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Abstract Class and Abstract Class and Packages Packages from Chapter 9from Chapter 9

Abstract Class and Abstract Class and Packages Packages from Chapter 9from Chapter 9

Lecture Lecture

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Review

Abstract classesUsing final with inheritancePackagesAccess protectionImporting packages

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Abstract ClassesSometimes, 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.

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More about Abstract ClassesThe 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)

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Example –page 217

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ExplanationIn 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.

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Final

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

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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.

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Example – with a compilation error

Because show() is declared as final,

it cannot be overridden.

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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.

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Example – with a compilation error

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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 statementAny number of import statementsA single public class declarationAny number of classes private to the package

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Create a packageTo 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

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Package Hierarchy

The package hierarch is:package java.awt.image;

It access the package in java/awt/image

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Manual Method Example (1)Make a directory called YourPackage under your current directorymkdir YourPackage

Use a notepad to create a file as follows and put it under the directory of YourPackage

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Example (2)Compile it

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Example (3)

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More Example (1)

Create a file on left-hand side.Put it into a directory called MypackCompile it

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More Example (2)

Creating Packages Using Eclipse

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Snap for Creating Package

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Viewing the newly created Package

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

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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)

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Examples of Importing Packages

import java.util.Date(import Date Package undre java, utility)

import java.io.*(import IO packages under java)

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Example (1)

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

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Your Class under YourPackage

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More Example

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More Example - ExplanationCreate a add class which has add and sub methodsCreate a lecture108 and import the add classPass the value from agrs[0] and pass to add class to perform add and sub.

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SummaryAbstract classes – used as a familyUsing final with inheritance – the last methodPackages – create class pathAccess protection – private, public protect (Packages)Importing packages – import other classes