A Case For Sealing Classes In Java

It is a well-known fact that inheritance as defined in most existing object-oriented languages breaks encapsulation in a very subtle way. For instance, Java provides facilities for encapsulation both at the class-level and at the packagelevel. But it also introduces language constructs (such as public/ protected modifier and inheritance) that lets clients to access the internals of a class in a subtle way. In this paper we introduce the notion of class sealing in Java that selectively prevent clients of a package from accessing the internals of a class via inheritance. Class sealing allows extension of classes within a package, but prevent clients from extending such classes outside of the package. We will discuss three areas of application of class sealing: maintenance, security, and program optimization. We provide empirical evidence, collected from large bodies of real-life code, showing that sealed classes fit well within existing coding practice.