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https://issues.apache.org/jira/browse/FLINK-3519?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15267587#comment-15267587
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ASF GitHub Bot commented on FLINK-3519:
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Github user fhueske commented on the pull request:
https://github.com/apache/flink/pull/1724#issuecomment-216378193
+1 to merge
> Subclasses of Tuples don't work if the declared type of a DataSet is not the
> descendant
> ---------------------------------------------------------------------------------------
>
> Key: FLINK-3519
> URL: https://issues.apache.org/jira/browse/FLINK-3519
> Project: Flink
> Issue Type: Bug
> Components: Type Serialization System
> Affects Versions: 1.0.0
> Reporter: Gabor Gevay
> Assignee: Gabor Gevay
> Priority: Minor
>
> If I have a subclass of TupleN, then objects of this type will turn into
> TupleNs when I try to use them in a DataSet<TupleN>.
> For example, if I have a class like this:
> {code}
> public static class Foo extends Tuple1<Integer> {
> public short a;
> public Foo() {}
> public Foo(int f0, int a) {
> this.f0 = f0;
> this.a = (short)a;
> }
> @Override
> public String toString() {
> return "(" + f0 + ", " + a + ")";
> }
> }
> {code}
> And then I do this:
> {code}
> env.fromElements(0,0,0).map(new MapFunction<Integer, Tuple1<Integer>>() {
> @Override
> public Tuple1<Integer> map(Integer value) throws Exception {
> return new Foo(5, 6);
> }
> }).print();
> {code}
> Then I don't have Foos in the output, but only Tuples:
> {code}
> (5)
> (5)
> (5)
> {code}
> The problem is caused by the TupleSerializer not caring about subclasses at
> all. I guess the reason for this is performance: we don't want to deal with
> writing and reading subclass tags when we have Tuples.
> I see three options for solving this:
> 1. Add subclass tags to the TupleSerializer: This is not really an option,
> because we don't want to loose performance.
> 2. Document this behavior in the javadoc of the Tuple classes.
> 3. Make the Tuple types final: this would be the clean solution, but it is
> API breaking, and the first victim would be Gelly: the Vertex and Edge types
> extend from tuples. (Note that the issue doesn't appear there, because the
> DataSets there always have the type of the descendant class.)
> When deciding between 2. and 3., an important point to note is that if you
> have your class extend from a Tuple type instead of just adding the f0, f1,
> ... fields manually in the hopes of getting the performance boost associated
> with Tuples, then you are out of luck: the PojoSerializer will kick in anyway
> when the declared types of your DataSets are the descendant type.
> If someone knows about a good reason to extend from a Tuple class, then
> please comment.
> For 2., this is a suggested wording for the javadoc of the Tuple classes:
> Warning: Please don't subclass Tuple classes, but if you do, then be sure to
> always declare the element type of your DataSets to your descendant type.
> (That is, if you have a "class A extends Tuple2", then don't use instances of
> A in a DataSet<Tuple2>, but use DataSet<A>.)
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