code: public boolean containsValue(Object value) {
          if (value == null) {
              value = new Null();
          }
  
          Entry tab[] = mTable;
          
          for (int i = tab.length ; i-- > 0 ;) {
              for (Entry e = tab[i], prev = null; e != null; e = e.mNext) {
                  Object entryValue = e.getValue();
  
                  if (entryValue == null) {
                      // Clean up after a cleared Reference.
                      mModCount++;
                      if (prev != null) {
                          prev.mNext = e.mNext;
                      }
                      else {
                          tab[i] = e.mNext;
                      }
                      mCount--;
                  }
                  else if (value.equals(entryValue)) {
                      return true;
                  }
                  else {
                      prev = e;
                  }
              }
          }
  
          return false;
      }
docs: Returns <tt>true</tt> if this map maps one or more keys to the
  specified value.
  
  @param value value whose presence in this map is to be tested.
  @return <tt>true</tt> if this map maps one or more keys to the
  specified value.