Java isInstance vs instanceOf operator

Question

The whole generics thing is kinda throwing me for a loop, and more so the RTT.

Specificis? Ah well here's the gist:

enum QueryHelper {
  query1,
  query2;
  static <T> QueryHelper getQueryHelper (Class<T> expectedReturn) {
    if (expectedReturn.isInstance (SomeRelatedClass.class))
      return query1;
    else
      return query2;
  }
}

and then I would call it like so:

...
QueryHelper helper = QueryHelper.getQueryHelper(SomeRelatedClass.class);
...

This is so that I can really flexibly assign the query return type in the actual helper. It does some casting and object creation. What I am seeing is that there is no match, should I be doing this some other way? Or is the whole idea just bad?

And the real heart of this is that I don't understand the difference between class.isInstance and the instanceOf operator? Should I be using the latter?

Answer 1

This is so that I can really flexibly assign the query return type in the actual helper.

There is nothing flexible about the return type of this method

static <T> QueryHelper getQueryHelper (Class<T> expectedReturn) {
    if (expectedReturn.isInstance (SomeRelatedClass.class))
      return query1;
    else
      return query2;
}

It will always return an instance of QueryHelper. If you want the return type to be flexible you would need to define it as something like:

static <T> T getQueryHelper (Class<T> expectedReturn) {
}

Now the return type is flexible, because it will depend on the type of the argument

And the real heart of this is that I don't understand the difference between class.isInstance and the instanceOf operator?

The difference is that instanceof does a type-check that is fixed at compile-time, for example:

static boolean isInstance(Object myVar) {
    return (myVar instanceof Foo);
}

will always check that myVar is an instance of Foo, whereas

static <T> boolean isInstance(Object myVar, Class<T> expectedType) {
    return expectedType.isInstance(myVar);
}

will check that myVar is an instance of expectedType, but expectedType can be a different type each time the method is called

Answer 2

Class.isInstance() doesn't work like your code expects. It tests whether the object you pass to it is an instance of the class. In you code:

expectedReturn.isInstance(SomeRelatedClass.class)

The object you're passing is a Class object. Try this instead, which returns true:

Class.class.isInstance(SomeRelatedClass.class);

What you're probably looking for is Class.isAssignableFrom(), e.g.:

Object.class.isAssignableFrom(Class.class);

Means you can do this:

Class klass = ...;
Object o = klass;

Answer 3

The expected argument of isInstance is an object that may be an Instance of the class that your class object represents. What you're comparing it to is an instance of the class... java.lang.Class! So it's not going to match.

e.g., would be true:

Class.class.isInstance(SomeRelatedClass.class);

Also would be true (without architectural commentary on the sanity of actually building your query helper this way)

expectedReturn.isInstance(new SomeRelatedClass());