FastMap and IdentityHashMap

@Grapes(
    @Grab(group='javolution', module='javolution', version='5.5.1')
)
import javolution.util.FastMap // also FastSet, FastList

Map labels1 = new FastMap().shared()
Map labels2 = [:] as IdentityHashMap // unsorted

100.times { i ->
    labels1[String.valueOf(i)] = i*i
    labels2[String.valueOf(i)] = i*i
}

assert labels1 == labels2.sort()

see http://javolution.org/target/site/apidocs/javolution/util/FastMap.html