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.

Answer 1

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) ) {
   //...
}

Answer 2

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);
   // ...
}

Answer 3

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....
}

Answer 4

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)) {
      ...
  }

Answer 5

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>.

Answer 6

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).