Abstract Class, Packages and interface from Chapter 9 Lecture 10

Review Abstract classes Using final with inheritance Packages Access protection Importing packages

Abstract Classes Sometimes, we want to define a superclass that declares the structure of a abstraction with a complete implementation of every method. That is, we want to create a superclass that only defines a generalised form that will be shared by all of its subclasses.

More about Abstract Classes The main purpose of an abstract class is to define a common interface for it's subclasses to which some or all implementation is deferred. Abstract classes may be used in designing a family of related subclasses. The general form is: abstract type name (parameter-list)

Example of Abstract In an object-oriented drawing application, we can draw circles, lines, squares, and so on. Each of these graphic objects share certain states (position), and behavior (move, draw). You can take advantage of these similarities and declare them all to inherit from the same parent Graphic Object. Graphic Object circles lines square

Example of Graphic Object abstract class GraphicObject { int x, y; . . . void moveTo(int newX, int newY) { } abstract void draw(); class circle extends GraphicObject { void draw() {

Example –page 217

Explanation In object-oriented programming, we want to model an abstract concept without being able to create an instance of it. Here, class A defines an abstract class callme() {This class also supports a concrete method callme1(). } callme() is implemented in Class B.

Final Final with inheritance has three uses. First, it can be used to create the equivalent of a named constant Secondly, to apply final to prevent overriding Thirdly, to use final to prevent inheritance

Using final to prevent overriding Overriding is one of java’s most powerful features, there will be times when we will want to prevent it from occurring. To disallow a method from being overridden, we specify final as a modifier at the start of its declaration.

Example – with a compilation error Because show() is declared as final, it cannot be overridden. error

Using final to prevent inheritance Sometimes, we want to prevent a class from being inherited. To do this, we can precede the class declaration with final. Declaring a class as final implies that all of its methods as final.

Example – with a compilation error

Package Packages are one of the basic components of a Java Program. In general, a Java source can contain any of the following: A single package statement Any number of import statements A single public class declaration Any number of classes private to the package

Name of the package (directory), not class Create a package To create a package is easy. The user simply includes a package command as the first statement in a Java source file. The package statement defines a name space in which classes are package, which has no name. The format is: package pkg; Name of the package (directory), not class

Package Hierarchy directory The package hierarch is: package java.awt.image; It access the package in java/awt/image directory

Example (1) Make a directory called YourPackage under your current directory mkdir YourPackage Use a notepad to create a file as follows and put it under the directory of YourPackage

use notepad to create another one Example (2) Compile it use notepad to create another one YourPackage

Example (3) input: 4, output 4*4 = 16

More Example (1) Create a file on left-hand side. Put it into a directory called Mypack Compile it

More Example (2) The Class Path

Access Protection In Java, there are at least three control mechanism. private, public and protected Package adds another dimension to access control. Java provides many levels of protection over the visibility of variables and methods within classes, subclasses and packages

Importing Packages Java includes the import statement to bring certain classes or entire packages into visibility. Once imported, a class can be referred to directly, using only its name. The format is: import pkg1[.pkg2](classname)

Examples of Importing Packages import java.util.Date (import Date Package undre java, utility) import java.io.* (import IO packages under java)

Your Class is created under YourPackage Example (1) Your Class is created under YourPackage

Example (2) Create a file using import YourPackage.* (it will import any classes under this directory)

Your Class under YourPackage

More Example class add

More Example - Explanation Create a add class which has add and sub methods Create a lecture108 and import the add class Pass the value from agrs[0] and pass to add class to perform add and sub. Here, the add class consists of add() and sub() method

Summary Abstract classes – used as a family Using final with inheritance – the last method Packages – create class path Access protection – private, public protect (Packages) Importing packages – import other classes