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Subject: [jira] [Commented] (FLINK-3519) Subclasses of Tuples don't work if
 the declared type of a DataSet is not the descendant
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    [ https://issues.apache.org/jira/browse/FLINK-3519?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15169202#comment-15169202 ] 

ASF GitHub Bot commented on FLINK-3519:
---------------------------------------

Github user ggevay commented on the pull request:

    https://github.com/apache/flink/pull/1724#issuecomment-189326695
  
    I have also added that a subclass of TupleN can only be a POJO. (see TypeExtractor.java:1686)


> 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
>            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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