Re: performance of HashSet with strings and integers

Kevin McMurtrie <>
Wed, 14 Oct 2009 22:45:53 -0700
In article
 Frederik <> wrote:


I thought of replacing strings with integers in some code I wrote,
because I believed it would benefit performance. But before doing so,
I made a small test class:

public class testStringSetVsIntSet {
    public static void main(String[] args) {
        long time;
        boolean b;
        Set set;

        Integer I1 = new Integer(100), I2 = new Integer(500);

        set = new HashSet();

        time = System.currentTimeMillis();
        for (int i=0; i<50000000; i++) {
            b = set.contains(I1);
            b = set.contains(I2);

        time = System.currentTimeMillis() - time;
        System.out.println("Time 1: " + time);

        String headStr = "Head";
        String agentStr = "Agent";
        String qualifStr = "Qualif";

        set = new HashSet();

        time = System.currentTimeMillis();
        for (int i=0; i<50000000; i++) {
            b = set.contains(headStr);
            b = set.contains(qualifStr);

        time = System.currentTimeMillis() - time;
        System.out.println("Time 2: " + time);

But to my surprise, the second loop with the strings appeared to be
twice as fast as the first one with the integers! (first loop 3
seconds, second 1.5 seconds)
I didn't expect this because calculating the hashcode for a string is
normally slower than for an integer (every string character is taken
into account) and I thought the "equals" method for a string should be
slower than for an Integer as well.
Can anybody explain this to me?

A few things:

- Strings cache their hash value in recent JVMs.
- The performance of hashing varies for individual keys based on
collisions and locations within the collision chain.
- All of your map hits can use the fast '==' test rather than equals(),
making collisions an unusually large factor of performance.
- Hotspot was compiling during loop 1. Call the test method twice and
the second results will probably be different.

nitpick: Integer.valueOf(n) and autoboxing returns pooled objects for a
range of values so it's sometimes much more efficient than the Integer
constructor. It's not in the loop so it doesn't matter here.
I won't see Goolge Groups replies because I must filter them as spam

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Putin Lights Menorah