个人银行账户管理系统（c++改写java）第8章：多态特性

import java.text.DecimalFormat;

public abstract class Account {
    private String id;    //帐号
    private double balance;    //余额
    private static double total = 0; //所有账户的总金额
    
        //供派生类调用的构造函数，id为账户
    protected Account(final Date date, final String id)
    {
        this.id=id;
        balance=0;
        date.show();
        System.out.println("\t#" + id + " created");
    }
        //记录一笔帐，date为日期，amount为金额，desc为说明
    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
        balance += amount;
        total += amount;
        date.show();
        System.out.println( "\t#" + id + "\t" + amount + "\t" + df.format(balance) + "\t" + desc );
    }
        //报告错误信息
    protected void error(final String msg)
    {
        System.out.println("Error(#" + id + "):" + msg);
    }
    static DecimalFormat df = new DecimalFormat("0.0");
    public final String getId() { return id; }
    public double getBalance() { return balance; }
    public static double getTotal() { return total; }
        //存入现金，date为日期，amount为金额，desc为款项说明
    abstract void deposit(final Date date, double amount,final String desc);
        //取出现金，date为日期，amount为金额，desc为款项说明
    abstract void withdraw(final Date date, double amount,final String desc);
        //结算（计算利息、年费等），每月结算一次，date为结算日期
    abstract void settle(final Date date);
        //显示账户信息
    public void show(){
        System.out.print(id + "\tBalance: " + df.format(balance));
        }
}

public class SavingsAccount extends Account {//储蓄账户类

    private Accumulator acc;    //辅助计算利息的累加器
    private double rate;        //存款的年利率
    
        //构造函数
    public SavingsAccount(final Date date,final String id, double rate)
    {
        super(date,id);
        this.rate = rate;
        acc=new Accumulator(date,0);
    }
    public double getRate() { return rate; }
        //存入现金
    public void deposit(final Date date, double amount,final String desc)
    {
        record(date, amount, desc);
        acc.change(date, getBalance());
    }
        //取出现金
    public void withdraw(final Date date, double amount,final String desc)
    {
        if (amount > getBalance()) {
            error("not enough money");
        } else {
            record(date, -amount, desc);
            acc.change(date, getBalance());
        }
    }
        //结算利息，每年1月1日调用一次该函数
    public void settle(final Date date)
    {
        if(date.getMonth() == 1) {
            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());
        }
    }
}

public class CreditAccount extends Account{//信用账户类

    private Accumulator acc;    //辅助计算利息的累加器
    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 double getDebt() {    //获得欠款额
            double balance = getBalance();
            return (balance < 0 ? balance : 0);
    }
    
        //构造函数
    public CreditAccount(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 double getCredit() { return credit; }
    public double getRate() { return rate; }
    public double getFee() { return fee; }
    public double getAvailableCredit() {    //获得可用信用
            if (getBalance() < 0) 
                return credit + getBalance();
            else
                return credit;
        }
        //存入现金
    public void deposit(final Date date, double amount,final String desc)
    {
        record(date, amount, desc);
        acc.change(date, getDebt());
    }
        //取出现金
    public void withdraw(final Date date, double amount,final String desc)
    {
        if (amount - getBalance() > credit) {
            error("not enough credit");
        } else {
            record(date, -amount, desc);
            acc.change(date, getDebt());
        }
    }
        //结算利息和年费，每月1日调用一次该函数
    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 void show()
    {
        super.show();
        System.out.println( "\tAvailable credit:" + getAvailableCredit());
    }
}

public class Accumulator {
    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 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;
        }
}

public class Date {
    //存储平年中某个月1日之前有多少天，为便于getMaxDay函数的实现，该数组多出一项
            final int DAYS_BEFORE_MONTH[] = { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365 };
            private int year;        //年
            private int month;        //月
            private int day;        //日
            private int totalDays;    //该日期是从公元元年1月1日开始的第几天

            boolean isLeapYear() {    //判断当年是否为闰年
                return year % 4 == 0 && year % 100 != 0 || year % 400 == 0;
            }
            public Date(int year, int month, int day)    //用年、月、日构造日期
            {
                this.day = day;
                this.month = month;
                this.year = year;
                if (day <= 0 || day > getMaxDay()) {
                    System.out.println("Invalid date: ");
                    show();
                    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 int getYear() { return year; }
            public int getMonth() { return month; }
            public int getDay() { return day; }
            public int getMaxDay()        //获得当月有多少天
            {
                if (isLeapYear() && month == 2)
                    return 29;
                else
                    return DAYS_BEFORE_MONTH[month]- DAYS_BEFORE_MONTH[month - 1];
            }
                
            public void show()            //输出当前日期
            {
                System.out.print(getYear() + "-" + getMonth() + "-" + getDay());
            }
                //计算两个日期之间差多少天    
            int distance(Date date)
            {
                return totalDays - date.totalDays;
            }
}

import java.io.IOException;
import java.text.DecimalFormat;
import java.util.Scanner;

public class main {
    public static void main(String[] args) throws IOException{
        Date date = new Date(2008, 11, 1);    //起始日期
        //建立几个账户
        SavingsAccount sa1 = new SavingsAccount(date, "S3755217", 0.015);
        SavingsAccount sa2 = new SavingsAccount(date, "02342342", 0.015);
        CreditAccount ca = new CreditAccount(date, "C5392394", 10000, 0.0005, 50);
        Account accounts[] = { sa1, sa2, ca };
        final int n = accounts.length;    //账户总数

        System.out.println("(d)deposit (w)withdraw (s)show (c)change day (n)next month (e)exit");
        char cmd;
        Scanner in=new Scanner(System.in); //使用Scanner类定义对象
        int index, day;
        double amount;
        String desc;
        DecimalFormat df = new DecimalFormat("0.0");
        do {
            //显示日期和总金额
            date.show();
            System.out.print("\tTotal: " + df.format(Account.getTotal()) + "\tcommand> ");

            cmd = (char)System.in.read();
            switch (cmd) {
            case 'd':    //存入现金
                index = in.nextInt();
                amount = in.nextDouble();
                desc = in.nextLine();
                accounts[index].deposit(date, amount, desc);
                break;
            case 'w':    //取出现金
                index = in.nextInt();
                amount = in.nextDouble();
                desc = in.nextLine();
                accounts[index].withdraw(date, amount, desc);
                break;
            case 's':    //查询各账户信息
                for (int i = 0; i < n; i++) {
                    System.out.print("[" + i + "] ");
                    accounts[i].show();
                    System.out.println();
                    if(i<(n-1)) {System.in.read();}
                }
                break;
            case 'c':    //改变日期
                day = in.nextInt();
                if (day < date.getDay())
                    System.out.print("You cannot specify a previous day");
                else if (day > date.getMaxDay())
                    System.out.print("Invalid day");
                else
                    date = new Date(date.getYear(), date.getMonth(), day);
                break;
            case 'n':    //进入下个月
                if (date.getMonth() == 12)
                    date = new Date(date.getYear() + 1, 1, 1);
                else
                    date = new Date(date.getYear(), date.getMonth() + 1, 1);
                for (int i = 0; i < n; i++) {
                    accounts[i].settle(date);
                    if(i<(n-1)) {System.in.read();}
                }
                break;
            }
        } while (cmd != 'e');
        in.close();
    }
    
}

总结：

1.java丢弃了c++中的指针，操作符重载等特性；

2.java支持接口之间的多继承，并支持类与接口之间的实现机制（关键字为implements）；

3.java文件编译出来的是class文件，具有强大的可移植性，而c++是直接可以运行的exe文件。