java - 如何从另一个类访问一个对象

Question

我正在尝试使用 LibraryCard 中的 toString 方法来访问姓名和电子邮件。但是，当我使用 owner.getName() 时，它会给我一个空指针异常错误。我也在努力把卡片做成特定的人。

public class Student{

  public String name;
  public String email;

  public Student(){
    name = "UNASSIGNED";
    email = "UNASSIGNED"; 
  }

  public String getEmail(){ 
    return email;
  }

  public String getName(){
    return name;
  }

  public void setEmail(String newEmail){
    email = newEmail;
  }

  public void setName(String newName){
    name = newName;
  }  
}

和

public class LibraryCard{

  public Student owner;
  public int borrowCount;

  public LibraryCard(){
    owner = null;
    borrowCount = 0;    
  }

  public void checkOut(int bookNum){
    borrowCount += bookNum;    
  }

  public int getNumberOfooks(){
    return borrowCount;
  }

  public Student getOwnerName(){
    return owner;   
  }

  public void setOwner(Student object){
    owner = object; 
  }

  public String toString(){
    return "\nCard1 Info:\n\t Owner Name: " + owner.getName() + "\n\t Email: " + owner.getEmail() + "\n\t Books borrowed: " + borrowCount ; 
  }
}

和测试人员

public class LibraryCardTester{

  public static void main(String args[]){

    Student student1 = new Student();

    student1.setName("James Johnson");
    student1.setEmail(student1.getName()+"@gmail.edu");
    System.out.println("Name: " + student1.getName());
    System.out.println("Email: " +student1.getEmail());

    Student student2 = new Student();

    student2.setName("Barack Obama");
    student2.setEmail(student1.Name()+"@gmail.edu");
    System.out.println("Name: " + student2.getName());
    System.out.println("Email: " +student2.getEmail());

    LibraryCard card1 = new LibraryCard();
    card1.checkOut(3);

    LibraryCard card2 = new LibraryCard();
    card2.checkOut(2);

    LibraryCard card3 = new LibraryCard();
    card3.checkOut(5);

    System.out.println(card1.toString());
    System.out.println(card2.toString());
    System.out.println(card3.toString());

  }
}

Answer 1

在我看来你还没有为借书证设置所有者:

card1.setOwner(student1);

您的构造函数将所有者设置为 null，因此除非您使用 setOwner() 方法设置所有者，否则所有者将始终为 null，这意味着您正在调用 getName() 空对象上的方法导致 NullPointerException:

  public LibraryCard(){
    owner = null;
    borrowCount = 0;    
  }