code: public void setWorkingSet(Set<K> workingSet) {
          synchronized(workingSet) {
              for (K key : workingSet) {
                  this.get(key);
              }
          }
      }
docs: Sets the current working set, ensuring that elements in this working set
  will not be ejected in the near future.
  
  @param workingSet set of K that makes up the current working set.