我已经阅读了这里的一些NullPointerException线程，并且已经将我的代码格式化为使用我找到的一个简单检查，但我仍然得到NullPointerException。
完整代码：请注意错误所在的****
public class Assignment11 {
public static void main(String[] args) {
ArrayList<Employee> employeeArray = new ArrayList<Employee>();
ArrayList<Book> booksToCirculate = new ArrayList<Book>();
ArrayList<Book> archivedbooks = new ArrayList<Book>();
int passAndCirc = 0;
int totalEmployees = 0;
addBook("Software Engineering", booksToCirculate);
addBook("Chemistry", booksToCirculate);
addEmployee("Adam", booksToCirculate, employeeArray);
addEmployee("Sam", booksToCirculate, employeeArray);
addEmployee("Ann",booksToCirculate, employeeArray);
circulateBook("Chemistry","1999 03 20",booksToCirculate,employeeArray);
//circulateBook("Software Engineering","1998 02 20",booksToCirculate,employeeArray);
passOn("Chemistry", "1999 03 25",employeeArray,archivedbooks,booksToCirculate);
passOn("Chemistry", "1999 03 29",employeeArray,archivedbooks,booksToCirculate);
passOn("Chemistry", "1999 03 30",employeeArray,archivedbooks,booksToCirculate);
passOn("Software Engineering","1998 02 21",employeeArray,archivedbooks,booksToCirculate);
// passOn("Software Engineering","1998 02 29",employeeArray,archivedbooks,booksToCirculate);
}
public static void addEmployee(String aName, ArrayList<Book> booksToCirculate, ArrayList<Employee> employeeArray)
{
Employee anEmployee = new Employee();
anEmployee.setName(aName);
employeeArray.add(anEmployee);
for (Book b : booksToCirculate)
{
b.getQueue().add(anEmployee); //adds employee to each queue
}
System.out.println(anEmployee.getName() + " has been added to each queue, and list of employees.");
}
public static void addBook(String aName, ArrayList<Book> booksToCirculate)
{
Book aBook = new Book();
aBook.setBookName(aName);
aBook.setArchived(false);
//aBook.setStartDate(LocalDate.now()); //lookin good
booksToCirculate.add(aBook);
System.out.println(aBook.getBookName() + " has been added to the library.");
}
public static void circulateBook(String aName, String aDate, ArrayList<Book> booksToCirculate, ArrayList<Employee> employeeArray)
{
DateTimeFormatter formatter = DateTimeFormatter.ofPattern ( "yyyy MM dd" );
LocalDate dateStarted = formatter.parse ( aDate , LocalDate :: from ); //have the date in 2015 11 05 format
Employee anEmployee = null;
Book aBook = null;
Queue<Employee> Employees = new LinkedList<>();
for (Book b : booksToCirculate)
{
if (b.getBookName().equals(aName))
{
b.setStartDate(dateStarted);
anEmployee = b.getQueue().poll();
aBook = b;
Employees = b.getQueue();
b.setQueue(Employees);
}
}
for (Book b: booksToCirculate)
{
if (!b.getBookName().equals(aName))
{
b.setQueue(Employees);
b.getQueue().add(anEmployee);
}
}
for (Employee e : employeeArray)
{
if (e.getName().equals(anEmployee.getName()))
{
//System.out.println(e.getName());
e.setaBook(aBook);
e.setRetainingTime(0);
System.out.println(e.getName()+" Has the book " + e.getaBook().getBookName()+" Given to him on: "+e.getaBook().getStartDate() + " setting his/her waiting time to " + e.getRetainingTime()+ ".");
}
}
}
public static void passOn(String aName, String aDate, ArrayList<Employee> employeeArray, ArrayList<Book> archivedBooks, ArrayList<Book> booksToCirculate)
{
Employee anEmployee = null;
Book aBook = null;
Queue<Employee> Employees = new LinkedList<>();
DateTimeFormatter formatter = DateTimeFormatter.ofPattern ( "yyyy MM dd" );
LocalDate datePassed = formatter.parse ( aDate , LocalDate :: from );
LocalDate dateReceived;
LocalDate startDate;
int daysElapsed = 0;
int daysElapsed2 = 0;
for (Employee e : employeeArray)
{
***************************************************************************
if (e.getaBook().getBookName() != null) //Where the error is
if (e.getaBook().getBookName().equals(aName)){
{
startDate = e.getaBook().getStartDate();
Period period = Period.between ( startDate, datePassed );
daysElapsed2 = period.getDays();
e.setRetainingTime(daysElapsed2);
System.out.println(e.getName() + " has passed on "+ e.getaBook().getBookName()+" changing his/her's retaining time to " + daysElapsed2+ ".");
}
}
}
//**************************************************************************
for (Book b : booksToCirculate)
{
outerloop:
if (b.getBookName().equals(aName))
{
if (b.getQueue().isEmpty())
{
b.setEndDate(datePassed);
archivedBooks.add(b);
b.setEndDate(datePassed);
b.setArchived(true);
System.out.println("The book " + b.getBookName() + " has been archived.");
break outerloop;
}
dateReceived = b.getStartDate();
Period period = Period.between ( dateReceived, datePassed );
daysElapsed = period.getDays();
anEmployee = b.getQueue().poll();
anEmployee.setWaitingTime(daysElapsed);
anEmployee.setaBook(b);
aBook = b;
Employees = b.getQueue();
b.setQueue(Employees);
}
}
for (Book b: booksToCirculate)
{
if (!b.getBookName().equals(aName))
{
b.setQueue(Employees);
b.getQueue().add(anEmployee);
}
}
for (Employee e : employeeArray)
{
if (e.getName().equals(anEmployee.getName()))
{
e.setaBook(aBook);
e.setRetainingTime(daysElapsed);
System.out.println(e.getName()+" Has the book " + e.getaBook().getBookName()+" passed to him on: "+datePassed + " setting his/her waiting time to " + e.getRetainingTime()+ ".");
}
}
//break maybe
}
}
class Employee{
String name;
int waiting_time;
int retaining_time;
int priority;
Book aBook;
ArrayList <Book> booksRead = new ArrayList<Book>();
public Employee()
{
this.waiting_time=0;
this.retaining_time=0;
}
public void setName(String aName)
{
name = aName;
}
public String getName()
{
return name;
}
public void setWaitingTime(int waitingtime)
{
waiting_time = waitingtime;
}
public int getWaitingTime()
{
return waiting_time;
}
public void setRetainingTime(int retainingtime)
{
retaining_time = retainingtime;
}
public int getRetainingTime()
{
return retaining_time;
}
public void setPriority()
{
priority = waiting_time - retaining_time;
}
public int getPriority()
{
return priority;
}
public void setaBook(Book book)
{
aBook = book;
}
public Book getaBook()
{
return aBook;
}
public void setbooksRead(ArrayList<Book> j)
{
booksRead = j;
}
public ArrayList<Book> getbooksRead()
{
return booksRead;
}
}
class Book{
String name;
LocalDate start_date;
LocalDate end_date;
boolean archived;
Queue<Employee> Employees = new LinkedList<>();
public Book()
{
}
public void setBookName(String aBook)
{
name = aBook;
}
public String getBookName()
{
return name;
}
public void setStartDate(LocalDate starting)
{
start_date = starting;
}
public LocalDate getStartDate()
{
return start_date;
}
public void setEndDate(LocalDate ending)
{
end_date = ending;
}
public LocalDate getEndDate()
{
return end_date;
}
public void setArchived(boolean ba)
{
archived = ba;
}
public boolean isArchived()
{
return archived;
}
public void setQueue(Queue<Employee> qa)
{
Employees = qa;
}
public Queue<Employee> getQueue()
{
return Employees;
}
}
我无法弄清楚为什么我在我的无效点检查器上获得nullpointerexception以确保事情不为空，一直盯着代码一小时并且看起来不会找到我在哪里出错。非常感谢任何和所有帮助！
仅供参考：我知道不好意思发布你的整个代码，但不知道哪里出现了什么/出了什么问题，这样就迫使我这么做了。
答案 0 :(得分：1)
e为null（ArrayList可以包含null）或getaBook()的调用返回null因此当您致电时e.getaBook().getBookName()抛出NullPointerException。
答案 1 :(得分：1)
e可能是null。 e.getaBook()也可能是null。
答案 2 :(得分：1)
请对e和e.getaBook()进行空检查。仅供参考，我更新了代码以适应单一陈述中的所有条件。
for (Employee e : employeeArray)
{
if (e!=null && e.getaBook()!=null &&e.getaBook().getBookName() != null && e.getaBook().getBookName().equals(aName)) //modified code
startDate = e.getaBook().getStartDate();
Period period = Period.between ( startDate, datePassed );
daysElapsed2 = period.getDays();
e.setRetainingTime(daysElapsed2);
System.out.println(e.getName() + " has passed on "+ e.getaBook().getBookName()+" changing his/her's retaining time to " + daysElapsed2+ ".");
}
}