규칙 6.9

A struct must be the members written in it, in one of three spellings (class, interface, a type over an object literal). On a class or an interface with an extends clause it must be the base’s members first and its own after, through a chain of any depth. A class’s members include the fields its constructor’s parameter properties declare, and a field written without a type takes the one its initializer names (Rule 8.14). An interface that declares a method is a contract and not a struct: a class may name it in implements and a type parameter in its constraint (<T extends Shape>), and a value typed by it must be refused with the remedy, once, where the interface declares the method. A call signature, an index signature, and an optional member must be refused.