code: public <KEY> Where<KEY> where(KeySelector<T1, KEY> keySelector)  {
  		Preconditions.checkNotNull(keySelector);
  		final TypeInformation<KEY> keyType = TypeExtractor.getKeySelectorTypes(keySelector, input1.getType());
  		return where(keySelector, keyType);
  	}
docs: Specifies a {@link KeySelector} for elements from the first input.
  
  @param keySelector The KeySelector to be used for extracting the first input's key for partitioning.