CODEWITHSUNDEEP
javainheritance
user4279125
user4279125

Reputation:

narrow down instanceof to a specific subclass

how do one narrow down instanceof to a specific child without ending up with the parent class .

for example if i should have a parent class StackoverflowUsers then i have direct subclass of TenKUsers,HundredKusers etc etc.. Now my question is how do i use instanceof to get if an Object is an instance of one of the direct subclasses of StackoverflowUsers and not end up with the option of that my object is an instance of StackoverflowUsers .. is that possible?

Upvotes: 2

Views: 440

Answers (3)

slartidan
slartidan

Reputation: 21576

You can exclude the superclass itself with checking .getClass(), but "grandchildren" will also be matched...

You could use reflection, but in general reflection should be avoided (is kind of "cheating").

public class T {

    static class StackoverflowUsers {}

    static class TenKUsers extends StackoverflowUsers {}
    static class HundredKusers extends StackoverflowUsers {}

    static class EtcEtc extends TenKUsers {}

    public static void main(String[] args) {

        System.out.println("Instance of String: "+checkInstance(new String()));
        System.out.println("Instance of StackoverflowUsers (superclass): "+checkInstance(new StackoverflowUsers()));
        System.out.println("Instance of TenKUsers (direct subclass): "+checkInstance(new TenKUsers()));
        System.out.println("Instance of HundredKusers (direct subclass): "+checkInstance(new HundredKusers()));
        System.out.println("Instance of EtcEtc (indirect subclass): "+checkInstance(new EtcEtc()));

        System.out.println();
        System.out.println("using reflection:");
        System.out.println("Instance of String: "+checkInstanceUsingReflection(new String()));
        System.out.println("Instance of StackoverflowUsers (superclass): "+checkInstanceUsingReflection(new StackoverflowUsers()));
        System.out.println("Instance of TenKUsers (direct subclass): "+checkInstanceUsingReflection(new TenKUsers()));
        System.out.println("Instance of HundredKusers (direct subclass): "+checkInstanceUsingReflection(new HundredKusers()));
        System.out.println("Instance of EtcEtc (indirect subclass): "+checkInstanceUsingReflection(new EtcEtc()));

    }

    private static boolean checkInstance(Object o) {
        return (o instanceof StackoverflowUsers) && (o.getClass() != StackoverflowUsers.class);
    }

    private static boolean checkInstanceUsingReflection(Object o) {
        Class<?> superclass = o.getClass().getSuperclass();
        return superclass == StackoverflowUsers.class;
    }
}

which returns:

Instance of String: false
Instance of StackoverflowUsers (superclass): false
Instance of TenKUsers (direct subclass): true
Instance of HundredKusers (direct subclass): true
Instance of EtcEtc (indirect subclass): true

using reflection:
Instance of String: false
Instance of StackoverflowUsers (superclass): false
Instance of TenKUsers (direct subclass): true
Instance of HundredKusers (direct subclass): true
Instance of EtcEtc (indirect subclass): false

Maybe you should rethink your classes. It seems like TenKUsers could also be a boolean flag stored inside of StackoverflowUsers. Or even better: give your StackoverflowUsers an EnumSet<Archievments> and make Archievments contain "TenKUser" and "HundredKusers".

Upvotes: 2

Michael Lang
Michael Lang

Reputation: 3992

In addition to the answer of Mena, if you want to use instanceof to check, you can make your superclass Stackoverflowusers abstract to ensure no instances can exist and then check with:

object instanceof Stackoverflowusers

Upvotes: 0

Mena
Mena

Reputation: 48404

Simply use the instanceof TenKUsers (etc.) idiom.

It will tell you if the object you are looking at is the child of that base class.

Since you imply TenKUsers is a child of StackoverflowUsers, instanceof StackoverflowUsers will also be true for that object, of course.

Upvotes: 4

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