public Builder(ReduceOp defaultOp){
  this.defaultOp=defaultOp;
}

public Builder ( ReduceOp defaultOp ) { this . defaultOp = defaultOp ; }
create a reducer builder , and set the default column reduction operation . for any columns that aren ' t specified explicitly , they will use the default reduction operation . if a column does have a reduction operation explicitly specified , then it will override the default specified here .
