JDK8HashSet.java

package com.dbobrov.benchmark;

import org.openjdk.jmh.annotations.*;

import java.util.HashSet;
import java.util.Set;
import java.util.concurrent.TimeUnit;

@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.MICROSECONDS)
public class JDK8HashSet {

    private static class Simple {
        private final int hc;

        public Simple(int hc) {
            this.hc = hc;
        }

        @Override
        public int hashCode() {
            return hc;
        }

        @Override
        public boolean equals(Object obj) {
            return this == obj;
        }
    }

    private static class Ordered extends Simple implements Comparable<Ordered> {
        private final int c;

        public Ordered(int hc, int c) {
            super(hc);
            this.c = c;
        }

        @Override
        public int compareTo(Ordered o) {
            return Integer.compare(c, o.c);
        }
    }

    @GenerateMicroBenchmark
    public Set<Simple> measureSimple() {
        Set<Simple> set = new HashSet<>();
        for (int i = 0; i < 100; i++) {
            set.add(new Simple(1));
        }
        return set;
    }

    @GenerateMicroBenchmark
    public Set<Ordered> measureOrdered() {
        Set<Ordered> set = new HashSet<>();
        for (int i = 0; i < 100; i++) {
            set.add(new Ordered(1, i));
        }
        return set;
    }

    @GenerateMicroBenchmark
    public Set<Simple> measurePartiallyOrdered1() {
        Set<Simple> set = new HashSet<>();
        for (int i = 0; i < 100; i++) {
            if (i % 2 == 0) {
                set.add(new Simple(1));
            } else {
                set.add(new Ordered(1, i));
            }
        }
        return set;
    }

    @GenerateMicroBenchmark
    public Set<Simple> measurePartiallyOrdered2() {
        Set<Simple> set = new HashSet<>();
        for (int i = 0; i < 100; i++) {
            if (i % 4 == 0) {
                set.add(new Simple(1));
            } else {
                set.add(new Ordered(1, i));
            }
        }
        return set;
    }


}

results.txt

c.d.b.JDK8HashSet.measureOrdered               avgt   1        10    1       21.373        1.085    us/op
c.d.b.JDK8HashSet.measurePartiallyOrdered1     avgt   1        10    1      100.075        0.586    us/op
c.d.b.JDK8HashSet.measurePartiallyOrdered2     avgt   1        10    1       72.480        0.221    us/op
c.d.b.JDK8HashSet.measureSimple                avgt   1        10    1      112.671        0.755    us/op

Would be even faster if you don't pay the upfront cost of System.iHC():

    private static class Ordered ... {
        private int id;

        public Ordered(int hc, int id) {
            ...
            this.id = id;
        }

        public int compareTo(Ordered o) {
            return Integer.compare(id, o.id);
        }
    }

    ...

    new Ordered(1, i);