《继承和多态》PPT课件.ppt

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 GeometricObject 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 java.util.Date dateCreated; public GeometricObject() dateCreated = new java.util.Date(); public String getColor() return color; public void setColor(String color) this.color = color; public boolean isFilled() return filled; ,public void setFilled(boolean filled) this.filled = filled; public java.util.Date 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 Circle 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 * radius; 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 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; public double getPerimeter() return 2 * (width + height); ,9.2 父类和子类TestCircle测试类,/ TestCircle.java: Demonstrate private modifier public 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 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) 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“); ,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 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“); ,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 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 constructor is invoked“); ,2. Invoke Faculty 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 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“); ,3. Invoke Employees no-arg 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 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“); ,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 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 no-arg constructor 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 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“); ,6. 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) 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“); ,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) 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.println(“(1) Persons 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) 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“); ,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9-5）,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 “Student“; 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 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).getArea(); 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 public static void main(String args) java.util.ArrayList cityList = new java.util.ArrayList(); 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“); 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) + “ “); System.out.println(); java.util.ArrayList list = new java.util.ArrayList(); list.add(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 java.util.ArrayList list = new java.util.ArrayList(); public boolean isEmpty() return list.isEmpty(); public int getSi