handle different instance variables in method

Question

So if I have a method where a variable can be an instance of a bunch of different classes where only some of them have a specific instance variable, how do I use this instance variable in the method without getting the cannot be resolved or is not a field error?

consider this code:

void method1(){
    SuperType randomInstance = getRandomInstance();
    if(randomInstance.stop == true) //do something
}

where SuperType is a super class to all possible instances that randomInstance can hold.

However, an instance doesn't necessarily have the variable stop so I get an error saying stop cannot be resolved or is not a field

So my question is, is there a way to get around this or would I have to create different methods for different instances depending on if they have the variable stop or not?

score 2 · Accepted Answer · answered Jun 11 '15 at 09:30

If having a stop property can be viewed as a behavior shared by some of the sub-classes of SuperType, you can consider defining an interface - let's call it Stoppable - having methods getStop (or perhaps isStopped if it's a boolean) and setStop.

Then your code can look like :

void method1(){
    SuperType randomInstance = getRandomInstance();
    if (randomInstance instanceof Stoppable) {
        Stoppable sInstance = (Stoppable) randomInstance;
        if(sInstance.getStop() == ...) //do something
    }
}

Aah that was actually exactly what I was looking for, I knew I had read about this somewhere! — PandaDeTapas, Jun 11 '15 at 09:39

score 1 · Answer 2 · answered Jun 11 '15 at 09:29

1

Give the classes in question a common supertype or interface (they seem, from your code, to have one — SuperType), and define the instance field (it's not a "variable") on the supertype or define a getter function on the interface. (Actually, even if the supertype is a class, it's commonly best practice to define the field using a getter anyway, even if you could make it a public or protected instance field.)

answered Jun 11 '15 at 09:29

T.J. Crowder

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Well yes they have a supertype but it is not something that I can change since I am using a framework. But I will just create a mutual interface, it will work fine. – PandaDeTapas Jun 11 '15 at 09:33
@PandaDeTapas: Yup, there you go. Adding an interface of your own with this in it is [Eran's answer](http://stackoverflow.com/a/30776852/157247), I believe. – T.J. Crowder Jun 11 '15 at 09:36

score 1 · Answer 3 · edited May 23 '17 at 12:22

1

If you cannot change your class hiearchy with the introdution of an Interface (Stoppable for example) can resort to reflection to detect if the class has a provate field named stop.

You can find an example of field "listing" from a class here and Field is documented here

edited May 23 '17 at 12:22

Community

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answered Jun 11 '15 at 09:35

Giovanni

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handle different instance variables in method

3 Answers3