Java tutorial
//package com.java2s; import java.util.Collections; import java.util.Set; import java.util.WeakHashMap; import com.google.common.collect.Sets; public class Main { /** * There is currently no class in Java for a weak set, but it can be created as a set wrapping a weak map as in the body of the method below. * @return a weak set */ public static <T> Set<T> createWeakSet() { return Collections.newSetFromMap(new WeakHashMap<T, Boolean>()); } public static <T> Set<T> createWeakSet(Iterable<T> items) { Set<T> set = createWeakSet(); set.addAll(Sets.newHashSet(items)); return set; } }