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Java port of Stefan Goessner JsonPath.

There is a newer version: 2.9.0

package com.jayway.jsonpath.internal.function.numeric;

import com.jayway.jsonpath.JsonPathException;
import com.jayway.jsonpath.internal.EvaluationContext;
import com.jayway.jsonpath.internal.PathRef;
import com.jayway.jsonpath.internal.function.Parameter;
import com.jayway.jsonpath.internal.function.PathFunction;

import java.util.List;

/**
 * Defines the pattern for processing numerical values via an abstract implementation that iterates over the collection
 * of JSONArray entities and verifies that each is a numerical value and then passes that along the abstract methods
 *
 *
 * Created by mattg on 6/26/15.
 */
public abstract class AbstractAggregation implements PathFunction {

    /**
     * Defines the next value in the array to the mathmatical function
     *
     * @param value
     *      The numerical value to process next
     */
    protected abstract void next(Number value);

    /**
     * Obtains the value generated via the series of next value calls
     *
     * @return
     *      A numerical answer based on the input value provided
     */
    protected abstract Number getValue();

    @Override
    public Object invoke(String currentPath, PathRef parent, Object model, EvaluationContext ctx, List parameters) {
        int count = 0;
        if(ctx.configuration().jsonProvider().isArray(model)){

            Iterable objects = ctx.configuration().jsonProvider().toIterable(model);
            for (Object obj : objects) {
                if (obj instanceof Number) {
                    Number value = (Number) obj;
                    count++;
                    next(value);
                }
            }
        }
        if (parameters != null) {
            for (Parameter param : parameters) {
                Object value = param.getValue();
                if (null != value && value instanceof Number) {
                    count++;
                    next((Number)value);
                }
            }
        }
        if (count != 0) {
            return getValue();
        }
        throw new JsonPathException("Aggregation function attempted to calculate value using empty array");
    }
}