Recall from the previous lesson that the extends
clause declares that
your class is a subclass of another. You can specify only one
superclass for your class (Java does not support multiple class
inheritance), and even though you can omit the extends
clause from your
class declaration, your class has a superclass. So, every class in Java
has one and only one immediate superclass. This statement leads to the
question, "Where does it all begin?"
As depicted in the following figure, the top-most class,
the class from which all other classes are derived,
is the
Object
class defined in java.lang.
The Object class defines and implements behavior
that every class in
the Java system needs. It is the most general of all classes. Its
immediate subclasses, and other classes near top of the hierarchy,
implement general behavior; classes near the bottom of the hierarchy
provide for more specialized behavior.
Definition:
A subclass is a class that extends another class.
A subclass inherits state and behavior from all of its ancestors.
The term "superclass" refers to a class's direct ancestor
as well as to all of its ascendant classes.
The next section,
Understanding Inheritance,
talks about the various issues involved in extending
any class, such as which members it inherits, which members it can
override or hide, and which it cannot. The section that follows,
Being a Descendent of Object,
describes what all classes inherit
from the Object class and how these classes
implement their inherited functionality.
