How fast is instanceof?

Question

How expensive is an instanceof operation? Is it in the order of simple arithmetic, or of reflection? Does it vary much?

Answer 1

How expensive is an instanceof operation? Is it in
the order of simple arithmetic, or of reflection?
Does it vary much?Between the two, and yes. It's rarely going to be as slow as reflection, but when you get a "hit" it's practically as fast as arithmetic.
So performance is rarely a reason to avoid "sanity tests" in code (remember, instanceof is essentially a check for != null as well as the type of the object).
Try the following code snippet:
import java.util.*;
public class iof
   public static void main(String[] argv)
      int c = 1000000;
      List content = new ArrayList(c);
      for(int i = 0; i < c; i++ )
         content.add(new String("" + i));
      boolean ignoreState = false;
      Iterator i;
      long start = new Date().getTime();
      i = content.iterator();
      while(i.hasNext())
         Object o = i.next();
      // Instance of (miss)
         ignoreState = (o instanceof List);
      long middle1 = new Date().getTime();
      i = content.iterator();
      while(i.hasNext())
         Object o = i.next();
      // Instance of (hit)
         ignoreState = (o instanceof String);
      long middle2 = new Date().getTime();
      i = content.iterator();
      while(i.hasNext())
         Object o = i.next();
      // Instance of (arithmetic)
         ignoreState = ((2 + 2) > 3);
      long middle3 = new Date().getTime();
      i = content.iterator();
      while(i.hasNext())
         Object o = i.next();
         ignoreState = ( o.getClass().getName().equals(STRINGNAME) );
      long end = new Date().getTime();
      System.out.println("Instanceof (Miss) : " + (middle1 - start) + "ms");
      System.out.println("Instanceof (Hit)  : " + (middle2 - middle1) + "ms");
      System.out.println("Arithmetic        : " + (middle3 - middle2) + "ms");
      System.out.println("Reflection        : " + (end - middle3) + "ms");
   private static String STRINGNAME = "java.lang.String";
}