尝试if(enemy instanceof Flying) {}

Answer 8

This is a design smell. The fundamental requirement is a job for the Visitor pattern but you should also be looking to use polymorphism better than this before you even get to the Visitor.