Java Reference
In-Depth Information
Chapter 7
The Object and Objects Classes
In this chapter, you will learn
•
About the hierarchical class structure in Java
Object
class being the superclass of classes
•
About the
Object
class with detailed examples
•
How to use methods of the
Object
class in your class
•
How to reimplement methods of the
•
How to check two objects for equality
•
The difference between immutable and mutable objects
Objects
class to deal with
null
values gracefully
•
How to use the utility methods of the
The Object Class
Java has an
Object
class in the
java.lang
package. All Java classes, those that are included in the Java class libraries
and those that you create, extend the
Object
class directly or indirectly. All Java classes are a subclass of the
Object
class and the
Object
class is the superclass of all classes. Note that the
Object
class itself does not have a superclass.
Classes in Java are arranged in a tree-like hierarchical structure, where the
Object
class is at the root (or top).
I will discuss class hierarchy in detail in the chapter on inheritance. I will discuss some details of the
Object
class in
this chapter.
There are two important rules about the
Object
class. I will not explain the reasons behind these rules here. The
reasons why you could do these things with the
Object
class will be clear after you read the chapter on inheritance.
Rule #1
A reference variable of the
Object
class can hold a reference of an object of any class. As any reference variable can
store a
null
reference, so can a reference variable of the
Object
type. Consider the following declaration of a reference
variable
obj
of the
Object
type:
Object obj;
You can assign a reference of any object in Java to
obj
. All of the following statements are valid:
// Can assign the null reference
obj = null;
