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Abstract Class and Packages from Chapter 9

Abstract Class and Packages from Chapter 9. Lecture. Review. Abstract classes Using final with inheritance Packages Access protection Importing packages. Abstract Classes.

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Abstract Class and Packages from Chapter 9

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  1. Abstract Class and Packages from Chapter 9 Lecture

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

  3. 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.

  4. 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)

  5. Example –page 217

  6. 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.

  7. 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

  8. 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.

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

  10. 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.

  11. Example – with a compilation error error

  12. 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

  13. 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

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

  15. Manual Method 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

  16. Example (2) • Compile it YourPackage

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

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

  19. More Example (2) The Class Path

  20. Creating Packages Using Eclipse

  21. Snap for Creating Package

  22. Viewing the newly created Package

  23. 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

  24. 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)

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

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

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

  28. Your Class under YourPackage

  29. More Example class add

  30. 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

  31. 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

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