"one class in each file can be made available to code that is external to that file. In other words, only one class in each file can be declared inside a package declaration. You must declare any additional classes outside your package definition, which makes those classes invisible to code outside that source file. The name of the class declared inside the package definition must match the name of the source file."
추가적인 클래스들을 패키지 밖에 정의할 수 있고 이렇게 정의된 클래스는 소스코드 밖에서는 보이지 않게 된다.
