Example for a LEFT OUTER JOIN in Apache Flink

LeftOuterJoinExample.java

package org.myorg.quickstart;

import org.apache.flink.api.common.functions.CoGroupFunction;
import org.apache.flink.api.common.functions.MapFunction;
import org.apache.flink.api.java.DataSet;
import org.apache.flink.api.java.ExecutionEnvironment;
import org.apache.flink.api.java.operators.DataSource;
import org.apache.flink.api.java.tuple.Tuple2;
import org.apache.flink.util.Collector;

public class LeftOuterJoinExample {
	
	public static class LeftOuterJoin implements CoGroupFunction<Tuple2<Integer, String>, Tuple2<Integer, String>, Tuple2<Integer, Integer>> {

		@Override
		public void coGroup(Iterable<Tuple2<Integer, String>> leftElements,
		                    Iterable<Tuple2<Integer, String>> rightElements,
		                    Collector<Tuple2<Integer, Integer>> out) throws Exception {

			final int NULL_ELEMENT = -1;

			for (Tuple2<Integer, String> leftElem : leftElements) {
				boolean hadElements = false;
				for (Tuple2<Integer, String> rightElem : rightElements) {
					out.collect(new Tuple2<Integer, Integer>(leftElem.f0, rightElem.f0));
					hadElements = true;
				}
				if (!hadElements) {
					out.collect(new Tuple2<Integer, Integer>(leftElem.f0, NULL_ELEMENT));
				}
			}

		}
	}
	
	public static void main(String[] args) throws Exception {

		ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment();


		DataSource<Integer> leftSide = env.fromElements(1, 2, 3, 4, 5);
		DataSet<Tuple2<Integer, String>> leftSide2 = leftSide.map(
				new MapFunction<Integer, Tuple2<Integer, String>>() {
					@Override
					public Tuple2<Integer, String> map(Integer integer) throws Exception {
						return new Tuple2<Integer, String>(integer, "some data");
					}
				});

		DataSource<Integer> rightSide = env.fromElements(4, 5, 6, 7, 8, 9, 10);
		DataSet<Tuple2<Integer, String>> rightSide2 = rightSide.map(
				new MapFunction<Integer, Tuple2<Integer, String>>() {
					@Override
					public Tuple2<Integer, String> map(Integer integer) throws Exception {
						return new Tuple2<Integer, String>(integer, "some other data");
					}
				});

		DataSet<Tuple2<Integer, Integer>> leftOuterJoin = leftSide2.coGroup(rightSide2)
				.where(0)
				.equalTo(0)
				.with(new LeftOuterJoin());

		leftOuterJoin.print();

		env.execute();
	}
}