《继承和多态》PPT课件.ppt

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1、第09章 继承和多态,章节内容,9.1 引言9.2父类和子类9.3 使用super关键字9.4 方法覆盖9.5 覆盖和重载9.6 对象类object和它的tostring方法9.7 多态性、动态绑定和一般程序设计9.8 对象转换和instanceof运算符9.9 数组线性表ArrayList类9.10自定义栈类9.11 protected数据和方法9.12 final类、方法和变量,9.1 引言,在面向对象中，从已有的类派生出新类，叫做继承。超类=父类=基类次类=子类=扩展类=派生类Java中使用关键字extends来实现类的继承。例如：class Circle extends Geometr

2、icObject 类体；创建了一个Circle类，它继承自GeometricObject类。子类继承了父类所有的成员，还可以包含自己的数据成员和方法成员,9.2父类和子类,9.2父类和子类父类GeometricObject,public class GeometricObject private String color=white;private boolean filled;private dateCreated;public GeometricObject()dateCreated=new();public String getColor()return color;public voi

3、d setColor(String color)this.color=color;public boolean isFilled()return filled;,public void setFilled(boolean filled)this.filled=filled;public getDateCreated()return dateCreated;public String toString()return created on+dateCreated+ncolor:+color+and filled:+filled;,9.2父类和子类子类Circle,public class Cir

4、cle extends GeometricObject private double radius;public Circle()public Circle(double radius)this.radius=radius;public double getRadius()return radius;public void setRadius(double radius)this.radius=radius;,public double getArea()return radius*radius*Math.PI;public double getDiameter()return 2*radiu

5、s;public double getPerimeter()return 2*radius*Math.PI;public void printCircle()System.out.println(The circle is created+getDateCreated()+and the radius is+radius);,9.2父类和子类 子类Rectangle,public class Rectangle extends GeometricObject private double width;private double height;public Rectangle()public

6、Rectangle(double width,double height)this.width=width;this.height=height;public double getWidth()return width;public void setWidth(double width)this.width=width;,public double getHeight()return height;public void setHeight(double height)this.height=height;public double getArea()return width*height;p

7、ublic double getPerimeter()return 2*(width+height);,9.2 父类和子类TestCircle测试类,/TestCircle.java:Demonstrate private modifierpublic class TestCircle/*Main method*/public static void main(String args)/Create a Circle with radius 5.0 Circle myCircle=new Circle(5.0);System.out.println(The area of the circle

8、 of radius+myCircle.getRadius()+is+myCircle.getArea();/Increase myCircles radius by 10%myCircle.setRadius(myCircle.getRadius()*1.1);System.out.println(The area of the circle of radius+myCircle.getRadius()+is+myCircle.getArea();,9.3 使用super关键字,super关键字的功能调用父类的构造方法：super();super(参数);必须出现在子类构造方法的第一行调用父

9、类的方法：super.方法名（参数）；return 2*super.findArea()+(2*getRadius()*Math.PI)*length;,9.3 使用super关键字,9.3.2 构造方法链创建派生类对象时，要首先创建其中包含的基类子对象，也就是说在派生类的构造方法中，要首先调用基类的构造方法。通常，派生类构造方法中的第一条语句都是使用关键字super调用基类的构造方法；如果没有使用关键字super显式地调用基类的构造方法，则编译器将向派生类构造方法中添加一条语句super(),调用基类默认的构造方法，它是派生类构造方法的第一条语句。在任何情况下，构造一个类的实例时，将会沿着继

10、承链调用所有基类的构造方法,public class Faculty extends Employee public static void main(String args)new Faculty();public Faculty()System.out.println(4)Facultys no-arg constructor is invoked);class Employee extends Person public Employee()this(2)Invoke Employees overloaded constructor);System.out.println(3)Emplo

11、yees no-arg constructor is invoked);public Employee(String s)System.out.println(s);class Person public Person()System.out.println(1)Persons no-arg constructor is invoked);,Trace Execution,public class Faculty extends Employee public static void main(String args)new Faculty();public Faculty()System.o

12、ut.println(4)Facultys no-arg constructor is invoked);class Employee extends Person public Employee()this(2)Invoke Employees overloaded constructor);System.out.println(3)Employees no-arg constructor is invoked);public Employee(String s)System.out.println(s);class Person public Person()System.out.prin

13、tln(1)Persons no-arg constructor is invoked);,1.Start from the main method,animation,Trace Execution,public class Faculty extends Employee public static void main(String args)new Faculty();public Faculty()System.out.println(4)Facultys no-arg constructor is invoked);class Employee extends Person publ

14、ic Employee()this(2)Invoke Employees overloaded constructor);System.out.println(3)Employees no-arg constructor is invoked);public Employee(String s)System.out.println(s);class Person public Person()System.out.println(1)Persons no-arg constructor is invoked);,2.Invoke Faculty constructor,animation,Tr

15、ace Execution,public class Faculty extends Employee public static void main(String args)new Faculty();public Faculty()System.out.println(4)Facultys no-arg constructor is invoked);class Employee extends Person public Employee()this(2)Invoke Employees overloaded constructor);System.out.println(3)Emplo

16、yees no-arg constructor is invoked);public Employee(String s)System.out.println(s);class Person public Person()System.out.println(1)Persons no-arg constructor is invoked);,3.Invoke Employees no-arg constructor,animation,Trace Execution,public class Faculty extends Employee public static void main(St

17、ring args)new Faculty();public Faculty()System.out.println(4)Facultys no-arg constructor is invoked);class Employee extends Person public Employee()this(2)Invoke Employees overloaded constructor);System.out.println(3)Employees no-arg constructor is invoked);public Employee(String s)System.out.printl

18、n(s);class Person public Person()System.out.println(1)Persons no-arg constructor is invoked);,4.Invoke Employee(String)constructor,animation,Trace Execution,public class Faculty extends Employee public static void main(String args)new Faculty();public Faculty()System.out.println(4)Facultys no-arg co

19、nstructor is invoked);class Employee extends Person public Employee()this(2)Invoke Employees overloaded constructor);System.out.println(3)Employees no-arg constructor is invoked);public Employee(String s)System.out.println(s);class Person public Person()System.out.println(1)Persons no-arg constructo

20、r is invoked);,5.Invoke Person()constructor,animation,Trace Execution,public class Faculty extends Employee public static void main(String args)new Faculty();public Faculty()System.out.println(4)Facultys no-arg constructor is invoked);class Employee extends Person public Employee()this(2)Invoke Empl

21、oyees overloaded constructor);System.out.println(3)Employees no-arg constructor is invoked);public Employee(String s)System.out.println(s);class Person public Person()System.out.println(1)Persons no-arg constructor is invoked);,6.Execute println,animation,Trace Execution,public class Faculty extends

22、 Employee public static void main(String args)new Faculty();public Faculty()System.out.println(4)Facultys no-arg constructor is invoked);class Employee extends Person public Employee()this(2)Invoke Employees overloaded constructor);System.out.println(3)Employees no-arg constructor is invoked);public

23、 Employee(String s)System.out.println(s);class Person public Person()System.out.println(1)Persons no-arg constructor is invoked);,7.Execute println,animation,Trace Execution,public class Faculty extends Employee public static void main(String args)new Faculty();public Faculty()System.out.println(4)F

24、acultys no-arg constructor is invoked);class Employee extends Person public Employee()this(2)Invoke Employees overloaded constructor);System.out.println(3)Employees no-arg constructor is invoked);public Employee(String s)System.out.println(s);class Person public Person()System.out.println(1)Persons

25、no-arg constructor is invoked);,8.Execute println,animation,Trace Execution,public class Faculty extends Employee public static void main(String args)new Faculty();public Faculty()System.out.println(4)Facultys no-arg constructor is invoked);class Employee extends Person public Employee()this(2)Invok

26、e Employees overloaded constructor);System.out.println(3)Employees no-arg constructor is invoked);public Employee(String s)System.out.println(s);class Person public Person()System.out.println(1)Persons no-arg constructor is invoked);,9.Execute println,animation,9.4 方法覆盖,当子类 从父类中继承方法，子类必须修改父类中定义的方法，就

27、叫做方法覆盖（methord overriding）,public class Circle extends GeometricObject/Other methods are omitted/*Override the toString method defined in GeometricObject*/public String toString()return super.toString()+nradius is+radius;,9.5 覆盖和重载,9.6 对象类Object和它的toString（）方法,如果定义一个类时,没有指明它的父类,则它的父类就是Object类。Object

28、中几个常用的实例方法：public String toString()public boolean equals(Object object)（见9.13）public Object clone()（见9.13）public int hashCode()（见9.13）public Class getClass()（见9.13）public String toString()返回值代表一个该对象的字符串，默认情况下，返回一个由该对象所属的类名、和该对象十六进制的散列码组成的字符串一般而言，每个类继承toString需要对该函数进行重载，如GeometricObject类中的toString 修改

29、如下：public String toString()return“color:”+color+“and is filled:”+filled;,9.7 多态性、动态绑定和一般程序设计,对于需要父类类型的参数，总可以传入子类的实例；实现的那一种实现由java虚拟机在运行时动态决定，称为动态绑定或者多态性。绑定机制如下：假设对象o是类c1，c2，.,Cn-1,Cn的实力，其中C1是C2的子类，C2是C3的子类，如果对象O调用一个方法p，JVM依次在类C1，C2，,Cn-1,Cn中查找方法P，直到找到为止。一旦找到，停止查找并调用这个第一次的实现。,9.7 多态性、动态绑定和一般程序设计（实例9-

30、5）,public class PolymorphismDemo public static void main(String args)m(new GraduateStudent();m(new Student();m(new Person();m(new Object();public static void m(Object x)System.out.println(x.toString();class GraduateStudent extends Student class Student extends Person public String toString()return S

31、tudent;class Person extends Object public String toString()return Person;,9.8 对象转换和instanceof运算符,原则：派生类对象可以复制给基类对象（向上转换），基类对象赋值给派生类对象时必须显式转换（向下转化）Object o=new Student()/隐式转换Student b=o/error Student b=(Student)o;为了确保转换的安全，好的关是转换前确保该对象是另一个对象的实例，使用instanceof运算符,9.8 对象转换和instanceof运算符（举例）,public class

32、TestPolymorphismCasting public static void main(String args)Object object1=new Circle(1);Object object2=new Rectangle(1,1);displayObject(object1);displayObject(object2);public static void displayObject(Object object)if(object instanceof Circle)System.out.println(The circle area is+(Circle)object).ge

33、tArea();System.out.println(The circle diameter is+(Circle)object).getDiameter();else if(object instanceof Rectangle)System.out.println(The rectangle area is+(Rectangle)object).getArea();,9.9 数组线性表ArrayList类,ArrayList：存储不限个数的对象9-7例要点：String是Object的子类，因此可以加入到ArrayList中,实例9-7,public class TestArrayList

34、 public static void main(String args)cityList=new();cityList.add(London);cityList.add(New York);cityList.add(Paris);cityList.add(Toronto);cityList.add(Hong Kong);cityList.add(Singapore);System.out.println(List size?+cityList.size();System.out.println(Is Toronto in the list?+cityList.contains(Toronto

35、);System.out.println(The location of New York in the list?+cityList.indexOf(New York);System.out.println(Is the list empty?+cityList.isEmpty();cityList.add(2,Beijing);cityList.remove(Toronto);cityList.remove(1);for(int i=0;i cityList.size();i+)System.out.print(cityList.get(i)+);();list=new();list.ad

36、d(new Circle(2);list.add(new Circle(3);System.out.println(The area of the circle?+(Circle)list.get(0).getArea();,9.10 自定义栈类（自学）,public class MyStack private list=new();public boolean isEmpty()return list.isEmpty();public int getSize()return list.size();public Object peek()return list.get(getSize()-1

37、);,public Object pop()Object o=list.get(getSize()-1);list.remove(getSize()-1);return o;public Object push(Object o)list.add(o);return o;public int search(Object o)return list.lastIndexOf(o);public String toString()return stack:+list.toString();,9.11 protected数据和方法（重要）,protectd修饰符：修饰类中的成员。保护的成员或方法可以被

38、子类或同一包中的任何类访问，即使子类在不同包中也可以。private默认protectedpublic，可见性依次增强private、protected仅仅用于成员public和默认修饰符用于成员，也可用于类。默认修饰符类不能被其他包里的类访问,9.11 protected数据和方法（重要）,9.12 final类、方法和变量,final：说明该类是终极类，不能做父类MathStringStringBuffer,9.13 对象Object中的方法（自学）,equals方法作用：检验两个Objcct类的派生类对象是否相等。equals方法在Objcct类中的默认实现如下：public boole

39、an equals(Object obj)return(this=obj);所以要让equals方法比较两个派生类对象的内容是否相等，必须在派生类中覆盖该方法。,9.13 对象Object中的方法（自学）,hashCode方法返回对象的散列代码。对象的散列码是一个整数，用于在散列集合中存储并快速地查找对象。Object类中实现的hashCode方法，返回对象的十六进制内存地址。一个类中若覆盖了equals方法，则应该同时覆盖hashCode方法，因为根据约定，如果两个对象的内容相等，它们的散列码一定相同。,9.13 对象Object中的方法,clone方法作用：把一个对象的内容复制给另一个对象。例如：newObj=new someObj.clone();不是所有类的对象都可以被克隆，要成为一个可克隆的对象，它的类必须实现接口。,9.13 对象Object中的方法,getClass方法一个类在使用时必须先装入。Java虚拟机装入类时，创建一个包含类信息的对象，这些信息有类名、构造方法和方法等。这个对象是类的一个实例，它描述有关类的信息，称为元对象。通过元对象，可以在运行时得到类的信息。每个对象都可以使用getClass()方法返回它的元对象。,结束,