The target interface should not be a @FunctionalInterface

[khatchad]

If we modify a @FunctionalInterface such that the method is now default, what happens to the lambda expressions using the @FunctionalInterface? My guess is that they won't work anymore but need to investigate.

[khatchad]

Here's the javadoc documentation:

java.lang.FunctionalInterface

@target(value={TYPE})
@retention(value=RUNTIME)
@documented
An informative annotation type used to indicate that an interface type declaration is intended to be a functional interface as defined by the Java Language Specification. Conceptually, a functional interface has exactly one abstract method. Since default methods have an implementation, they are not abstract. If an interface declares an abstract method overriding one of the public methods of java.lang.Object, that also does not count toward the interface's abstract method count since any implementation of the interface will have an implementation from java.lang.Object or elsewhere.

Note that instances of functional interfaces can be created with lambda expressions, method references, or constructor references.

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.

However, the compiler will treat any interface meeting the definition of a functional interface as a functional interface regardless of whether or not a FunctionalInterface annotation is present on the interface declaration.

Since:
1.8
@jls
4.3.2. The Class Object
@jls
9.8 Functional Interfaces
@jls
9.4.3 Interface Method Body

[khatchad]

Interesting, this is actually a real problem. An @FunctionalInterface must have one and only abstract method. So, for example, if we convert the abstract method into a default method and the interface is marked as a @FunctionalInterface, then we have a compile time error.

So, we should check that if the interface is marked as @FunctionalInterface, that we are not refactoring the abstract method. But, we don't refactor anything other than abstract methods inside interfaces. As such, it should be enough to simply check that the destination interface is not an @FunctionalInterface.

There's a larger problem here, however. That is, not all functional interfaces are marked as @FunctionalInterfaces. How do we handle those? How do we know that an interface is effectively a @FunctionalInterface?