Test if object is instanceof a parameter type

Question

Is there a way to determine if an object is an instance of a generic type?

public <T> test(Object obj) {
    if (obj instanceof T) {
        ...
    }
}

That clearly doesn't work. Is there an alternative? Like I want to use Java reflection to instantiate a class and then check to make sure it is of type generic T.

score 150 · Accepted Answer · answered Apr 20 '11 at 18:14

150

The only way you can do this check is if you have the Class object representing the type:

Class<T> type; //maybe passed into the method
if ( type.isInstance(obj) ) {
   //...
}

answered Apr 20 '11 at 18:14

Mark Peters

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If you don't want to get `type` as a method parameter, maybe you want to initialize it: `Class type = ((T) new Object()).getClass();` – JordiVilaplana Jan 14 '16 at 00:18
10

@JordiVilaplana That is not right, it will give you java.lang.Object as the class. Check this out: http://ideone.com/bxt9Jq – Sri Harsha Chilakapati Apr 13 '16 at 08:55
4

Use: `Class type = (Class) ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[0];` – Ton Snoei Feb 02 '18 at 07:58

Puce · Answer 2 · 2017-11-18T20:02:16.673

27

To extend the sample of Mark Peters, often you want to do something like:

Class<T> type; //maybe passed to the method
if ( type.isInstance(obj) ) {
   T t = type.cast(obj);
   // ...
}

edited Nov 18 '17 at 20:02

answered Apr 20 '11 at 18:25

Puce

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score 15 · Answer 3 · answered Feb 02 '18 at 07:58

15

If you don't want to pass Class type as a parameter as mentioned by Mark Peters, you can use the following code. Kudos to David O'Meara.

  Class<T> type = (Class<T>) ((ParameterizedType) getClass().getGenericSuperclass())
                  .getActualTypeArguments()[0];
  if (type.isInstance(obj)) {
      ...
  }

answered Feb 02 '18 at 07:58

Ton Snoei

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7

It gives `Type safety: Unchecked cast from Type to Class` – Marco Sulla Apr 25 '19 at 19:53

Georgios Syngouroglou · Answer 4 · 2017-01-16T20:56:25.650

5

You could try this,

// Cast your object to the generic type.
T data = null;
try {
    data = (T) obj;
} catch (ClassCastException cce) {
    // Log the error.
}

// Check if the cast completed successfully.
if(data != null) {
    // whatever....
}

edited Jan 16 '17 at 20:56

answered Apr 23 '15 at 15:36

Georgios Syngouroglou

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**This does not work!** The cast succeeds for **all** types. Which is why it gives an "Unchecked cast" compile warning. – Lii Feb 02 '18 at 08:47

score 3 · Answer 5 · answered Apr 20 '11 at 18:14

3

It would make more sense to put the restriction on where the type T is used to parametrise the Class type. When you pass the type in, instead of using something like Class<?>, you should use Class<? extends T>.

answered Apr 20 '11 at 18:14

Chris Dennett

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If you want to know the exact type of T at runtime, it is important to require that the caller pass in a Class, not a Class extends T>. For example, Puce's code would not compile with Class extends T>. – Theodore Murdock May 21 '14 at 20:38

score 1 · Answer 6 · answered Apr 20 '11 at 18:16

1

This will only work (partly) if you have an object of type T. Then you can get the class of that object, see java.lang.Class<T> and find if it's the same as the object in question.

But note that this goes counter the very reason we have genrics: using a generic type is a way to say that you don't care what type it really is (up to upper and lower bounds that may be specified).

answered Apr 20 '11 at 18:16

Ingo

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The Class> returned from a .getClass() call on a non-null T instance is not guaranteed to be a Class. The best you can guarantee is that it's a Class extends T>. – Theodore Murdock May 21 '14 at 20:43

Test if object is instanceof a parameter type

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