sample.java

interface IBill {
    String getType();
    void execute(); //假設你要做野
}

class ElectricityBill implements IBill {
    int billDay;
    int kw;
    String region;
    Double paidAmount;

    public ElectricityBill(String inRegion, int inBillDay, int inKW) {
        paidAmount = 0.0;
        region = inRegion;
        billDay = inBillDay;
        kw = inKW;
    }

    @Override
    public String getType() {
        return "E";
    }

    @Override
    public void execute() {
        //做咩都得
    }
}

class GasBill implements IBill {
    int billDay;
    int kJ;
    String rateType;
    Double paidAmount;

    public GasBill(String inRateType, int inBillDay, int inKJ) {
        paidAmount = 0.0;
        rateType = inRateType;
        billDay = inBillDay;
        kJ = inKJ;
    }

    @Override
    public String getType() {
        return "G";
    }

    @Override
    public void execute() {
        //做你想做既野,好似係
    }
}

class Bill {
    String type;
    IBill bill;

    public Bill(String inputType, IBill inBill) {
        type = inputType;
        bill = inBill;
    }
}

class CustomerAccount {
    String custName, custAdd, custContact;
    private int maxSize = 10;

    //如果限定10張單,正路做法係唔好用dynamic allocate size既list/vector, 如果一定要用, 手動check size再做處理
    //如果係指一個月會有10種唔同類型既單,係另一個玩法
    //1.唔用list/vector,用array
    IBill[] custBills1 = new IBill[maxSize];
    int index = 0;

    //2.用list/vector
    List<IBill> custBills2 = new ArrayList<>(maxSize); //或者用LinkedList,睇你需要邊個
    Vector<IBill> custBills3 = new Vector<>(maxSize);

    public CustomerAccount(String inCustName, String inCustAdd, String inCustContact)
    {
        custName = inCustName;
        custAdd = inCustAdd;
        custContact = inCustContact;
    }

    //1. 用array需要手動指定index
    private int getArrayIndex() {
        if (index == maxSize) {
            index = 0;
            return maxSize;
        } else {
            return index++;
        }
    }

    public void addBill(IBill bill) {
        //1. array方法
        custBills1[getArrayIndex()] = bill;

        //2. list/vector
        custBills2.add(bill);
        custBills3.add(bill);

        //做size control
        if (custBills2.size() == maxSize) {
            custBills2.remove(0);//清最頭果個,位置可以控制
        }
        if (custBills3.size() == maxSize) {
            custBills3.removeElementAt(0);
        }
    }

    public IBill getBill(int index) {
        //1. 如果用array, 需要指定特定index
        IBill ret = custBills1[index];

        //2. list/vector都係指定index, 但有其他方法拎
        ret = custBills2.get(index);
        ret = custBills3.elementAt(index);

        //2. vector仲可以拎頭尾
        ret = custBills3.firstElement();
        ret = custBills3.lastElement();

        return ret;
    }

    public void killBill() {
        //做個例子,點樣call 唔同class入面同樣名既method (名由interface IBill控制)
        //直接call IBill就得,理撚得佢實際咩樣
        for (int i = 0; i < custBills1.length; i++) {
            custBills1[i].execute();
            custBills1[i].getType();
        }

        for (int i = 0; i < custBills2.size(); i++) {
            custBills2.get(i).getType();
            //....
        }
        //仲有iterator玩法
        Iterator<IBill> it = custBills2.iterator();
        while (it.hasNext()) {
            IBill temp = it.next();
            temp.getType();
            temp.execute();
        }
    }
}