Question

这是使用BlueJ的Java。

我有四个名为Person，Letter，Address和PhoneNumber的课程。在每一个中，我重写了toString（）方法，以从类中返回我想要的值的连接字符串。当调用Letter toString（）时，它对所有值都返回null。

我们的想法是使用硬编码信息，将其传递到适当的类中，然后以标准字母格式返回。

我是朝着正确的方向前进，打印出硬编码的信息，还是应该走另一条路？这是一个家庭作业问题，但我觉得我碰到了砖墙。

以下是课程：

public class Person
{
private static String aPerson;
private String first;
private String middle;
private String last;    
private Address address;
private PhoneNumber phone;

public String getFirst()
{
    return this.first;
}

public void setFirst(String FirstName)
{
    this.first = FirstName;
}

    public String getMiddle()
{
    return this.middle;
}

public void setMiddle(String MiddleName)
{
    this.middle = MiddleName;
}

    public String getLast()
{
    return this.last;
}

public void setLast(String LastName)
{
    this.last = LastName;
}

public Address getMyAddress()
{
    return this.address;
}
public void setMyAddress(Address Address)
{
    this.address = Address;
}

public PhoneNumber getMyPhoneNum()
{
    return this.phone;
}

public void setMyPhoneNum(PhoneNumber Number)
{
    this.phone = Number;
}

public Person()
{
    aPerson = getFirst() + getMiddle() + getLast() + getMyAddress() + 
    getMyPhoneNum();
}

public String toString()
{
    return aPerson;
}
}

电话号码：

public class PhoneNumber
{
private String number;
private int areaCode = 0;
private int phonePrefix = 0;
private int phoneLineNum = 0;
private int phoneExtension = 0;

public String getNumber()
{
    return number;
}

public void setNumber(String Number)
{
    number = Number;
}

public int getAreaCode()
{
    return areaCode;
}

public void setAreaCode(int AreaCode)
{
    areaCode = AreaCode;
}

public int getPrefix()
{
    return phonePrefix;
}

public void setPrefix(int Prefix)
{
    phonePrefix = Prefix;
}

public int getPhoneLineNumber()
{
    return phoneLineNum;
}

public void setLineNum(int PhoneNumber)
{
    phoneLineNum = PhoneNumber;
}

public int getExtension()
{
    return phoneExtension;
}

public void setExtension(int Extension)
{
    phoneExtension = Extension;
}
}

地址：

public class Address
{
private String state;
private String anAddress;
private String address;
private String city;
private int zip = 0;

public String getState()
{
   return state;
}

public void setState(String State)
{
   state = State;
}

   public String getAddress()
{
   return address;
}

   public void setAddress(String Address)
{
   address = Address;
}

   public String getCity()
{
   return city;
}

   public void setCity(String City)
{
   city = City;
}

   public int getZip()
{
   return zip;
}

   public void setZip(int Zip)
{
   zip = Zip;
}

public Address()
{
   anAddress = getState() + getAddress() + getCity() + getZip();
}

public String toString()
{
   return this.anAddress;
}
}

字母：

public class Letter
{
private Person to;
private Person from;
private String body;
private String finishedLetter;

public Person getTo()
{
    return to;
}

public void setTo(Person newValue)
{
    to = newValue;
}

public Person getFrom()
{
    return from;
}

public void setFrom(Person newValue)
{
    from = newValue;
}

public String getBody()
{
    return body;
}

public void setBody(String newValue)
{
    body = newValue;
}

public Letter()
{
    finishedLetter = getTo() + " \n" + getFrom() + " \n" + getBody();
}

public String toString()
{
    return finishedLetter;
}
}

主要：

public class MainClass
{
public static void main(String args[])
{
    PhoneNumber phone1 = new PhoneNumber();
    phone1.setAreaCode(417);
    phone1.setPrefix(447);
    phone1.setLineNum(7533);
    phone1.setExtension(0);

    PhoneNumber phone2 = new PhoneNumber();
    phone2.setAreaCode(210);
    phone2.setPrefix(336);
    phone2.setLineNum(4343);
    phone2.setExtension(9850);

    Address address1 = new Address();

    address1.setState("MO");
    address1.setAddress("1001 East Chestnut Expressway");
    address1.setCity("Springfield");
    address1.setZip(65807);

    Address address2 = new Address();

    address2.setState("TX");
    address2.setAddress("4800 Calhoun Road");
    address2.setCity("Houston");
    address2.setZip(77004);

    Person person1 = new Person();

    person1.setFirst("Shane");
    person1.setMiddle("Carroll");
    person1.setLast("May");
    person1.setMyAddress(address1);
    person1.setMyPhoneNum(phone1);

    Person person2 = new Person();

    person2.setFirst("Ted");
    person2.setMiddle("Anthony");
    person2.setLast("Nugent");
    person2.setMyAddress(address2);
    person2.setMyPhoneNum(phone2);

    Letter aLetter = new Letter();

    aLetter.setTo(person2);
    aLetter.setFrom(person1);
    aLetter.setBody("This is the body");

    System.out.println(aLetter.toString());
}
}

Answer 1

您的Letter构造函数在填充这些字段之前调用getTo()和getFrom()等方法。不要这样做，因为您的finishedLetter字符串永远不会正确地“完成”。即，

public Letter()
{
    finishedLetter = getTo() + " \n" + getFrom() + " \n" + getBody();
}

始终会生成null + "\n" + null + "\n" + null

也许这类代码应该在toString()方法中。

Answer 2

当您使用new Letter()构建信件时，它会使用多个finishedLetter值初始化其实例字段null。由于尚未使用相应的setter设置to，from和body，因此其getter返回null，导致finishedLetter等于“null \ nnull \ nnull”。

要解决此问题，我的一种方法是在finishedLetter方法本身中定义toString()。这将解决问题，并采用更加面向对象的方法进行程序设计。

// remove constructor (if you wish) and finishedLetter field

public String toString() {
    return getTo() + " \n" + getFrom() + " \n" + getBody();
}

更好的方法是将to，from和body作为Letter构造函数中的参数。

// remove finishedLetter field

public Letter(Person to, Person from, String body) {
    this.to = to;
    this.from = from;
    this.body = body;
}

public String toString() {
    return getTo() + " \n" + getFrom() + " \n" + getBody();
}

java toString（）从类返回null

2 个答案: