The performance impact of using instanceof in Java

Question

I am working on an application and one design approach involves extremely heavy use of the instanceof operator. While I know that OO design generally tries to avoid using instanceof, that is a different story and this question is purely related to performance. I was wondering if there is any performance impact? Is is just as fast as ==?

For example, I have a base class with 10 subclasses. In a single function that takes the base class, I do checks for if the class is an instance of the subclass and carry out some routine.

One of the other ways I thought of solving it was to use a "type id" integer primitive instead, and use a bitmask to represent categories of the subclasses, and then just do a bit mask comparison of the subclasses "type id" to a constant mask representing the category.

Is instanceof somehow optimized by the JVM to be faster than that? I want to stick to Java but the performance of the app is critical. It would be cool if someone that has been down this road before could offer some advice. Am I nitpicking too much or focusing on the wrong thing to optimize?

Answer 1

Instanceof is very fast. It boils down to a bytecode that is used for class reference comparison. Try a few million instanceofs in a loop and see for yourself.

Answer 2

Generally the reason why the "instanceof" operator is frowned upon in a case like that (where the instanceof is checking for subclasses of this base class) is because what you should be doing is moving the operations into a method and overridding it for the appropriate subclasses. For instance, if you have:

if (o instanceof Class1)
   doThis();
else if (o instanceof Class2)
   doThat();
//...

You can replace that with

o.doEverything();

and then have the implementation of "doEverything()" in Class1 call "doThis()", and in Class2 call "doThat()", and so on.

Answer 3

I'll get back to you on instanceof performance. But a way to avoid problem (or lack thereof) altogether would be to create a parent interface to all the subclasses on which you need to do instanceof. The interface will be a super set of all the methods in sub-classes for which you need to do instanceof check. Where a method does not apply to a specific sub-class, simply provide a dummy implementation of this method. If I didn't misunderstand the issue, this is how I've gotten around the problem in the past.

Answer 4

You should measure/profile if it's really a performance issue in your project. If it is I'd recommend a redesign - if possible. I'm pretty sure you can't beat the platform's native implementation (written in C). You should also consider the multiple inheritance in this case.

You should tell more about the problem, maybe you could use an associative store, e.g. a Map<Class, Object> if you are only interested in the concrete types.

Answer 5

It's hard to say how a certain JVM implements instance of, but in most cases, Objects are comparable to structs and classes are as well and every object struct has a pointer to the the class struct it is an instance of. So actually instanceof for

if (o instanceof java.lang.String)

might be as fast as the following C code

if (objectStruct->iAmInstanceOf == &java_lang_String_class)

assuming a JIT compiler is in place and does a decent job.

Considering that this is only accessing a pointer, getting a pointer at a certain offset the pointer points to and comparing this to another pointer (which is basically the same as testing to 32 bit numbers being equal), I'd say the operation can actually be very fast.

It doesn't have to, though, it depends a lot on the JVM. However, if this would turn out to be the bottleneck operation in your code, I'd consider the JVM implementation rather poor. Even one that has no JIT compiler and only interprets code should be able to make an instanceof test in virtually no time.