Does instanceof operator generate a lot of overhead ? Why?
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It does generate some overhead, combined with the subsequent casting. With recent version of Java the overhead has decreased. But anyway that's microoptimization - ie you should not worry about it in the general case.
The real argument against instanceof is that in many cases there are better OOP ways to achieve the desired behaviour.
It may or may NOT generate any overhead if the compiler can prove the instance. Even if the compiler can not prove the target immediately the overhead is very little. A few cpu clocks (esp. if instanceof jump is properly predicted).
Following casts after instanceof are usually free.
(note: by the compiler I mean the JIT one)
There is no serious overhead. It's almost certainly cheaper than a home-grown getType()-style solution. Casting, while not free, is also very cheap.
As noted by Bozho it can be indicative of a flawed design, but in some situations it is the most pragmatic choice and so shouldn't be disregarded out of hand.
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