Java(第8章面向对象(多态性for reading)

PrivateOverride.java StaticPolymorphism.java
Java编程基础
Downward Cast

向下转型Downward Cast

父类对象子类对象 必须明确地指明要转型的子类类型
子类 子类对象=(子类)父类实例

向下转型的前提条件

必须是曾经做过向上转型的对象，才能再向下转 型
Java编程基础
抽象类与接口

抽象类的实际应用——模板设计
Person Student Worker
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package ch08.abstract2; abstract class Person{ private String name; private int age; public Person(String name,int age){ =name; this.age=age; } public String getName() { return name;} public int getAge() {return age;} public abstract String getContent(); public void say(){ System.out.println(this.getContent()); } }
package ch08.abstract2; class Worker extends Person{ private float salary; public Worker(String name,int age,float salary){ super(name,age); this. salary = salary; } public String getContent(){ return "工人信息：姓名"+super.getName()+ "年龄："+super.getAge()+ "工资："+this. salary; } }


子类对象父类对象 程序会自动完成 向上转型损失了子类新扩展的属性和方法， 仅剩下父类中声明过的属性和方法 如果对象发生了向上转型关系后，所调用的 方法一定是被子类覆盖过的方法
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Java编程基础
Upcasting上溯造型
Upcastingtest.java
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Java编程基础
“overriding” private methods
package ch08.abstract2; public class AbstractDemo3 { public static void main(String[] args){ Person per1=new Student("张三",20,98); Person per2=new Worker("李四",33,3000); per1.say(); per2.say(); } }
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Java编程基础
接口
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Java编程基础
接口

Fields in interfaces、

接口中所有变量默认为public static final Fields defined in interfaces cannot be "blank finals," but they can be initialized with non-constant expressions
PolDemo2.java
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Java编程基础

instanceof操作符

instanceof操作符

判断一个对象到底是哪个类的实例 返回boolean类型
对象 instanceof 类
PolDemo3.java
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Java编程基础
instanceof操作符

在进行对象的向下转型关系前，最好先 判断后再进行相应的转换操作

“overriding” private methods—— private 方法能否被重写


private method is hidden from the derived class only non-private methods may be overridden overriding private methods, which generates no compiler warnings
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Java编程基础
package ch08.abstract1; public class AbstractDemo1 { public static void main(String[] args){ B b=new B(); b.print(); } }
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Java编程基础
抽象类

思考

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Java编程基础
接口
public interface Flyer { public void takeOff(); public void land(); public void fly(); } public class Airplane implements Flyer { public void takeOff() {……...} public void land() {……..} public void fly() {……..} }
public void doSomething(Employee e) { if (e instanceof Manager) { // Process a Manager } else if (e instanceof Engineer) { // Process an Engineer } else {// Process any other type of Employee} }

抽象类


包含抽象方法方法的类必须是抽象类 抽象类必须被继承，子类必须覆写override抽 象类中的全部抽象方法
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package ch08.abstract1; Java编程基础 abstract class A{ public static final String FLAG="China"; private String name="yangwei"; public String getName() {return name;} public void setName(String name) { = name; } public abstract void print(); } package ch08.abstract1; public class B extends A { public void print() { System.out.println("FLAG="+FLAG); System.out.println("姓名="+super.getName()); } }

通过对象的多态为抽象类实例化
Java编程基础
Java编程基础
——接口
杨巍
yangwei@
Java编程基础
接口Interfaces

接口Interfaces

特殊的类 由全局常量和公共的抽象方法所组成

接口定义
interface 接口名称{ 全局常量; public abstract 返回值类型 方法名称(参数); }
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Java编程基础
Covariant return types

Method Override

重写方法必须和被重写方法具有相同方法名 称、参数列表和返回类型 an overridden method in a derived class can return a type derived from the type returned by the base-class method
Java编程基础
Java编程基础
——对象的多态
杨巍
yangwei@
Java编程基础
多态性

多态性

方法的重载overload与覆写override 对象的多态性

具有继承关系的引用类型变量才能相互转换 子类对象父类对象 父类对象子类对象
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向上转型Upcasting
package ch08.abstract2; class Student extends Person{ private float score; public Student(String name,int age,float score){ super(name,age); this.score=score; } public String getContent(){ return "学生信息：姓名"+super.getName()+ "年龄："+super.getAge()+ "成绩："+this.score; } }
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Java编程基础
接口

接口

使用关键字interface定义 接口中定义的变量都是public static final 接口中定义的成员方法都是抽象方法 没有构造方法
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package ch08.interfacetest; interface A { public static final String AUTHOR="yangwei"; public abstract void print(); public abstract String getInfo(); } package ch08.interfacetest; interface { String AUTHOR="yangwei"; public void print(); public String getInfo(); }