Java 11 Predicate lambda expression generic type ClassCastException

Question

I have problem on java 11 predicate lambda expression generic type , when I write predicate as lambda expression is not returning exactly generic parameter ,and ClassCastException occurs , code seen below

Explaination When we write lambda expression , Generic Parameter method getGenericInterfaces()[0] -> ClassCastException occurs this line because there no generic parameter seen when I debugged code , seen as just Condition not Condition<<Integer>>, anyway when we define predicate as seen below (working case) , its working

Working Case

Condition<Integer> oddValueCondition = new Condition<Integer>() {
    @Override
    public boolean test(Integer integer) {
        return integer % 2 == 0;
    }
};

Not Working Case

Condition<Integer> oddValueCondition = integer -> integer % 2 == 0;

interface definition

@FunctionalInterface
public interface Condition<T> extends Predicate<T> {
    default Class<T> getParameterType() {
        ParameterizedType parameterizedType = (ParameterizedType) this.getClass().getGenericInterfaces()[0];
        Type[] typeArguments = parameterizedType.getActualTypeArguments();
        Class<T> type = (Class<T>) typeArguments[0];
        return type;
    }
}

Test

oddValueCondition.getParameterType();

So what do you think problem is , when write lambda expression ?

And also I created issue on the github OpenJDK

if they says TypeReference Hack is not work on anonymous class and lambda expression , I will accept answer

Works fine on my environment, I'd suggest to check the java language level in project settings — edwgiz, Jan 08 '20 at 15:18
Have you tried casting `Condition oddValueCondition = (Condition) integer -> integer % 2 == 0; ` — Karim, Jan 08 '20 at 15:18
@edwgiz and you are certain the Lambda version works. Can you add the following into the interface and see what it prints with both versions. Lambda and anoymous class `System.out.println(this.getClass().getGenericInterfaces()[0]);` — WJS, Jan 08 '20 at 16:42
@WJS sorry guys, my mistake, it prints `interface Condition` without generics — edwgiz, Jan 08 '20 at 17:59
I think @sleiman answer is correct , Lambda expressions and anonymous inner classes dont compile same byte code . — Mithat Konuk, Jan 08 '20 at 18:41

Sleiman Jneidi · Accepted Answer · 2020-01-08T15:48:31.630

1

Lambda expressions and anonymous inner classes don't compile to the same byte-code.

The TypeReference hack doesn't work with lambda as the type information is not available through this API. And by the way, this has been like this since Java 1.8.

edited Jan 08 '20 at 15:48

answered Jan 08 '20 at 15:37

Sleiman Jneidi

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hmm , i couldnt find any document on it , so could you show offical document on this issue , otherwise if anyone is accept that , we can sign as answer. – Mithat Konuk Jan 08 '20 at 15:52
And also I created issue on the github https://github.com/AdoptOpenJDK/openjdk-build/issues/1462 – Mithat Konuk Jan 08 '20 at 16:09
1

@MithatKonuk it isn't a bug and was discussed here sometime ago – Sleiman Jneidi Jan 08 '20 at 16:12
i did not created as bug , opened as issue and okay then we can take as a answer – Mithat Konuk Jan 08 '20 at 16:23

Java 11 Predicate lambda expression generic type ClassCastException

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