3.第6章——增添字符串、对象数组
改动的一些点：本章实例中增加了Date类，对照前两章做出java的一些基本类格式的改动即可。此外的改动有：
（1）java中字符串类型不需要像C++那样还要调用std::string，而直接使用默认包lang中String类。
（2）在使用Date对象引用作参数时，由于java没有引用类型的符号&，所以在作函数形参时直接使用Date对象。
（3）在创建Date对象时，java不能直接使用类名创建类的对象，而需要用Date date=new Date(int,int,int)才能声明并初始化一个Date对象，同样当需要创建一个Date对象作参数时也需要new一个Date对象。
（4）java里面对象数组有多种写法，其中的每一个对象也都需要和上面说的方法一样一个一个new。最简单的写法就是:SavingsAccount[]accounts={new SavingsAccount(date,String,double),new ......}
（5）C++中的sizeof()函数用来计算对象所占内存空间大小，在java里没有。所以将n=sizeof(accounts)/sizeof(SavingsAccount)改成了accounts.length()。其中length()方法返回数组长度，实现的功能相同。
源代码：
Data类：public class Date {
private int year,month,day,totalDays;//该日期是从公元元年1月1日开始的第几天
private final int[] DAYS_BEFORE_MONTH={0,31,59,90,120,151,181,212,243,273,304,334,365};
public Date(int year,int month,int day){
this.year=year;
this.month=month;
this.day=day;
if(day<=0||day>getMaxDay()){
System.out.print("Invalid date");
show();
System.out.println("");
System.exit(1);
}
int years=year-1;
totalDays=years*365+years/4-years/100+years/400+DAYS_BEFORE_MONTH[month-1]+day;
if(isLeapYear()&&month>2) totalDays++;
}
public final int getMaxDay() {
if(isLeapYear() && month==2) return 29;
else return DAYS_BEFORE_MONTH[month]-DAYS_BEFORE_MONTH[month-1];
}
public final void show() {
System.out.print(getYear()+"-"+getMonth()+"-"+getDay());
}
public final int getYear() {return year;}
public final int getMonth() {return month;}
public final int getDay() {return day;}
public final boolean isLeapYear() {
return year%4==0&&year%100!=0||year%400==0;
}
public final int distance(final Date date) {
return totalDays-date.totalDays;
}
}
SavingAccount6类：
public class SavingsAccount6 {
private static double total=0;private String id;
private Date lastDate;//账号，上一次余额变动日期
private double balance,rate,accumulation;//余额，年利率，余额按日累加值
public static double getTotal(){return total;}
public SavingsAccount6(final Date date,final String id,double rate){
lastDate=date;
this.id=id;
this.rate=rate;
balance=0;
accumulation=0;
date.show();
System.out.println("\t#"+id+" created");
}
private final void error(final String msg) {
System.out.println("Error(#"+id+"):"+msg);
}
//记录一笔账，有日期、金额和说明（desc）
private void record(final Date date,double amount,final String desc){
accumulation=accumulate(date);
lastDate=date;
amount=Math.floor(amount*100+0.5)/100;//保留小数点后两位
total+=amount;
balance+=amount;
date.show();
System.out.println("\t#"+id+"\t"+amount+"\t"+balance+"\t"+desc);
}
//获得到指定日期为止的存款金额按日累积值
private final double accumulate(final Date date){
return accumulation+balance*date.distance(lastDate);
}
//存入和取出现金
public void deposit(final Date date,double amount,final String desc){
record(date,amount,desc);
}
public void withdraw(final Date date,double amount,final String desc){
if(amount>getBalance())
System.out.println("not enough money");
else
record(date,-amount,desc);
}
//显示账户信息
public void settle(final Date date){
Date date0=new Date(date.getYear()-1,1,1);
double interest=accumulate(date)*rate/date.distance(date0);//计算年息
if(interest!=0)
record(date,interest,"interest");
accumulation=0;
}
public final void show(){
System.out.print(id+"\tBalance: "+balance);
}
public final String getId(){return id;}
public final double getBalance(){return balance;}
public final double getRate(){return rate;}
}
SavingsAccount61类：
public class SavingsAccount61 {
public static void main(String args[]){
Date date=new Date(2008,11,1);
SavingsAccount6[] accounts={new SavingsAccount6(date,"03755217",0.015),new SavingsAccount6(date,"02342342",0.015)};
final int n=accounts.length;
//11月份的几笔账目
accounts[0].deposit(new Date(2008,11,5), 5000, "salary");
accounts[1].deposit(new Date(2008,11,25),10000,"sell stock 0323");
//12月份的几笔账目
accounts[0].deposit(new Date(2008,12,5), 5500, "salary");
accounts[1].withdraw(new Date(2008,12,20),4000,"buy a laptop");
System.out.println("");
for(int i=0;i<n;i++){
accounts[i].settle(new Date(2009,1,1));
accounts[i].show();
System.out.println("");
}
System.out.println("Total:"+SavingsAccount6.getTotal());
}
}
4.第7章——继承与派生，抽象出父类，增添子类
改动的一些点：从本章开始，多个类的继承派生关系使得类越来越多，也越来越复杂。
（1）java中类的继承用的是extends关键字，而非C++里的class SavingsAccount:public Account{};而且java没有C++里的继承类型。
（2）在C++中将原有的record和error成员函数访问控制权限修改为protected,而且设置了一个保护的构造函数供派生类调用，在java中不作改动，仍保留protected。
(3)由于两类账户计息对象和周期不同，但都需要将某个数值（余额或欠款金额）按日累加，所以选择建立新类Accumulator。同上一章类似，在调用Accumulator对象时注意使用new。
（4）由于子类2中的show（）函数显示的与父类不尽相同，在调用父类show（），之后还要将额外的信息输出。
（5）对于继承自父类的构造方法或者方法中需要调用父类同名方法，java与C++很大的不同是引入了关键字super，并且在子类构造方法中需要首先用super（Date，String）去初始化继承自父类那部分的域变量。
注：此程序的不足之处在于，两个子类中的deposit、withdraw、settle方法是彼此独立的，只有明确知道一个对象的具体类型之后才能调用它的这些方法，这也是不能将main（）方法中的3个账户放在一个对象数组中进行操作的内在原因。在下一章将通过C++中的虚函数来解决（而相应的，在java中也有机制实现）。
源代码：
Date类同上。
Accumulator类：
public class Accumulator { //将某个数值按日累加
private Date lastDate; //上次变更数值的时期
private double value; //数值的当前值
private double sum; //数值按日累加之和
//构造函数，date为开始累加的日期，value为初始值
public Accumulator(final Date date, double value){
lastDate=date;
this.value=value;
sum=0;
}
//获得到日期date的累加结果
public final double getSum(final Date date){return sum+value * date.distance(lastDate);
}
//在date将数值变更为value
public void change(final Date date, double value) {
sum = getSum(date);
lastDate = date;
this.value = value;
}
//初始化，将日期变为date，数值变为value，累加器清零
public void reset(final Date date, double value) {
lastDate = date;
this.value = value;
sum = 0;
}
}
Account类（父类）：public class SavingsAccount7 {
private String id;
private double balance;
private static double total=0;//所有账户的总金额
protected SavingsAccount7(final Date date,final String id){
this.id=id;
balance=0;
date.show();
System.out.println("\t#"+id+" created");
}
protected final void error(final String msg) {
System.out.println("Error(#"+id+"):"+msg);
}
//记录一笔账，有日期、金额和说明（desc）
protected void record(final Date date,double amount,final String desc){
amount=Math.floor(amount*100+0.5)/100;//保留小数点后两位
total+=amount;
balance+=amount;
date.show();
System.out.println("\t#"+id+"\t"+amount+"\t"+balance+"\t"+desc);
}
public void show(){
System.out.print(id+"\tBalance: "+balance);
}
public final String getId(){return id;}
public final double getBalance(){return balance;}
public static double getTotal(){return total;}
}
SavingsAccount类（子类1）：
public class SavingsAccount71 extends SavingsAccount7{
private Accumulator acc;//辅助计算利息的累加器
private double rate;//存款的年利率
public SavingsAccount71(final Date date,final String id,double rate){
super(date,id);
this.rate=rate;
acc=new Accumulator(date,0);
}
public final double getRate(){return rate;}
//存入和取出现金
public void deposit(final Date date,double amount,final String desc){
record(date,amount,desc);
acc.change(date, getBalance());
}
public void withdraw(final Date date,double amount,final String desc){
if(amount>getBalance())
System.out.println("not enough money");
else{
record(date,-amount,desc);
acc.change(date, getBalance());
}
}
public void settle(final Date date){
double interest=acc.getSum(date)*rate/date.distance(new Date(date.getYear()-1,1,1));//计算年息
if(interest!=0)
record(date,interest,"interest");
acc.reset(date, getBalance());
}
}
CreditAccount类（子类2）：
public class SavingsAccount72 extends SavingsAccount7{
private Accumulator acc;
private double credit;//信用额度
private double rate;//欠款的日利率
private double fee;//信用卡年费
private final double getDebt(){
double balance=getBalance();
return (balance<0?balance:0);
}
public SavingsAccount72(final Date date,final String id,double credit,double rate,double fee){
super(date,id);
this.credit=credit;
this.rate=rate;
this.fee=fee;
acc=new Accumulator(date,0);
}
public final double getCredit(){return credit;}
public final double getRate(){return rate;}
public final double getFee(){return fee;}
public final double getAvailableCredit(){//获得可用信用额度
if(getBalance()<0)
return credit+getBalance();
else
return credit;
}
public void deposit(final Date date,double amount,final String desc){
record(date,amount,desc);
acc.change(date, getDebt());
}
public void withdraw(final Date date,double amount,final String desc){
if(amount-getBalance()>credit)
System.out.println("not enough credit");
else{
record(date,-amount,desc);
acc.change(date, getDebt());
}
}
public void settle(final Date date){
double interest=acc.getSum(date)*rate;
if(interest!=0)
record(date,interest,"interest");
if(date.getMonth()==1)
record(date,-fee,"annual fee");
acc.reset(date, getDebt());
}
public final void show(){
super.show();
System.out.print("\tAvailable credit: "+getAvailableCredit());
}
public static void main(String args[]){
Date date=new Date(2008,11,1);
SavingsAccount71 sa1=new SavingsAccount71(date,"S3755217",0.015);
SavingsAccount71 sa2=new SavingsAccount71(date,"02342342",0.015);
SavingsAccount72 ca=new SavingsAccount72(date,"C5392394",10000,0.0005,50);
//11月份的几笔账目
sa1.deposit(new Date(2008,11,5), 5000, "salary");
ca.withdraw(new Date(2008,11,15), 2000, "buy a cell");
sa2.deposit(new Date(2008,11,25),10000,"sell stock 0323");
//结算信用卡
ca.settle(new Date(2008,12,1));
//12月份的几笔账目
ca.deposit(new Date(2008,12,1),2016,"repay the credit");
sa1.deposit(new Date(2008,12,5), 5500, "salary");
//结算所有账户
sa1.settle(new Date(2009,1,1));
sa2.settle(new Date(2009,1,1));
ca.settle(new Date(2009,1,1));
System.out.println("");
sa1.show();System.out.println("");
sa2.show();System.out.println("");
ca.show();System.out.println("");
System.out.println("Total:"+SavingsAccount7.getTotal());
}
}