Small test program for checking the performance impact of an instanceof call (tl;dr: not much)

TestInstanceOf.java

class A {}
class B extends A {}
class C {}

public class TestInstanceOf {
  public static void main(String[] args) {
    final B b = new B();
    final C c = new C();
    int i = 0, nA = 0;

    final long start = System.nanoTime();
    for (; i < 1000000000; ++i) {
      // just in case, try both positive and negative cases
      final Object o = (i & 1) == 0 ? b : c;
      if (o instanceof A) {
        // do something here so the compiler doesn't try to optimize it out
        ++nA;
      }
    }
    System.out.println("Total time in ns: " + (System.nanoTime() - start)/((double)i));

    System.out.println("Number of As (half of instanceof calls): " + nA);
  }
}

And more results, this time on an i5-3470 CPU @ 3.20GHz :

% java -version
java version "1.6.0_27"
OpenJDK Runtime Environment (IcedTea6 1.12.5) (6b27-1.12.5-1ubuntu1)
OpenJDK 64-Bit Server VM (build 20.0-b12, mixed mode)

% java -cp . TestInstanceOf
Total time in ns: 0.863790555
Number of As (half of instanceof calls): 500000000