Have you read about an interface declaration as public abstract interface InterfaceName in Java? At the first glance, is it a weird declaration? Why should we declare an interface as abstract? For example, we may see below code in some projects:public abstract interface MyInterface { public void check(); public abstract boolean check(boolean really);}Actually here abstract is redundant, an interface is implicitly abstract, we no need to put an abstract in front of interface. But it does no harm if you do.You can find the below statement about abstract interface in JLS：9.1.1.1 abstra...