Why isn’t @FunctionalInterface used on all the interfaces in the JDK that qualify?

Well, an annotation documenting an intention would be useless if you assume that there is always that intention given.

You named the example AutoCloseable which is obviously not intended to be implemented as a function as there’s Runnable which is much more convenient for a function with a ()->void signature. It’s intended that a class implementing AutoCloseable manages an external resource which anonymous classes implemented via lambda expression don’t do.

A clearer example is Comparable, an interface not only not intended to be implemented as a lambda expression, it’s impossible to implement it correctly using a lambda expression.

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Possible reasons for not marking an interface with @FunctionalInterface by example:

• The interface has programming language semantics, e.g. AutoClosable or Iterable (that’s unlikely to happen for your own interfaces)
• It’s not expected that the interface has arbitrary implementations and/or is more an identifier than the actual implementation, e.g. java.net.ProtocolFamily, or java.lang.reflect.GenericArrayType (Note that the latter would also inherit a default implementation for getTypeName() being useless for lambda implementations as relying on toString())

• The instances of this interface should have an identity, e.g. java.net.ProtocolFamily, java.nio.file.WatchEvent.Modifier, etc. Note that these are typically implemented by an enum

  Another example is java.time.chrono.Era which happens to have only a single abstract method but its specification says “Instances of Era may be compared using the == operator.”

• The interface is intended to alter the behavior of an operation for which an implementation of the interface without inheriting/implementing anything else makes no sense, e.g. java.rmi.server.Unreferenced
• It’s an abstraction of common operations of classes which should have more than just these operations, e.g. java.io.Closeable, java.io.Flushable, java.lang.Readable
• The expected inheritance is part of the contract and forbids lambda expression implementations, e.g. in java.awt: ActiveEvent should be implemented by an AWTEvent, PrinterGraphics by a Graphics, the same applies to java.awt.print.PrinterGraphics (hey, two interfaces for exactly the same thing…), wheras javax.print.FlavorException should be implemented by a javax.print.PrintException subclass
• I don’t know whether the various event listener interfaces aren’t marked with @FunctionalInterface for symmetry with other multi-method event listener that can’t be functional interfaces, but actually event listeners are good candidates for lambda expressions. If you want remove a listener at a later time, you have to store the instance but that’s not different to, e.g. inner class listener implementations.

• The library maintainer has a large codebase with more than 200 candidate types and not the resources to discuss for every interface whether it should be annotated and hence focuses on the primary candidates for being used in a functional context. I’m sure, that, e.g. java.io.ObjectInputValidation, java.lang.reflect.InvocationHandler, juc RejectedExecutionHandler & ThreadFactory wouldn’t be bad as @FunctionalInterface but I have no idea whether, e.g. java.security.spec.ECField makes a good candidate. The more general the library is, the more likely users of the library will be able to answer that question for a particular interface they are interested in but it would be unfair to insist on the library maintainer to answer it for all interfaces.

  In this context it makes more sense to see the presence of a @FunctionalInterface as a message that an interface is definitely intended to be usable together with lambda expressions than to treat the absence of the annotation as an indicator for it’s being not intended to be used this way. This is exactly like the compiler handles it, you can implement every single abstract method interface using a lambda expression, but when the annotation is present it will ensure that you can use this interface in this way.