I know it can be applied on interface but as Interfaces with default methods ( Java 8+ Interfaces ) are very close now to abstract classes, Just wondering now can we use @FunctionalInterface with Abstract classes?
If not why we can't?
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Andronicus
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Pramod S. Nikam
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3Did you try it? – Andronicus Mar 10 '19 at 14:19
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1as appear its name it uses just for interfaces. and you can test it. – Hadi J Mar 10 '19 at 14:19
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Possible duplicate of [What is use of Functional Interface in Java 8?](https://stackoverflow.com/questions/36881826/what-is-use-of-functional-interface-in-java-8) – Naman Mar 10 '19 at 16:53
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No, FI was solely created to facilitate lambdas. – Vishwa Ratna Mar 11 '19 at 08:28
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You can not apply @FunctionalInterface to abstract class, enum or another annotation. From java doc:
If a type is annotated with this annotation type, compilers are required to generate an error message unless:
- The type is an interface type and not an annotation type, enum, or class.
- The annotated type satisfies the requirements of a functional interface.
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2@PraJazz see here for more details https://stackoverflow.com/questions/24610207/abstract-class-as-functional-interface – Ruslan Mar 10 '19 at 14:33
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As the name indicates it should be used for interfaces. Applying it to class results in the following:
Error:(10, 1) java: Unexpected @FunctionalInterface annotation xxx is not a functional interface
But this is not much of a problem, as this is just a marker interface for interfaces with one method.
Andronicus
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The functional interface is there for the specific purpose - It's used with interfaces only which have strictly one and only one abstract method. if not then the compiler complains -
Invalid '@FunctionalInterface' annotation; TestInterface is not a functional interface
And there is no point in using this with any other class, enum stuffs
Arvind Kumar
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