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The main module contains the GeoTools public interfaces that are used by other GeoTools modules (and GeoTools applications). Where possible we make use industry standard terms as provided by OGC and ISO standards. The formal GeoTools public api consists of gt-metadata, jts and the gt-main module. The main module contains the default implementations that are available provided to other GeoTools modules using our factory system. Factories are obtained from an appropriate FactoryFinder, giving applications a chance configure the factory used using the Factory Hints facilities. FilterFactory ff = CommonFactoryFinder.getFilterFactory(); Expression expr = ff.add( expression1, expression2 ); If you find yourself using implementation specific classes chances are you doing it wrong: Expression expr = new AddImpl( expression1, expressiom2 );

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/*
 *    GeoTools - The Open Source Java GIS Toolkit
 *    http://geotools.org
 *
 *    (C) 2016, Open Source Geospatial Foundation (OSGeo)
 *
 *    This library is free software; you can redistribute it and/or
 *    modify it under the terms of the GNU Lesser General Public
 *    License as published by the Free Software Foundation;
 *    version 2.1 of the License.
 *
 *    This library is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 *    Lesser General Public License for more details.
 */
package org.geotools.feature.visitor;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import org.geotools.data.util.NullProgressListener;
import org.geotools.factory.CommonFactoryFinder;
import org.opengis.feature.simple.SimpleFeatureType;
import org.opengis.feature.type.AttributeDescriptor;
import org.opengis.filter.FilterFactory;
import org.opengis.filter.expression.Expression;
import org.opengis.util.ProgressListener;

/**
 * Helper class to help building a valid group by visitor.
 *
 * A valid group by visitor requires an aggregate attribute, an aggregate visitor and at least
 * one group by attribute.
 */
public final class GroupByVisitorBuilder {

    private Expression aggregateAttribute;
    private Aggregate aggregateVisitor;
    private List groupByAttributes = new ArrayList<>();
    private ProgressListener progressListener;

    public GroupByVisitorBuilder withAggregateAttribute(
            int attributeTypeIndex, SimpleFeatureType type) {
        aggregateAttribute = toExpression(attributeTypeIndex, type);
        return this;
    }

    public GroupByVisitorBuilder withAggregateAttribute(
            String attributeName, SimpleFeatureType type) {
        aggregateAttribute = toExpression(attributeName, type);
        return this;
    }

    public GroupByVisitorBuilder withAggregateAttribute(Expression aggregateAttribute) {
        this.aggregateAttribute = aggregateAttribute;
        return this;
    }

    public GroupByVisitorBuilder withAggregateVisitor(Aggregate aggregateVisitor) {
        this.aggregateVisitor = aggregateVisitor;
        return this;
    }

    public GroupByVisitorBuilder withAggregateVisitor(String aggregateVisitorName) {
        this.aggregateVisitor = Aggregate.valueOfIgnoreCase(aggregateVisitorName);
        return this;
    }

    public GroupByVisitorBuilder withGroupByAttribute(
            int attributeTypeIndex, SimpleFeatureType type) {
        groupByAttributes.add(toExpression(attributeTypeIndex, type));
        return this;
    }

    public GroupByVisitorBuilder withGroupByAttribute(
            String attributeName, SimpleFeatureType type) {
        groupByAttributes.add(toExpression(attributeName, type));
        return this;
    }

    public GroupByVisitorBuilder withGroupByAttributes(
            Collection attributesNames, SimpleFeatureType type) {
        for (String attributeName : attributesNames) {
            withGroupByAttribute(attributeName, type);
        }
        return this;
    }

    public GroupByVisitorBuilder withGroupByAttribute(Expression groupByAttribute) {
        groupByAttributes.add(groupByAttribute);
        return this;
    }

    public GroupByVisitorBuilder withProgressListener(ProgressListener progressListener) {
        this.progressListener = progressListener;
        return this;
    }

    private Expression toExpression(int attributeTypeIndex, SimpleFeatureType type) {
        FilterFactory filterFactory = CommonFactoryFinder.getFilterFactory(null);
        AttributeDescriptor attribute = type.getDescriptor(attributeTypeIndex);
        if (attribute == null) {
            throw new IllegalArgumentException(
                    "Attribute index '" + attributeTypeIndex + "' is not valid.");
        }
        return filterFactory.property(attribute.getLocalName());
    }

    private Expression toExpression(String attributeName, SimpleFeatureType type) {
        FilterFactory filterFactory = CommonFactoryFinder.getFilterFactory(null);
        AttributeDescriptor attribute = type.getDescriptor(attributeName);
        if (attribute == null) {
            throw new IllegalArgumentException("Attribute '" + attributeName + "' is not valid.");
        }
        return filterFactory.property(attribute.getLocalName());
    }

    /**
     * Create a group by visitor checking that all mandatory values are present.
     *
     * @return a new group by visitor
     */
    public GroupByVisitor build() {
        if (aggregateAttribute == null) {
            throw new IllegalArgumentException("An aggregate attribute is required.");
        }
        if (aggregateVisitor == null) {
            throw new IllegalArgumentException("An aggregate visitor is required.");
        }
        if (groupByAttributes == null || groupByAttributes.isEmpty()) {
            throw new IllegalArgumentException("At least one group by attribute is required.");
        }
        if (progressListener == null) {
            progressListener = new NullProgressListener();
        }
        return new GroupByVisitor(
                aggregateVisitor, aggregateAttribute, groupByAttributes, progressListener);
    }
}