import org.openrdf.query.algebra.GroupElem; //导入依赖的package包/类
/**
 * Make an instance of {@link AggregationsEvaluator} based on a {@link Group} node.
 *
 * @param aggStateStore - The mechanism for storing aggregation state. (not null)
 * @param aggNode - Defines which aggregation functions need to be performed.
 * @param groupByVars - The names of the binding whose values are used to group aggregation results. (not null)
 * @return The evaluator that handles the node's aggregations.
 */
public static  AggregationsEvaluator make(final AggregationStateStore aggStateStore, final Group aggNode, final List<String> groupByVars) {
    requireNonNull(aggStateStore);
    requireNonNull(aggNode);
    requireNonNull(groupByVars);

    // The aggregations that need to be performed are the Group Elements.
    final List<AggregationElement> aggregations = new ArrayList<>();
    for(final GroupElem groupElem : aggNode.getGroupElements()) {
        // Figure out the type of the aggregation.
        final AggregateOperator operator = groupElem.getOperator();
        final Optional<AggregationType> type = AggregationType.byOperatorClass( operator.getClass() );

        // If the type is one we support, create the AggregationElement.
        if(type.isPresent()) {
            final String resultBindingName = groupElem.getName();

            final AtomicReference<String> aggregatedBindingName = new AtomicReference<>();
            groupElem.visitChildren(new QueryModelVisitorBase<RuntimeException>() {
                @Override
                public void meet(final Var node) {
                    aggregatedBindingName.set( node.getName() );
                }
            });

            aggregations.add( new AggregationElement(type.get(), aggregatedBindingName.get(), resultBindingName) );
        }
    }

    return new AggregationsEvaluator(aggStateStore, aggregations, groupByVars);
}
 

import org.openrdf.query.algebra.GroupElem; //导入依赖的package包/类
@Override
public void meet(final GroupElem n) {
    final ProjectionElem e = new ProjectionElem();
    e.setTargetName(n.getName());
    e.setSourceName(n.getName());
    e.setSourceExpression(new ExtensionElem(n.getOperator(), n.getName()));
    meet(e);
}
 

import org.openrdf.query.algebra.GroupElem; //导入依赖的package包/类
@Override
public void meet(GroupElem node) throws RuntimeException {
	setSeen(node);
	super.meet(node);
}
 

import org.openrdf.query.algebra.GroupElem; //导入依赖的package包/类
@Override
public void meet(GroupElem node) throws RuntimeException {
	check(node);
	super.meet(node);
}
 

import org.openrdf.query.algebra.GroupElem; //导入依赖的package包/类
@Override
public void meet(GroupElem arg0) throws Exception {
	arg0.visitChildren(this);
}
 

import org.openrdf.query.algebra.GroupElem; //导入依赖的package包/类
@Override
public void meet(GroupElem arg0) throws SparqlParserException
{
   // NO-OP
}