CODEWITHSUNDEEP
javatypesinterfacecasting
lukas.coenig
lukas.coenig

Reputation: 551

How to find out if a Type implements an interface in Java?

This seems to be very easy, but I was not able to solve it for days now... I must be too stupid, and so I hope to get some help here (couldn't find the answer though).

I simply want to know if a given Type implements some interface. For example, does type X implement the Serializable interface? It should give true for types X like JComponent.class, and false for Thread.class.

Or, stated differently, can an object of type X be cast to the given interface?

I tried instanceof and isAssignableFrom, but nothing seems to work.

EDIT:

Ok, maybe I really was too stupid :-( I have tried the following:

This code does not compile:

    Type t1 = JComponent.class;
    Type t2 = Thread.class;
    Serializable.class.isAssignableFrom(t1);
    Serializable.class.isAssignableFrom(t2);

This code yields true for both t1 and t2 (which makes sense, of course):

    Type t1 = JComponent.class;
    Type t2 = Thread.class;
    Serializable.class.isAssignableFrom(t1.getClass());
    Serializable.class.isAssignableFrom(t2.getClass());

(And similar things with instanceOf do not work either.)

But what DOES seem to work is this:

    Type t1 = JComponent.class;
    Type t2 = Thread.class;
    Serializable.class.isAssignableFrom((Class<?>) t1);
    Serializable.class.isAssignableFrom((Class<?>) t2);

As a Type that implements an interface should always be a Class (right?), the cast should work in all cases... (EDIT: There is an even simpler solution - I used the wrong method for retreiving the constructor parameters (getGenericParameterTypes); using just getParameterTypes gives a list of Class objects rather than Type objects and there is no problem at all.)

Thank you for the very quick help!

Lukas

Upvotes: 0

Views: 778

Answers (2)

Scary Wombat
Scary Wombat

Reputation: 44824

object instanceof object_interface will yield true

Upvotes: 0

AlexR
AlexR

Reputation: 115328

No, both instanceof and isAssignablefrom() work. Try the following example:

List<String> list = new ArrayList<>(); // it is serializable
System.out.println(list instanceof Serializable); // should print true
System.out.println(Serializable.class.isAssignableFrom(list.getClass())); // should print true too

EDIT

Concerning to the second question. Yes, you can use both techniques to check whether the object can be cast.

Upvotes: 2

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