code: public boolean containsKey(Object key) {
          if (key == null) {
              return false;
          }
  
          Entry tab[] = mTable;
          int hash = System.identityHashCode(key);
          int index = (hash & 0x7FFFFFFF) % tab.length;
  
          for (Entry e = tab[index], prev = null; e != null; e = e.mNext) {
              Object entryKey = e.getKey();
  
              if (entryKey == 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 == entryKey) {
                  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.