Java Programming Fourth Edition. Chapter 12 Advanced Inheritance Concepts

Transcription

1 Java Programming Fourth Edition Chapter 12 Advanced Inheritance Concepts

2 Objectives Create and use abstract classes Use dynamic method binding Use a superclass as a method parameter Create arrays of subclass objects Java Programming, Fourth Edition 2

3 Objectives (continued) Use the Object class and its methods Use inheritance to achieve good software design Create and use interfaces Create and use packages Java Programming, Fourth Edition 3

4 Abstract class Creating and Using Abstract Classes Cannot create any concrete objects Can inherit Usually has one or more empty abstract methods Use keyword abstract Provide superclass from which other objects can inherit public abstract class Animal Java Programming, Fourth Edition 4

5 Creating and Using Abstract Classes (continued) Abstract method does not have: Body Curly braces Method statements To create abstract method Keyword abstract Header including method type, name, and arguments Include semicolon at end of declaration public abstract void speak(); Java Programming, Fourth Edition 5

6 Creating and Using Abstract Classes (continued) Subclass of abstract class Inherits abstract method from parent Must provide implementation for inherited method Or be abstract itself Code subclass method to override empty superclass method Java Programming, Fourth Edition 6

7 Using Dynamic Method Binding Every subclass object is a superclass member Convert subclass objects to superclass objects Can create reference to superclass object Create variable name to hold memory address Store concrete subclass object Animal ref; ref = new Cow(); Java Programming, Fourth Edition 7

8 Using Dynamic Method Binding (continued) Dynamic method binding Also called late binding Application s ability to select correct subclass method Makes programs flexible When application executes Correct method attached to application based on current one Java Programming, Fourth Edition 8

9 The AnimalReference Application Java Programming, Fourth Edition 9

10 Using a Superclass as a Method Parameter Use superclass as method argument Pass in subclass Use dynamic method binding public static void talkinganimal(animal animal) Dog dog = new Dog(); talkinganimal(dog); Java Programming, Fourth Edition 10

11 The TalkingAnimalDemo Class Java Programming, Fourth Edition 11

12 Output of TalkingAnimalDemo Application Java Programming, Fourth Edition 12

13 Creating Arrays of Subclass Objects Create superclass reference Treat subclass objects as superclass objects Create array of different objects Share same ancestry Creates array of three Animal references Animal[] ref = new Animal[3]; Reserve memory for three Animal object references Java Programming, Fourth Edition 13

14 Object Class Using the Object Class and Its Methods Every Java class extension of Object class Defined in java.lang package Imported automatically Includes methods to use or override Java Programming, Fourth Edition 14

15 Using the tostring() Method tostring() method Converts Object into String Contains information about Object Output Class sign Hash code Java Programming, Fourth Edition 15

16 Using the tostring() Method (continued) Write overloaded version of tostring() method Display some or all data field values for object Can be very useful in debugging a program Display tostring() value Examine contents Java Programming, Fourth Edition 16

17 The TestBankAccount Application Java Programming, Fourth Edition 17

18 Using the equals() Method equals() method Takes single argument Same type as invoking object Returns boolean value Indicates whether objects are equal Considers two objects of same class to be equal Only if they have same hash code Java Programming, Fourth Edition 18

19 Using the equals() Method (continued) Example of equals() method if(someobject.equals(someotherobjectofth esametype)) System.out.println("The objects are equal"); To consider objects to be equal based on contents Must write own equals() method Java Programming, Fourth Edition 19

20 Using the equals() Method (continued) Object method hashcode() Returns integer representing hash code Whenever you override equals() method Should override hashcode() method as well Equal objects should have equal hash codes Java Programming, Fourth Edition 20

21 The BankAccount Class Containing Its Own equals() Method Java Programming, Fourth Edition 21

22 Using Inheritance To Achieve Good Software Design Create powerful computer programs more easily If components used as is or slightly modified Make programming large systems more manageable Java Programming, Fourth Edition 22

23 Using Inheritance To Achieve Good Software Design (continued) Advantages of extendable superclasses Save development time Much code already written Save testing time Superclass code already tested Programmers understand how superclasses works Java Programming, Fourth Edition 23

24 Creating and Using Interfaces Multiple inheritance Inherit from more than one class Prohibited in Java Variables and methods in parent classes might have identical names Creates conflict Which class should super refer when child class has multiple parents? Java Programming, Fourth Edition 24

25 Interface Creating and Using Interfaces (continued) Alternative to multiple inheritance Looks like a class Except all methods and data items implicitly public, abstract, and final Description of what class does Declares method headers Java Programming, Fourth Edition 25

26 The Animal and Dog Classes and a Worker Interface Java Programming, Fourth Edition 26

27 The WorkingDog Class Java Programming, Fourth Edition 27

28 Creating and Using Interfaces Create an interface (continued) public interface Worker Implement an interface Keyword implements Requires subclass to implement own version of each method Use interface name in class header Requires class objects to include code public class WorkingDog extends Dog implements Worker Java Programming, Fourth Edition 28

29 Creating and Using Interfaces (continued) Abstract classes versus interfaces Cannot instantiate concrete objects of either Abstract classes Can contain nonabstract methods Provide data or methods that subclasses can inherit Subclasses maintain ability to override inherited methods Can include methods that contain actual behavior object performs Java Programming, Fourth Edition 29

30 Creating and Using Interfaces (continued) Abstract classes versus interfaces (continued) Interface Methods must be abstract Programmer knows what actions to include Every implementing class defines behavior that must occur when method executes Class can implement behavior from more than one parent Java Programming, Fourth Edition 30

31 Creating Interfaces to Store Related Constants Interfaces can contain data fields Must be public, static, and final Interfaces contain constants Provide set of data that many classes can use without having to redeclare values Java Programming, Fourth Edition 31

32 The PizzaConstants Interface Java Programming, Fourth Edition 32

33 Creating and Using Packages Package Named collection of classes Easily imports related classes into new programs Encourages other programmers to reuse software Helps avoid naming conflicts or collisions Give every package a unique name Java Programming, Fourth Edition 33

34 Creating and Using Packages (continued) Create classes for others to use Protect your work Do not provide users with source code in files with.java extensions Provide users with compiled files with.class extensions Include package statement at beginning of class file Place compiled code into indicated folder Java Programming, Fourth Edition 34

35 Creating and Using Packages (continued) Compile file to place in package Use Compiler option with javac command -d option places generated.class file in folder Use type-import-on-demand declaration import com.course.animals.* Or use separate import statements for each class Cannot import more than one package in one statement Java Programming, Fourth Edition 35

36 Creating and Using Packages Java ARchive (JAR) file Delivered to users (continued) Compress and store data Reduce size of archived class files Based on Zip file format Java Programming, Fourth Edition 36

37 You Do It Creating an abstract class Extending an abstract class Extending an abstract class with a second subclass Instantiating objects from subclasses Using object references Java Programming, Fourth Edition 37

38 You Do It (continued) Overriding the Object class equals() method Eliminating duplicate user entries Creating a package Java Programming, Fourth Edition 38

39 Summary Abstract class Class that you create only to extend from, but not to instantiate from Usually contain abstract methods Methods with no method statements Can convert subclass objects to superclass objects Dynamic method binding Create a method that has one or more parameters that might be one of several types Create an array of superclass object references but store subclass instances in it Java Programming, Fourth Edition 39

40 Summary (continued) Interface Similar to a class All methods implicitly public and abstract All of its data implicitly public, static, and final Create class that uses interface Include keyword implements and interface name in class header Place classes in packages Convention uses Internet domain names in reverse order Java Programming, Fourth Edition 40