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https://issues.apache.org/jira/browse/FLINK-2542?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Ufuk Celebi resolved FLINK-2542.
--------------------------------
Resolution: Won't Fix
I think it's OK to assume that people follow the general Object contract:
{code}
Note that it is generally necessary to override the {@code hashCode} method
whenever this method is overridden, so as to maintain the general contract for
the {@code hashCode} method, which states that equal objects must have equal
hash codes.
{code}
If more people run into this, we can revisit this issue.
> It should be documented that it is required from a join key to override
> hashCode(), when it is not a POJO
> ---------------------------------------------------------------------------------------------------------
>
> Key: FLINK-2542
> URL: https://issues.apache.org/jira/browse/FLINK-2542
> Project: Flink
> Issue Type: Bug
> Components: Gelly, Java API
> Reporter: Gabor Gevay
> Priority: Minor
> Fix For: 0.10, 0.9.1
>
>
> If the join key is not a POJO, and does not override hashCode, then the join
> silently fails (produces empty output). I don't see this documented anywhere.
> The Gelly documentation should also have this info separately, because it
> does joins internally on the vertex IDs, but the user might not know this, or
> might not look at the join documentation when using Gelly.
> Here is an example code:
> {noformat}
> public static class ID implements Comparable<ID> {
> public long foo;
> //no default ctor --> not a POJO
> public ID(long foo) {
> this.foo = foo;
> }
> @Override
> public int compareTo(ID o) {
> return ((Long)foo).compareTo(o.foo);
> }
> @Override
> public boolean equals(Object o0) {
> if(o0 instanceof ID) {
> ID o = (ID)o0;
> return foo == o.foo;
> } else {
> return false;
> }
> }
> @Override
> public int hashCode() {
> return 42;
> }
> }
> public static void main(String[] args) throws Exception {
> ExecutionEnvironment env =
> ExecutionEnvironment.getExecutionEnvironment();
> DataSet<Tuple2<ID, Long>> inDegrees = env.fromElements(Tuple2.of(new
> ID(123l), 4l));
> DataSet<Tuple2<ID, Long>> outDegrees = env.fromElements(Tuple2.of(new
> ID(123l), 5l));
> DataSet<Tuple3<ID, Long, Long>> degrees = inDegrees.join(outDegrees,
> JoinOperatorBase.JoinHint.REPARTITION_HASH_FIRST).where(0).equalTo(0)
> .with(new FlatJoinFunction<Tuple2<ID, Long>, Tuple2<ID,
> Long>, Tuple3<ID, Long, Long>>() {
> @Override
> public void join(Tuple2<ID, Long> first,
> Tuple2<ID, Long> second, Collector<Tuple3<ID, Long, Long>> out) {
> out.collect(new Tuple3<ID, Long,
> Long>(first.f0, first.f1, second.f1));
> }
>
> }).withForwardedFieldsFirst("f0;f1").withForwardedFieldsSecond("f1");
> System.out.println("degrees count: " + degrees.count());
> }
> {noformat}
> This prints 1, but if I comment out the hashCode, it prints 0.
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