我奇怪地遇到了一個為我的聯系人類打印 null 的問題。它似乎不喜歡 PhoneNumberList 的數字構造函數。我只是想返回跟蹤數字等的計數，但它只為 getNumber 和 getNumberCount 返回 null。嘗試將計數從私有更改為公共以直接訪問以及更改名稱和構造函數。  public class Contact{    private String name;    private PhoneNumberList numbers;    // Purpose:    //  initialize this instance of Contact    //  with no PhoneNumber    //    public Contact (String theName)    {        // You must allocate a PhoneNumberList here        numbers = new PhoneNumberList();        name = theName;    }    // Purpose:    //  initialize this instance of Contact    //  add p to the list of phone numbers associated with     //  this Contact    //    public Contact (String theName, PhoneNumber p)    {        // You must allocate a PhoneNumberList here        PhoneNumberList numbers = new PhoneNumberList();        name = theName;        numbers.add(p);    }    // Purpose:     //  return the name associated with this instance    //    public String getName ()    {        return name;    }    // Purpose:    //  change the name associated with this instance to be newName    //    public void setName(String newName)    {        name = newName;    }    // Purpose:    //  add a new PhoneNumber to this contact    //  there is no maximum number of phone numbers that can be    //  assigned to a contact.    //    public void addNumber (PhoneNumber p)    {        numbers.add(p);    }    // Purpose:    //  remove p from the list of PhoneNumbers associated with this contact    //  if p is not in the list, do nothing.    //    public void removeNumber (PhoneNumber p)    {        int index = numbers.find(p);        numbers.remove(index);    }    // Purpose:    //  return the count of PhoneNumbers associated with this contact    //    public int getNumberCount()    {        return numbers.count;    }