As we wrap up our whirlwind tour of classes, we want you think a bit about how to go about building good classes. Doing so is more of an art than a science, and it will take you lots of practice and time. However, there are a few rules that we’ve pointed out to help guide you. Here’s one more.
From Wikipedia:
The single responsibility principle states that every module or class should have responsibility over a single part of the functionality provided by the software, and that responsibility should be entirely encapsulated by the class.
It isn’t always clear what “responsibility over a single part of the
functionality” means. However, it is often very clear what it doesn’t
mean. For example, we wouldn’t think of adding functionality to the
Student class that tracked all of the data for each of the student’s
classes, such as catalog number, instructor, and so on. Those are
clearly different areas of responsibility. One way to interpret the
single responsibility principle is to say that “classes should be
small.”
As you go forth and create classes, the main thing to keep in mind is that your skill and judgement in creating Java classes will improve over time. The best way to improve is to write lots of code, ask lots of questions, and continue learning!
If you are interested in learning more about Single Responsibility Principle, you can check out the entry on Wikipedia.