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;