课后作业

课后作业

“类型转换”知识点考核-2

程序源代码：

class Mammal{}
class Dog extends Mammal {}
class Cat extends Mammal{}
 
public class TestCast
{
     public static void main(String args[])
     {
            Mammal m;
            Dog d=new Dog();
            Cat c=new Cat();
            m=d;
            //d=m;
            d=(Dog)m;
            //d=c;
            //c=(Cat)m;
 
     }
}

下列语句哪一个将引起编译错误?为什么?哪一个会引起运行时错误?为什么?

             m=d;

             d=m;

             d=(Dog)m;

             d=c;

             c=(Cat)m;

先进行自我判断，得出结论后，运行TestCast.java实例代码，看看你的判断是否正确。

答： d=m;和d=c;会引起错误。d = m错误的是没有对M强制类型转化。d=c错误是因为d 和c类型不同导致错误。

实验二

程序源代码：

public class ParentChildTest {
     public static void main(String[] args) {
            Parent parent=new Parent();
            parent.printValue();
            Child child=new Child();
            child.printValue();
            
            parent=child;
            parent.printValue();
            
            parent.myValue++;
            parent.printValue();
            
            ((Child)parent).myValue++;
            parent.printValue();
            
     }
}
 
class Parent{
     public int myValue=100;
     public void printValue() {
            System.out.println("Parent.printValue(),myValue="+myValue);
     }
}
class Child extends Parent{
     public int myValue=200;
     public void printValue() {
            System.out.println("Child.printValue(),myValue="+myValue);
     }
}

回答问题：

1.    左边的程序运行结果是什么？

2.    如何解释会得到这样的输出？

当子类与父类拥有一样的方法，并且让一个父类变量引用一个子类对象时，到底调用哪个方法，由对象自己的“真实”类型所决定，这就是说：对象是子类型的，它就调用子类型的方法，是父类型的，它就调用父类型的方法。

这个特性实际上就是面向对象“多态”特性的具体表现。

3.   计算机是不会出错的，之所以得到这样的运行结果也是有原因的，那么从这些运行结果中，你能总结出Java的哪些语法特性？

如果子类与父类有相同的字段,则子类中的字段会代替或隐藏父类的字段，子类方法中访问的是子类中的字段（而不是父类中的字段）。如果子类方法确实想访问父类中被隐藏的同名字段，可以用super关键字来访问它。

如果子类被当作父类使用,则通过子类访问的字段是父类的!

用多态的方法模拟ATM操作流程。

程序源代码：

package ATM;
import java.util.Scanner;
abstract class drawmoney{
       public abstract double getMoney(double a,double n);
}
class drawmoney100 extends drawmoney{
       public double getMoney(double a,double n){
              if(a<100){
                     System.out.println("余额不足，请选择其他业务");
              return a;
              }
              else {
                     System.out.println("已吐出100元");
                     return a-=100;
              }
       }
}
class drawmoney500 extends drawmoney{
       public double getMoney(double a,double n){
              if(a<500){
                     System.out.println("余额不足，请选择其他业务");
              return a;
              }
              else {
                     System.out.println("已吐出500元");
                     return a-=500;
              }
       }
}
class drawmoney1000 extends drawmoney{
       public double getMoney(double a,double n){
              if(a<1000){
                     System.out.println("余额不足，请选择其他业务");
              return a;
              }
              else {
                     System.out.println("已吐出1000元");
                     return a-=1000;
              }
       }
}
class drawmoney1500 extends drawmoney{
       public double getMoney(double a,double n){
              if(a<1500){
                     System.out.println("余额不足，请选择其他业务");
              return a;
              }
              else {
                     System.out.println("已吐出1500元");
                     return a-=1500;
              }
       }
}
class drawmoney2000 extends drawmoney{
       public double getMoney(double a,double n){
              if(a<2000){
                     System.out.println("余额不足，请选择其他业务");
              return a;
              }
              else {
                     System.out.println("已吐2000元");
                     return a-=2000;
              }
       }
}
class drawmoney5000 extends drawmoney{
       public double getMoney(double a,double n){
              if(a<5000){
                     System.out.println("余额不足，请选择其他业务");
              return a;
              }
              else {
                     System.out.println("已吐出5000元");
                     return a-=5000;
              }
       }
}
class drawmoneyn extends drawmoney{
       public double getMoney(double a,double n){
              if(a<n){
                     System.out.println("余额不足，请选择其他业务");
              return a;
              }
              else {
                     System.out.println("已吐出"+n+"元");
                     return a-=n;
              }
       }
}
class Account{
       String account;
       String usename;
       int password;
       String date;
       double balance;
       int Operation;
       public Account(){
               account="accout1805231593";
               usename="王凤彬";
               password=521114;
               date="1996.11.11";
               balance=230.15;
               Operation=0;
       }
       public Account(String a,String b,int c,String d,double e,int f){
               account=a;
               usename=b;
               password=c;
               date=d;
               balance=e;
               Operation=f;
       }
}
public class ATM {
 
        public Account h=new Account();
        public boolean shurumina(int a){
               if(a==h.password) { 
               System.out.println("密码正确");
               return true;}
               else System.out.println("密码错误,请重新输入");
               return false;
        }
        public void jiemian(boolean a){
               Scanner cin=new Scanner(System.in);
       while(a){
                 System.out.println("1.存款");
                     System.out.println("2.取款");
                     System.out.println("3.转账汇款");
                     System.out.println("4.修改密码");
                     System.out.println("5.查询余额");
                     System.out.println("6.退卡");
                     int b=cin.nextInt();
                     switch (b){
                     case 1:deposit(b);break;
                     case 2:withdrawals(b);break;
                     case 3:transfer(b);break;
                     case 4:Passwordservice(b);break;
                     case 5:System.out.println(h.balance);break;
                     case 6:a=false;break;
                     }
       }
                     
}
        public int withdrawals(int a){
               Scanner cin=new Scanner(System.in);
                     System.out.println("1.100元");
                     System.out.println("2.500元");
                     System.out.println("3.1000元");
                     System.out.println("4.1500元");
                     System.out.println("5.2000元");
                     System.out.println("6.5000元");
                     System.out.println("7.其他金额");
                     System.out.println("8.返回");
                     int b=cin.nextInt();
                     double c=0;
                     drawmoney m=null;
                     switch (b){
                     case 1:m= new drawmoney100();m.getMoney(h.balance, b);break;
                     case 2:m= new drawmoney500();m.getMoney(h.balance, b);break;
                     case 3:m= new drawmoney1000();m.getMoney(h.balance, b);break;
                     case 4:m= new drawmoney1500();m.getMoney(h.balance, b);break;
                     case 5:m= new drawmoney2000();m.getMoney(h.balance, b);break;
                     case 6:m= new drawmoney5000();m.getMoney(h.balance, b);break;
                     case 7:{
                            System.out.println("输入金额");
                            c=cin.nextDouble();
                            m= new drawmoneyn();
                            m.getMoney(h.balance, c);break;                  }
                     case 8: return a;
                     }
               return a;
        }
       public int deposit(int a){
               System.out.println("放入人民币");
               System.out.println("输入金额");
               Scanner cin=new Scanner(System.in);
               int b=cin.nextInt();
               h.balance+=b;
               System.out.println("已成功");
               jiemian(true);
               return a;
        }
        public int transfer(int a){
               System.out.println("请输入银行账号");
               Scanner cin=new Scanner(System.in);
              String b=cin.next();
              System.out.println("输入转账的金额");
              int c=cin.nextInt();
              h.balance-=c;
              System.out.println("已成功");
              jiemian(true);
              return 0;
        }
        public int Passwordservice(int a){
               System.out.println("请输入新密码");
               Scanner cin=new Scanner(System.in);
              int b=cin.nextInt();
              h.password-=b;
              System.out.println("已成功");
              return 0;
        }
       public static void main(String[] args) {
              // TODO Auto-generated method stub
              ATM mm=new ATM();
              
              String out=new String();
              out+="请插入银行卡";
              System.out.println(out);
              System.out.println("已成功");
              out+="请输入密码";
              System.out.println(out);
              Scanner cin=new Scanner(System.in);
              int a=cin.nextInt();
              while (!mm.shurumina(a)){
                      a=cin.nextInt();
                      
              }
           mm.jiemian(true);
              
       }
 
}

程序分析：ATM机程序的取款进行多态重写，采用了父类调用子类的方法进行重写。

程序截图：