In this module, you will be taught to test your code to get it working. The best way to do this is to write a set of unit tests that you can run against your program, at any time. In this module, we'll learn how to do unit testing in Java, and why we want to do it. We’ll also learn all about Java Arrays, which are collections of items, and ArrayLists, which are kind of like arrays, but much more flexible. These work just like lists in Python. ArrayLists are part of Java’s larger Collections Framework, which we’ll continue to reference throughout this course.

So far, we’ve been defining variables as instance variables in our Java classes. Instance variables are just variables defined inside a class, and every instance of a class can have a different value for an instance variable. In this module, we’ll look at defining static variables in our Java classes. Static variables are also defined as variables inside a class, but with the keyword 'static'. Static variables have the same value for every instance of a class. In fact, you don’t even need to create an instance of a class to use a static variable! We’ll also look at static methods. And just like static variables, you don’t need to create an instance of a class to use, or call, a static method. In this module, we’re also going to introduce polymorphism, which refers to overloading and overriding. We’ll focus on overloading, which is having two or more methods in the same class, with the same name.