code: public JoinOperatorSetsPredicateBase where(String... fields) {
  		return new JoinOperatorSetsPredicateBase(new Keys.ExpressionKeys<>(fields, input1.getType()));
  	}
docs: Continues a Join transformation.
  
  <p>Defines the fields of the first join {@link DataSet} that should be used as grouping keys. Fields
  are the names of member fields of the underlying type of the data set.
  
  @param fields The  fields of the first join DataSets that should be used as keys.
  @return An incomplete Join transformation.
  Call {@link org.apache.flink.api.java.operators.join.JoinOperatorSetsBase.JoinOperatorSetsPredicateBase#equalTo(int...)} or
  {@link org.apache.flink.api.java.operators.join.JoinOperatorSetsBase.JoinOperatorSetsPredicateBase#equalTo(KeySelector)}
  to continue the Join.
  
  @see Tuple
  @see DataSet