public void setCombinable(boolean combinable) {
		// sanity check
		if (combinable && !GroupCombineFunction.class.isAssignableFrom(this.userFunction.getUserCodeClass())) {
			throw new IllegalArgumentException("Cannot set a UDF as combinable if it does not implement the interface " +
					GroupCombineFunction.class.getName());
		} else {
			this.combinable = combinable;
		}
	}
public void setCombinable ( boolean combinable ) { if ( combinable && ! GroupCombineFunction . class . isAssignableFrom ( this . userFunction . getUserCodeClass ( ) ) ) { throw new IllegalArgumentException ( "Cannot set a UDF as combinable if it does not implement the interface " + GroupCombineFunction . class . getName ( ) ) ; } else { this . combinable = combinable ; } }
Marks the group reduce operation as combinable. Combinable operations may pre-reduce the
data before the actual group reduce operations. Combinable user-defined functions
must implement the interface {@link GroupCombineFunction}.

@param combinable Flag to mark the group reduce operation as combinable.