5.1 继承
class A
{ int i,j;void showIJ(){ System.out.println("I and J: "+i+" "+j);}}class B extends A
{void showK(int k){System.out.println("I+J+K="+(i+j+k)); }}
class SimpleInher
{public static void main(String args[]){ A superOb=new A();B subOb=new B();superOb.i=12; superOb.j=121;superOb.showIJ();subOb.showIJ(); subOb.showK(3);}}二、子类的继承性
- 若子类与父类在同一个包中，则子类可以继承父类非私有的成员变量或成员方法，并保持访问权限不变。
- 若子类与父类不在同一个包中，则子类可以继承父类共有的和受保护的成员变量或成员方法，并保持访问权限不变。
访问权限
不能，因为main是静态方法，只能调用同一个类中的其它静态方法，对同一个类中的非静态方法的调用必须使用对象名。
输出结果是：1000
1000
若f()方法的money=1000；改为 int money=1000；则输出结果为1000 / 120
package tom.jiafei;
public class Father
{ int height;protected int money=120;protected int getMoney() { return money;}void setMoney(int newMoney) { money=newMoney;}
}
package sun.com;
import tom.jiafei.Father;
public class Jerry extends Father
{ void f(){ money=1000; height=1.89f; System.out.println(money); setMoney(300); int number=getMoney(); System.out.println(number);}public static void main(String args[]) { Jerry jerry=new Jerry();jerry.f();}
}
class SuperClass
{ public int a(int x){ return x*x;}}class SubClass extends SuperClass
{ public int a(int y){ return super.a(y)+y; }}class Recover
{ public static void main(String args[]){int m=-12;SubClass sub_a=new SubClass();System.out.println(sub_a.a(m));}
}
class CallingCons
{ public static void main(String args[]){C xxxx=new C(); }
}
class A
{A(){ System.out.println("Inside A's constructor.");}}
class B extends A
{B(){ System.out.println("Inside B's constructor.");}
}
class C extends B
{C(){ System.out.println("Inside C's constructor.");}
}
class CallingCons
{ public static void main(String args[]){C xxxx=new C(); }
}
class A
{ A(int x){ System.out.println("x="+x);}
}
class B extends A
{B(){ super(3); System.out.println("Inside B's constructor.");}
}
class C extends B
{C(){ System.out.println("Inside C's constructor.");}
}三、super关键字
访问权限
访问权限与继承
5.2 对象的上转型对象
二、上转型对象的特点
很重要！ 如果子类重写了父类的静态方法，那么子类对象的上转型对象不能调用子类重写的静态方法，只能调用父类的静态方法。
例1：
abstract class Figure
{ double dim1,dim2;Figure(double a,double b){ dim1=a;dim2=b;}abstract double area();}
class Rectangle extends Figure
{ Rectangle(double a,double b){ super(a,b); }double area(){ System.out.println("Inside Area for Rectangle.");return dim1*dim2;}
}
class Triangle extends Figure
{ Triangle(double a,double b){ super(a,b); }double area(){ System.out.println("Inside Area for Triangle.");return dim1*dim2/2;}
}
public class AbstractAreas
{ public static void main(String args[]){ Rectangle r=new Rectangle(9,5);Triangle t=new Triangle(10,8);Figure figref;figref=r;System.out.println("Area is "+figref.area());figref=t;System.out.println("Area is "+figref.area()); }
}例2：
class Box
{ double width,height,depth;Box(){ width=1; height=1; depth=1; }double volume(){ return width*height*depth; }
}
class BoxWeight extends Box
{ double weight;BoxWeight(double w,double h,double d,double m){ width=w; height=h; depth=d; weight=m;}double getWeight(){ return weight*10; }
}
class RefDome
{ public static void main(String args[]){ Box plainbox=new Box();BoxWeight weightbox=new BoxWeight(3,5,7,8.37);System.out.println("Volume of weightbox is "+weightbox.volume());System.out.println("Weight of weightbox is "+weightbox.weight);System.out.println("Weight of weightbox is "+weightbox.getWeight()); System.out.println(" Volume of plainbox is "+ plainbox.volume() ); plainbox=weightbox; System.out.println("Volume of plainbox is "+plainbox.volume() );System.out.println("Weight of plainbox is "+plainbox.weight); //错System.out.println("Weight of plainbox is "+plainbox.getWeight()); //错}
}注：
- 不要将父类创建的对象和子类创建的对象的上转型对象混淆；
- 可以将对象的上转型对象再强制转换到一个子类对象，这时，该子类对象又具备了子类所有的属性和功能；
- 不可以将父类创建的对象的引用赋值给子类声明的对象。
A r;改成：B r;或C r; 不可以！！！
class A
{ void callme(){System.out.println("Inside A's callme methods"); }}
class B extends A
{ void callme(){System.out.println("Inside B's callme methods"); }}
class C extends A
{ void callme(){System.out.println("Inside C's callme methods"); }}
class Dispatch
{ public static void main(String args[]){ A r;r=new A();r.callme();r=new B();r.callme();r=new C();r.callme();}}
5.3 多态与重载
构造方法重载
class Sub_Over
{ int i;Sub_Over(){System.out.println("this is the first constructor");}Sub_Over(int n){ this();i=n;System.out.println("this is another constructor");System.out.println("the parameter n value is: "+i);}}public class OverLoad
{ public static void main(String args[]){ Sub_Over mm=new Sub_Over(12); }
}输出结果：
this is the first constructor
this is another constructor
the parameter n value is: 12
二、方法重载
- 同一个类（或子类与父类）中的两个或两个以上的方法共用一个名字，但参数不同，这样的方法被称为重载。（overloaded）；
- 构造方法也可以重载，这些构造方法可以互相调用，一个方法调用另一个构造方法时，使用关键字this，同时，这个调用语句应该是该构造方法的第一个可执行语句。
【练习1】
class A
{ double f(double x, float y){return x+y;}double f(float x,float y){return x*y;}
}public class E
{ public static void main(String args[]){ A a=new A();System.out.println(“**”+a.f(10,10));System.out.println(“##”+a.f(10.0d,10.0f));}
}说出运行结果：
**100
##10
【练习2】
class A
{ double f(double x, double y){return x+y;}
}class B extends A
{ double f(int x,int y){return x*y;}
}
public class E
{ public static void main(String args[ ]){ B b=new B();System.out.println(b.f(3,5));System.out.println(b.f(3.0,5.0));}
}运行结果：
15.0
8.0
【练习3】
class A
{ double f(double x, double y){return x+y;}static int g(int n){return n*n;}
}class B extends A
{ double f(double x,double y){ double m=super.f(x,y);return m+x*y;}static int g(int n){ int m=A.g(n);return m+n;}
}
public class E
{ public static void main(String args[]){ B b=new B();System.out.println(b.f(10.0,8.0));System.out.println(b.g(b.g(3)));}
}运行结果：
98.0
12
上机实践4-实验1 继承
Example.java
class People
{protected double weight,height;public void speakHello(){System.out.println("yayawawa");} public void averageHeight(){ height=173;System.out.println("average height:"+height);}public void averageWeight(){weight=70;System.out.println("average weight:"+weight);}
}
class ChinaPeople extends People
{
【代码1】
//重写public void speakHello()方法，要求输出类似“你好，吃了吗”这样的汉语信息
【代码2】 //重写public void averageHeight()方法，要求输出类似中国人的平均身高：168.78厘米”这样的汉语信息
【代码3】 //重写public void averageWeight()方法，//要求输出类似“中国人的平均体重：65公斤”这样的汉语信息public void chinaGongfu(){【代码4】//输出中国武术的信息，例如："坐如钟,站如松,睡如弓"等}
}
class AmericanPeople extends People
{
【代码5】 //重写public void speakHello()方法，要求输出类似 “How do you do”这样的英语信息。
【代码6】 //重写public void averageHeight()方法
【代码7】 //重写public void averageWeight()方法public void americanBoxing(){【代码8】//输出拳击的信息，例如，“直拳”、“钩拳”等}
class BeijingPeople extends ChinaPeople
{
【代码9】 //重写public void speakHello()方法，要求输出类似“您好”这样的汉语信息
【代码10】 //重写public void averageHeight()方法
【代码11】 //重写public void averageWeight()方法public void beijingOpera() {【代码12】//输出京剧的信息}
}
public class Example
{public static void main(String args[]){ChinaPeople chinaPeople=new ChinaPeople();AmericanPeople americanPeople=new AmericanPeople();BeijingPeople beijingPeople=new BeijingPeople();chinaPeople.speakHello();americanPeople.speakHello();beijingPeople.speakHello();chinaPeople.averageHeight();americanPeople.averageHeight();beijingPeople.averageHeight();chinaPeople.averageWeight();americanPeople.averageWeight();beijingPeople.averageWeight();chinaPeople.chinaGongfu();americanPeople.americanBoxing();beijingPeople.beijingOpera() ;beijingPeople.chinaGongfu();}
}上机实践4-实验2 上转型对象
HardWork.java
abstract class Employee
{public abstract double earnings();
}
class YearWorker extends Employee
{【代码1】 //重写earnings()方法
}
class MonthWorker extends Employee
{【代码2】 //重写earnings()方法。
}
class WeekWorker extends Employee
{【代码3】 //重写earnings()方法。
}
class Company
{Employee[] employee;double salaries=0;Company(Employee[] employee){this.employee=employee;}public double salariesPay(){salaries=0;【代码4】 //计算salaries。return salaries;}
}
public class HardWork
{public static void main(String args[]){Employee[] employee=new Employee[20];for(int i=0;i<employee.length;i++){if(i%3==0)employee[i]=new WeekWorker();else if(i%3==1)employee[i]=new MonthWorker();else if(i%3==2)employee[i]=new YearWorker();} Company company=new Company(employee);System.out.println("公司年工资总额:"+company.salariesPay());}
}