Classes define objects

We have seen that classes are the organizational unit of a Java program. Source code for methods is always within a class, and we might expect related methods to be in the same class.

However, classes do much more than this, and are the heart of object-oriented programming in Java:

1. A class is used to define the custom data structure that makes up an object.
2. Classes define methods that act on those objects.

Here’s an example of creating a custom data type, Circle, to store information about a circle: