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Q: What other operations are surprisingly slow?

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Q: What other operations are surprisingly slow?

Answer: Where do I begin? Here are a few that are most useful to know about. I wrote a timing utility that runs snippets of code in a loop, reporting the results in terms of thousands of iterations per second (K/sec) and microseconds per iteration (uSecs). Timing was done on a Sparc 20 with the JDK 1.1.4 JIT compiler. I note the following:
  • These were all done in 1998. Compilers have changed since then.
  • Counting down (i.e. for (int i=n; i>0; i--)) is twice as fast as counting up: my machine can count down to 144 million in a second, but up to only 72 million.
  • Calling Math.max(a,b) is 7 times slower than (a > b) ? a : b. This is the cost of a method call.
  • Arrays are 15 to 30 times faster than Vectors. Hashtables are 2/3 as fast as Vectors.
  • Using bitset.get(i) is 60 times slower than bits & 1 << i. This is the cost of a synchronized method call, mostly. Of course, if you want more than 64 bits, you can't use my bit-twiddling example. Here's a chart of times for getting and setting elements of various data structures:

      K/sec     uSecs          Code           Operation 
    =========  ======= ====================  ===========
      147,058    0.007 a = a & 0x100;        get element of int bits
          314    3.180 bitset.get(3);        get element of Bitset
       20,000    0.050 obj = objs[1];        get element of Array
        5,263    0.190 str.charAt(5);        get element of String
          361    2.770 buf.charAt(5);        get element of StringBuffer
          337    2.960 objs2.elementAt(1);   get element of Vector
          241    4.140 hash.get("a");        get element of Hashtable
    
          336    2.970 bitset.set(3);        set element of Bitset
        5,555    0.180 objs[1] = obj;        set element of Array
          355    2.810 buf.setCharAt(5,' ')  set element of StringBuffer
          308    3.240 objs2.setElementAt(1  set element of Vector
          237    4.210 hash.put("a", obj);   set element of Hashtable

  • Java compilers are very poor at lifting constant expressions out of loops. The C/Java for loop is a bad abstraction, because it encourages re-computation of the end value in the most typical case. So for(int i=0; i is three times slower than int len = str.length(); for(int i=0; i


This tip is reprinted on JavaFAQ.nu by by courtesy of Peter Norvig I am thankful for his important contributions to my site - 21 Infrequently Answered Java Questions. Alexandre Patchine


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