Find specific object in an arraylist

Question

Currently have an issue getting a specific object in an arraylist. So I have multiple classes that implements the same interface, and I create objects of the different classes. The problem is that I don't know how to differentiate the classes in the arraylist.

ArrayList<Interface> arraylist = new ArrayList<>();

public static void main(String[] args) {

    addInterface(new interfaceA());
    addInterface(new interfaceB());
    addInterface(new interfaceC());

}

public static void addInterface(Interface foo) {
    arraylist.add(foo);
}

Let say that I want to get interfaceA(), I could call it by arraylist.get(0) but I don't want to hardcode it. Each class has the same methods but the code is different.

Lot of problems with the above code: 1. interface is a reserved keyword and can not be used as a variable Interface interface 2. You can not instantiate an interface new interfaceA(). — Aniket Sahrawat
– Aniket Sahrawat, Commented Apr 13, 2018 at 13:33
One, consider a design pattern, such as a Chain of Responsibility and remove the specific attempt to retrieve it; allow each implementation to decide it wants to handle the request or not. If not possible, what are the criteria by which you will select? Perhaps a Map with the appropriate key. Finally, consider looking at this question Type Handler Map which, leaving aside the generics and discussion on Exceptins, shows something similar, which is a registry. — KevinO
– KevinO, Commented Apr 13, 2018 at 13:37
If all those classes have overridden equals correct you can just use the methods indexOf or lastIndexOf to find the indexes of specific object — user6073886
– user6073886, Commented Apr 13, 2018 at 13:38
@JayLee that's not what I asked. If you care about this difference in implementation, why are you putting them together in the same list? — Tim
– Tim, Commented Apr 13, 2018 at 13:42

OldCurmudgeon · Accepted Answer · 2018-04-13 13:52:54Z

2

I would use a Map instead of a List. In this case an IdentityHashMap is a good fit.

interface Thing {

}

IdentityHashMap<Class<? extends Thing>, Thing> things = new IdentityHashMap<>();

class ThingA implements Thing {
    @Override
    public String toString() {
        return "ThingA{}";
    }
}

class ThingB implements Thing {
    @Override
    public String toString() {
        return "ThingB{}";
    }
}

class ThingC implements Thing {
    @Override
    public String toString() {
        return "ThingC{}";
    }
}

public void registerThing(Thing thing) {
    things.put(thing.getClass(), thing);
}

public void test(String[] args) {
    registerThing(new ThingA());
    registerThing(new ThingB());
    registerThing(new ThingC());

    System.out.println(things.get(ThingB.class));
}

answered Apr 13, 2018 at 13:52

OldCurmudgeon

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ernest_k · Accepted Answer · 2018-04-13 13:39:35Z

2

You could filter using a predicate, by checking runtime classes:

List<Interface> interfaceAList = arraylist.stream()
                         .filter(e -> InterfaceA.class.isInstance(e))
                         .collect(Collectors.toList());

answered Apr 13, 2018 at 13:39

ernest_k

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Arya · Accepted Answer · 2018-04-13 13:47:48Z

0

    public Interface getInterfaceA(List<Interface> interfaces) {
    for (Interface i : interfaces) {
        if (i instanceof InterfaceA)
            return i;
    }
    return null;
    }

answered Apr 13, 2018 at 13:47

Arya

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1 Comment

Riyafa Abdul Hameed Over a year ago

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