code: public boolean containsKey(Object key) {
          Entry tab[] = mTable;
  
          if (key != null) {
              int hash = key.hashCode();
              int index = (hash & 0x7FFFFFFF) % tab.length;
              for (Entry e = tab[index], prev = null; e != null; e = e.mNext) {
                  if (e.getValue() == null) {
                      // Clean up after a cleared Reference.
                      mModCount++;
                      if (prev != null) {
                          prev.mNext = e.mNext;
                      }
                      else {
                          tab[index] = e.mNext;
                      }
                      mCount--;
                  }
                  else if (e.mHash == hash && key.equals(e.mKey)) {
                      return true;
                  }
                  else {
                      prev = e;
                  }
              }
          }
          else {
              for (Entry e = tab[0], prev = null; e != null; e = e.mNext) {
                  if (e.getValue() == null) {
                      // Clean up after a cleared Reference.
                      mModCount++;
                      if (prev != null) {
                          prev.mNext = e.mNext;
                      }
                      else {
                          tab[0] = e.mNext;
                      }
                      mCount--;
                  }
                  else if (e.mKey == null) {
                      return true;
                  }
                  else {
                      prev = e;
                  }
              }
          }
  
          return false;
      }
docs: Returns <tt>true</tt> if this map contains a mapping for the specified
  key.
  
  @return <tt>true</tt> if this map contains a mapping for the specified
  key.
  @param key key whose presence in this Map is to be tested.