What characteristics must a method have to make a functional interface truly functional?

The method should be:

• Abstract: It should be declared without implementation, i.e., without a body.
• Single: A functional interface can contain only one abstract method. Default (default) and static methods are allowed, but only one abstract method.
• Not from Object: The method should not override a public method from java.lang.Object, as such methods can be implemented by any class.