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Programming with Java
1
Chapter 12
More OOP, Interfaces, and Inner
Classes
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
2
Objectives
•
Create your own interfaces.
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Identify the types of inner classes available in Java.
•
Understand and use inner classes.
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Programming with Java
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Encapsulation
•
The term encapsulation refers to the fact that all methods and
variables are declared inside of a class, which it can hide or
expose, as necessary.
•
When you declare variables and methods private, no other
classes have access to the data.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
4
Inheritance
•
Inheritance provides the ability to create a class based on
another class.
•
You gain all the functionality of a superclass (also known as
base class) and add your own functionality.
•
One superclass can have several subclasses that inherit from
it.
•
A subclass on the other hand can only inherit from one
superclass.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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Hierarchy of
Classes
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Programming with Java
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Polymorphism
•
Many methods have the same name such as setText, which can
be used for a label as well as for a text component.
•
You can strive for the same thing, for example in having
different constructors.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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Inheriting from Your Own Classes
•
So far we have inherited classes that only exist in the Java
language.
•
You can extend a class that you have written.
•
Often the technique of inheritance is used intentionally at design
stage to keep similar objects with a single superclass.
•
Consider a profile of a person : Last Name, First Name, Street,
City, State, ….
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A profile of person can be used for employee, customer, or
vendor.
•
We can create a superclass of a Person then inherit subclasses for
employee, vendor, or customer.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
8
Superclass and Subclasses
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Programming with Java
9
Protected Variables
•
Private variables are not inherited.
•
If you plan to inherit from a class, you should make the
variables either public or protected.
•
Variables declared as protected act as private in the superclass,
but can be inherited to a subclass.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
10
Interfaces
•
Interfaces are Java’s answer to its lack of multiple inheritance.
•
The implements clause can list several interfaces.
•
Your program can have access to the methods and constants in
many interfaces.
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Interfaces have several similarities to classes.
 Each is a separate file saved with a .java extension.
 Both contain coding to declare variables and methods.
•
Certain limitations apply to interfaces:
 Methods do not contain a body.
 All variables must be declared as final (constants).
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
11
Abstract Classes and Methods
•
An abstract class is strictly written for inheritance.
•
You cannot instantiate an object of an abstract class, but you can
inherit from the class.
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Abstract classes can contain methods with code along with other
methods that are empty, which are called abstract methods.
•
This requires a subclass to override an empty class by including
a method with the same name.
•
You also have the option of overriding any other non-abstract
methods in the base class.
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Programming with Java
12
Abstract Classes and Methods
Continued
•
You can declare a class to be abstract by adding the keyword
abstract in the class header.
•
If you include any abstract (empty) methods in the class, the
class must be declared abstract.
•
An easy error to generate is to accidentally place a semicolon at
the end of the line declaring a new method.
•
Java considers this an abstract method and warns you that you
must declare the class abstract.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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Interface Abstract Methods
•
The methods in the interface are public by default, whether or
not you include the keyword public.
•
All fields will be public, final and static, whether or not you
include the keywords.
•
However, it is good practice to include the keywords.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
14
Creating an Interface
•
You can create your own interface using the keyword interface
instead of class.
•
You can now refer to the constants in any application or applet
that implements the interface.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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Inner Classes
•
•
•
•
•
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An inner class is class that is declared in another class.
Some refer to this as a nested class.
Why would you want to declare a class inside another class?
The most common reason is to organize code.
You can also take advantage of scoping rules for variables and
methods.
Code within the inner class can directly reference variables and
methods within the scope of the class – the code in the outer
class can directly reference variables and methods inside the
inner class.
An important thing is writing inner classes are merely a
different style of coding.
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Programming with Java
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Inner Classes
Type of Inner
Class
Applies To
Declared
Can be Used
Static Member
Classes and
interfaces
Inside a class as
static
By any class
Member (nonstatic)
Classes
Inside a class
(non-static)
Within the
member class
Local (named)
Classes
Inside a method
Within the method
Anonymous
(local unnamed)
Classes
Inside a method
with no name
Within the method
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Programming with Java
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Inner Classes
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Programming with Java
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Static Member Classes and
Interfaces
•
You can declare an inner member class to be static.
•
Just as the class variables, the outer class has only one copy of
the inner class, not a copy per instance.
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You can declare a static inner class to keep summary
information.
•
A static member class follows the scoping rules for a class and
instance variables.
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Programming with Java
19
Static Member Classes and
Interfaces Continued
•
You declare the class at the top level (not in a method), and it can
be referenced by code anywhere inside the class.
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A static member class can access all of the static members of its
containing class, including private members.
•
In a program with many methods, it is difficult to determine the
ones that overriding from the interface and/or superclass and
those that belong specifically to the class.
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Programming with Java
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Member Classes
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•
•
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•
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A member class is an inner class that is not declared static and
also not declared inside a method.
A member class – one copy exists for each instance of the class.
The inner class only exists for the use of the outer class and
cannot be used by other top-level classes.
The nested class has access to all instance and class variables
and objects of the outer class or any other inner classes,
including members declared as private.
The outer class has access to all the variables and methods in the
inner class.
An inner class declared in an applet has access to all of the
components.
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Programming with Java
21
Local Classes
•
A local inner class is an inner class declared inside a method.
•
Local inner classes are closely related to the local variables,
which have a local scope and can be used only within that
method.
•
A common use of the local inner class is to implement an event
handling interface or extend another class, such as the
MouseAdapter or WindowAdapter.
•
Since the code for the event handling appears in only one
method, it makes sense to code it as a local inner class.
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Programming with Java
22
Anonymous Classes
•
An anonymous class is a local class that you define without
giving it a name.
•
You can declare an anonymous inner class as an expression,
which may be an argument for a method call or assigned to a
variable.
•
If you declare a local class (with a name), you define the class
and then instantiate an object of that class.
•
With an anonymous class, you won't be referring to the class
object by name, so you can combine the two steps (definition
and instantiation).
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
23
Anonymous Classes
Continued
For example (used in Chapter 10 and 11)
frmMain.addWindowListener(new WindowAdapter()
{
public void windowClosing(WindowEvent event)
{
//Action for window close box
System.exit(0);
}
});
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.
Programming with Java
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Distributing Inner Classes
•
When you distribute an applet or application that uses inner
classes, you must make sure to include all .class files.
•
Although you write one class inside another, the compiler
produces separate .class files for each class.
•
For example, the PayrollApplet has only one .java file—
PayrollApplet.java, but the compiler generates two .class
files—PayrollApplet.class and PayrollApplet$Payroll.class.
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.