Mysterious Java performance

main.cc

#include <iostream>
#include <unordered_map>
#include <unordered_set>
#include <map>
#include <set>
#include <memory>

using namespace std;

int main()
{
  unordered_set<long> s;
  for ( long i=0; i<25000; ++i )
    s.insert(i);

  typedef pair<long,double> Object;
  typedef shared_ptr<Object> Ptr;

  unordered_map<long, Ptr > z1;
  multimap<double, Ptr > z2;
  for ( long i=0; i<25000; ++i ) {
    Ptr p( new Object(i,double(i)) );
    z1.insert( make_pair(i,p ));
    z2.insert( make_pair(double(i),p ));
  }


  for ( int n=0; n<1000; ++n )
  {
    unordered_map<long,Ptr> r1;
    multimap<double, Ptr> r2;

    for ( auto i=z1.begin(); i != z1.end(); ++i )
      if ( s.find( i->first ) != s.end() )
      {
        r1.insert( *i );
        r2.insert( make_pair(i->second->second, i->second ));
      }
  }

  return 0;
}

main.java

import java.util.*;

class Obj {
  public long val;
  public double score;

  public Obj( long v, double s ) { val = v; score = s; }
}

class Test {

    public static void main(String[] args) {

      HashSet<Long> s = new HashSet<Long>();
      for ( long i=0; i<25000; ++i )
        s.add(i);

      HashMap<Long,Obj> z1 = new HashMap<Long,Obj>();
      TreeMap<Double,Obj> z2 = new TreeMap<Double,Obj>();
      for ( long i=0; i<25000; ++i ) {
        Obj p = new Obj( i, i );
        z1.put( i, p );
        z2.put( (double) i, p );
      }

      for ( int n=0; n<1000; ++n )
      {
        loop(s,z1,z2);
      }

      System.gc();
    }

    public static void loop( HashSet<Long> s, HashMap<Long,Obj> z1, TreeMap<Double,Obj> z2 )
    {
      long count = 0;

      HashMap<Long,Obj> r1 = new HashMap<Long,Obj>();
      TreeMap<Double,Obj> r2 = new TreeMap<Double,Obj>();
      for ( Map.Entry<Long,Obj> x : z1.entrySet() )
      {
        if ( s.contains( x.getKey() ) )
        {
          r1.put( x.getKey(), x.getValue() );
          r2.put( x.getValue().score, x.getValue() );
        }
      }

      count += r1.size() + r2.size();
    }
}

Run.txt

$ g++ --version
g++ (SUSE Linux) 4.5.0 20100604 [gcc-4_5-branch revision 160292]

$ g++ -std=c++0x -O2 main.cc
$ time ./a.out

real    0m33.838s
user    0m30.784s
sys     0m2.930s


$ java -version
java version "1.6.0_22"

$ javac main.java
$ time java Test

real    0m17.783s
user    0m17.853s
sys     0m0.318s


WTF?