Till Rohrmann created FLINK-3681:
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Summary: CEP library does not support Java 8 lambdas as select
function
Key: FLINK-3681
URL: https://issues.apache.org/jira/browse/FLINK-3681
Project: Flink
Issue Type: Bug
Components: CEP
Affects Versions: 1.0.0
Reporter: Till Rohrmann
Assignee: Till Rohrmann
Priority: Minor
Fix For: 1.0.1
Currently, the CEP library does not support Java 8 lambdas to be used as
{{select}} or {{flatSelect}} function. The problem is that the
{{TypeExtractor}} has different semantics when calling
{{TypeExtractor.getUnaryOperatorReturnType}} either with a Java 8 lambda or an
instance of an UDF function.
To illustrate the problem assume we have the following UDF function
{code}
public interface MyFunction[T, O] {
O foobar(Map<String, T> inputElements);
}
{code}
When calling the {{TypeExtractor}} with an anonymous class which implements
this interface, the first type parameter is considered being the input type of
the function, namely {{T}}.
In contrast, when providing a Java 8 lambda for this interface, the
{{TypeExtractor}} will see an input type of {{Map<String, T>}}.
This problem also occurs with a {{FlatMapFunction}} whose first type argument
is {{T}} but whose first parameter of a Java 8 lambda is {{Iterable<T>}}. In
order to solve the problem here, the
{{TypeExtractor.getUnaryOperatorReturnType}} method has the parameters
{{hasIterable}} and {{hasCollector}}. If these values are {{true}}, then a
special code path is taken (in case of a Java 8 lambda), where the input type
is compared to the first type argument of the first input parameter of the
lambda (here an {{Iterable<T>}}). This hand-knitted solution does not
generalize well, as it will fail for all parameterized types which have the
input type at a different position (e.g. {{Map<String, T>}}.
In order to solve the problem, I propose to generalize the
{{getUnaryOperatorReturnType}} a little bit so that one can specify at which
position the input type is specified by a parameterized type.
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