What is the purpose of access modifiers?

Question

I know this applies to many languages, and not just Java, but that is the language I\'m most familiar with.

I understand what the modifiers do, and how to use them.

Answer 1

Making fields and methods private keeps other classes from improperly depending on the specific details of how a class works. The public interface (and the best case of all, an actual interface) describes how client code should interact with a library based on the semantics of the work being done. The implementer is then free to use whatever appropriate techniques to implement that interface and can make significant behind-the-scenes changes knowing that the client code will keep working.

An everyday example is the Collections group of interfaces. Most of the time, it's not important logically for code to know what particular kind of Set is in use, just that it's a collection that supports certain operations and doesn't have duplicates. This means that a method that accepts a Set will work with any Set, including HashSet and ImmutableSet, because the person who wrote the interface wasn't poking around in the implementation's internals.

An example where this breaks down is the unfortunate tendency of some programmers to use packages in the com.sun namespace, especially when using cryptography. Upgrading to a new version of the JRE routinely breaks this code, which would have worked fine if the programmer had used the proper javax.crypto interfaces and factory methods instead of poking around in the JVM internals.