Relationships and Finger Pointing in Java

We can correctly refer to HelloComponent as a JComponent because subclassing can be thought of as creating an “is a” relationship, in which the subclass “is a” kind of its superclass. HelloComponent is therefore a kind of JComponent. When we refer to a kind of object, we mean any instance of that object’s class or any of its subclasses. Later, we will look more closely at the Java class hierarchy and see that JComponent is itself a subclass of the Container class, which is further derived from a class called Component, and so on, as shown below. In this sense, a HelloComponent object is a kind of JComponent, which is a kind of Container, and each of these can ultimately be considered to be a kind of Component. It’s from these classes that HelloComponent inherits its basic GUI functionality and (as we’ll discuss later) the ability to have other graphical components embedded within it as well.

Component is a subclass of the top-level Object class, so all these classes are types of Object. Every other class in the Java API inherits behavior from Object, which defines a few basic methods, as you’ll see. We’ll continue to use the word object (lowercase o) in a generic way to refer to an instance of any class; we’ll use Object to refer specifically to the type of that class.