Re: performance of HashSet with strings and integers
Frederik wrote:
Hi,
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();
set.add(I1);
set.add(900);
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();
set.add(headStr);
set.add(agentStr);
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?
1. Since Strings are immutable, their hash code is calculated only
once and then stored in the String object.
2. Since your strings are of different lengths, they're discovered to
be unequal almost immediately (the second thing equals() checks is the
number of characters).
3. When contains() returns true, its parameter is nott just equal to
the one in the set but the same object; thus it's discovered to be
equal even faster (since the first thing equals() checks is "this ==
param").
4.This one is just a guess, but I wouldn't be surprised if the Integer
version became faster if you reversed the order. In general, Java
gets faster as it goes and the JIT can optimize the code.
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