User-defined aggregation functions

package example;

import org.neo4j.procedure.Description;
import org.neo4j.procedure.Name;
import org.neo4j.procedure.UserAggregationFunction;
import org.neo4j.procedure.UserAggregationResult;
import org.neo4j.procedure.UserAggregationUpdate;

public class LongestString
{
    @UserAggregationFunction
    @Description( "org.neo4j.function.example.longestString(string) - aggregates the longest string found" )
    public LongStringAggregator longestString()
    {
        return new LongStringAggregator();
    }

    public static class LongStringAggregator
    {
        private int longest;
        private String longestString;

        @UserAggregationUpdate
        public void findLongest(
                @Name( "string" ) String string )
        {
            if ( string != null && string.length() > longest)
            {
                longest = string.length();
                longestString = string;
            }
        }

        @UserAggregationResult
        public String result()
        {
            return longestString;
        }
    }
}

package example;

import org.junit.Rule;
import org.junit.Test;
import org.neo4j.driver.v1.*;
import org.neo4j.harness.junit.Neo4jRule;

import static org.hamcrest.core.IsEqual.equalTo;
import static org.junit.Assert.assertThat;

public class LongestStringTest
{
    // This rule starts a Neo4j instance
    @Rule
    public Neo4jRule neo4j = new Neo4jRule()

            // This is the function to test
            .withAggregationFunction( LongestString.class );

    @Test
    public void shouldAllowIndexingAndFindingANode() throws Throwable
    {
        // This is in a try-block, to make sure you close the driver after the test
        try( Driver driver = GraphDatabase.driver( neo4j.boltURI() , Config.build().withEncryptionLevel( Config.EncryptionLevel.NONE ).toConfig() ) )
        {
            // Given
            Session session = driver.session();

            // When
            String result = session.run( "UNWIND ["abc", "abcd", "ab"] AS string RETURN example.longestString(string) AS result").single().get("result").asString();

            // Then
            assertThat( result, equalTo( "abcd" ) );
        }
    }
}