数组空指针异常错误
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【中文标题】数组空指针异常错误【英文标题】：Array null pointer exception error 【发布时间】：2013-03-22 18:25:39 【问题描述】：我正在继续我的学校项目，但似乎遇到了另一个错误。所以发生的事情基本上是我收到一个空指针异常，即使代码看起来不错。我相信我的数组有问题，即使经过数小时的搜索，我似乎也找不到错误。再次，任何帮助/解决方案将不胜感激。
import java.util.*;
public class library
private static students stu1 = new students(65435, "Bob", "Ted");
private static students stu2 = new students(45546, "Guy", "Sid");
private static students stu3 = new students(78688, "Tim", "Cas");
private static students stu4 = new students(45387, "Guy", "Jim");
private static students stu5 = new students(12367, "Dom", "Lam");
private static students stu6 = new students(45905, "Kid", "Loo");
private static students stu7 = new students(65484, "Mut", "Hum");
private static students stu8 = new students(34578, "Kim", "Hay");
private static students stu9 = new students(20457, "Roy", "Boy");
private static students stu0 = new students(15678, "Kil", "Bil");
private static students[] studentArray;
private static students[] stuArrayAlt;
private static boolean firstArrayStu = true;
// private static books bookName = new books("title",
// "author","category","isbn", cost, rating out of 10);
private static books book1 = new books(
"Harry Potter and the Deathly Hallows", "JK Rowling", "fantasy",
"9780739360385", 30.00, 9.0);
private static books book2 = new books("Angels and Demons", "Dan Brown",
"fiction", "9780828855495", 25.00, 8.5);
private static books book3 = new books("The Hunger Games",
"Suzanne Collins", "science fiction", "9780439023481", 20.00, 8.0);
private static books book4 = new books("A Game of Thrones",
"George R R Martin", "fantasy", "9780002245845", 54.50, 12.5);
private static books book5 = new books("title2", "author2", "category2",
"isbn2", 54.50, 12.5);
private static books book6 = new books("title2", "author2", "category2",
"isbn2", 54.50, 12.5);
private static books book7 = new books("title2", "author2", "category2",
"isbn2", 54.50, 12.5);
private static books book8 = new books("title2", "author2", "category2",
"isbn2", 54.50, 12.5);
private static books book9 = new books("title2", "author2", "category2",
"isbn2", 54.50, 12.5);
private static books book0 = new books("title2", "author2", "category2",
"isbn2", 54.50, 12.5);
private static books[] bookArray;
private static books[] bookArrayAlt;
private static boolean firstArrayBook;
private static int year1;
private static int month1;
private static int date1;
public library()
bookArray = new books[] book1, book2, book3, book4, book5, book6,
book7, book8, book9, book0 ;
firstArrayBook = true;
studentArray = new students[] stu1, stu2, stu3, stu4, stu5, stu6,
stu7, stu8, stu9, stu0 ;
firstArrayStu = true;
public static void main(String[] args)
// System.out.println(stu1.getStuNum() + " " + stu1.getFirstName() +
// " "+ stu1.getLastName());
// books[] bookReturn = stu1.insertBook(book1);
// System.out.println(book1.getCheckInLibrary());
// System.out.println(book1.getCheckInLibrary());
// System.out.println(bookReturn[0].getName());
// books[] bookReturn2 = stu1.insertBook(book2);
// System.out.println(book2.getCheckInLibrary());
// System.out.println(book2.getCheckInLibrary());
// stu1.insertBook(book1);
// checkOutBook(stu1,book1);
// System.out.println(stu1);
// stu1=null;
// System.out.println(stu1);
/*
* stu1.lostBookFine(book1); System.out.println(stu1.getFineBalance());
* stu1.lostBookFine(book2); System.out.println(stu1.getFineBalance());
*/
int[] a = new int[] 1, 2, 3, 4, 5 ;
int[] b = new int[(a.length) + 1];
System.arraycopy(a, 0, b, 0, a.length);
b[a.length] = 6;
// for(int i=0;i<b.length;i++)
//
// System.out.println(b[i]);
//
/*
* int[] c = new int[]1,2,3,4,5,6; b = new int[(c.length)+1];
*
* System.arraycopy( c, 0, b, 0, c.length ); b[c.length]=7;
*
* for(int i=0;i<b.length;i++) System.out.println(b[i]);
*/
// int[] date = enterDate();
// int[] date2 = book1.addTwoWeeks(date);
// int[]date2= book1.getDateDue();
System.out.println(studentArray[0].getFirstName());
//boolean array=checkStuNum(45456);
//System.out.println(array);
//students[]array=lookUpLastName("sid");
//array[0].getFirstName();
public static void returnBook(students borrower, books bookReturn)
public static books[] lookUpTitleBooks(String title)// //////////////interchange
// the array-boolean
// firstArrayBook--two
// if statements
int counter = 0;
for (int i = 0; i < bookArray.length; i++)
if (title.equalsIgnoreCase(bookArray[i].getTitle())
|| ((bookArray[i].getTitle().toLowerCase())).contains(title
.toLowerCase()))
counter++;
books[] booksLookUp = new books[counter];
int counterS = 0;
for (int i = 0; i < bookArray.length; i++)
if (title.equalsIgnoreCase(bookArray[i].getTitle())
|| ((bookArray[i].getTitle().toLowerCase())).contains(title
.toLowerCase()))
booksLookUp[counterS] = bookArray[i];
counterS++;
return booksLookUp;
// look up last name of student
public static students[] lookUpLastName(String lName)
students[] studentlName = new students[1] ;
if (firstArrayStu == true)
int counter = 0;
System.out.println("check");
for (int i = 0; i < studentArray.length; i++)
if (lName.equalsIgnoreCase(studentArray[i].getLastName()) || ((studentArray[i].getLastName()).contains(lName.toLowerCase())) )
counter++;
System.out.println("check if");
System.out.println("check for");
studentlName = new students[counter];
int counterS = 0;
for (int i = 0; i < studentArray.length; i++)
if (lName.equalsIgnoreCase(studentArray[i].getFirstName())
|| ((studentArray[i].getFirstName().toLowerCase()))
.contains(lName.toLowerCase()))
studentlName[counterS] = studentArray[i];
counterS++;
if (firstArrayStu == false)
int counter = 0;
for (int i = 0; i < stuArrayAlt.length; i++)
if (lName.equalsIgnoreCase(stuArrayAlt[i].getFirstName())
|| ((stuArrayAlt[i].getFirstName().toLowerCase()))
.contains(lName.toLowerCase()))
counter++;
studentlName = new students[counter];
int counterS = 0;
for (int i = 0; i < stuArrayAlt.length; i++)
if (lName.equalsIgnoreCase(stuArrayAlt[i].getFirstName())
|| ((stuArrayAlt[i].getFirstName().toLowerCase()))
.contains(lName.toLowerCase()))
studentlName[counterS] = stuArrayAlt[i];
counterS++;
return studentlName;
public static void checkOutBook(students borrower, books bookBorrow)
boolean canBorrow1 = checkFine(borrower);
boolean canBorrow2 = checkBorrowedBooks(borrower);
boolean canBorrow3 = checkBorrowedBooks(bookBorrow);
if (canBorrow1 == false)
System.out.println("Your fine is too damn high");// alert window and
// redirect to
// main menu-so
// he/she can
// pay it if he
// wants to
if (canBorrow2 == false)
System.out.println("Your already have 3 books checked out");// alert
// window
// and
// redirect
// to
// main
// menu-
// so
// he/she
// can
// return
// a
// book
// if he
// wants
// to
if (canBorrow1 == false)
System.out.println("This book has been checkd out");// alert window
// and redirect
// to main menu-
// so he/she can
// look for
// another book
// and check it
// out
if (canBorrow1 && canBorrow2 && canBorrow3)
borrower.insertBook(bookBorrow);
bookBorrow.checkOutStatusChange();
// alert window to show successful check out and redirect to main
// menu
public static boolean checkFine(students borrower)
boolean canBorrow1 = borrower.checkBorrowedBooks(borrower
.getBookArray());
return canBorrow1;
public static boolean checkBorrowedBooks(students borrower)
boolean canBorrow2 = borrower.checkFine(borrower);
return canBorrow2;
public static boolean checkBorrowedBooks(books bookBorrow)
boolean canBorrow3 = bookBorrow.getCheckInLibrary();
return canBorrow3;
public static int[] enterDate()
Scanner input = new Scanner(System.in);
boolean dateTrue = false;
int year, month, date;
while (dateTrue == false)
System.out.println("Enter year");
year = input.nextInt();
System.out.println("Enter month");
month = input.nextInt();
System.out.println("Enter date");
date = input.nextInt();
// checking first date----------------------------------------
year1 = ((year >= 2010) ? year : 0);
month1 = ((month >= 1 && month <= 12) ? month : 0);
if (month1 == 1)
date1 = ((date >= 1 && date <= 31) ? date : 0);
if (month1 == 2 && (year1 % 4) != 0)
date1 = ((date >= 1 && date <= 28) ? date : 0);
if (month1 == 2 && (year1 % 4) == 0)
date1 = ((date >= 1 && date <= 29) ? date : 0);
if (month1 == 3)
date1 = ((date >= 1 && date <= 31) ? date : 0);
if (month1 == 4)
date1 = ((date >= 1 && date <= 30) ? date : 0);
if (month1 == 5)
date1 = ((date >= 1 && date <= 31) ? date : 0);
if (month1 == 6)
date1 = ((date >= 1 && date <= 30) ? date : 0);
if (month1 == 7)
date1 = ((date >= 1 && date <= 31) ? date : 0);
if (month1 == 8)
date1 = ((date >= 1 && date <= 31) ? date : 0);
if (month1 == 9)
date1 = ((date >= 1 && date <= 30) ? date : 0);
if (month1 == 10)
date1 = ((date >= 1 && date <= 31) ? date : 0);
if (month1 == 11)
date1 = ((date >= 1 && date <= 30) ? date : 0);
if (month1 == 12)
date1 = ((date >= 1 && date <= 31) ? date : 0);
if (month1 == 0 || date1 == 0 || year1 == 0)
// do nothing boolean remains false
// put alert window here
else
dateTrue = true;
int[] dates = year1, month1, date1 ;
return dates;
public void createStudent(int stuNum, String fName, String lName)
if (firstArrayStu == true)
for (int i = 0; i < studentArray.length; i++)
if (studentArray[i] == null)
studentArray[i] = new students(stuNum, fName, lName);
else if (i == (studentArray.length - 1))
stuArrayAlt = new students[studentArray.length + 1];
System.arraycopy(studentArray, 0, stuArrayAlt, 0,
studentArray.length);
stuArrayAlt[studentArray.length] = new students(stuNum,
fName, lName);
firstArrayStu = false;
else if (firstArrayStu == false)
for (int i = 0; i < stuArrayAlt.length; i++)
if (stuArrayAlt[i] == null)
stuArrayAlt[i] = new students(stuNum, fName, lName);
else if (i == (stuArrayAlt.length - 1))
studentArray = new students[stuArrayAlt.length + 1];
System.arraycopy(stuArrayAlt, 0, stuArrayAlt, 0,
stuArrayAlt.length);
studentArray[stuArrayAlt.length] = new students(stuNum,
fName, lName);
firstArrayStu = true;
public static void createBook(String name, String author, String category,
String isbn, double cost, double sRating)
if (firstArrayBook == true)
for (int i = 0; i < bookArray.length; i++)
if (bookArray[i] == null)
bookArray[i] = new books(name, author, category, isbn,
cost, sRating);
else if (i == (bookArray.length - 1))
bookArrayAlt = new books[bookArray.length + 1];
System.arraycopy(bookArray, 0, bookArrayAlt, 0,
bookArray.length + 1);
bookArrayAlt[bookArray.length] = new books(name, author,
category, isbn, cost, sRating);
firstArrayBook = false;
else if (firstArrayBook == false)
for (int i = 0; i < bookArrayAlt.length; i++)
if (bookArrayAlt[i] == null)
bookArrayAlt[i] = new books(name, author, category, isbn,
cost, sRating);
else if (i == (bookArrayAlt.length - 1))
bookArray = new books[bookArrayAlt.length + 1];
System.arraycopy(bookArrayAlt, 0, bookArray, 0,
bookArrayAlt.length + 1);
bookArray[bookArrayAlt.length] = new books(name, author,
category, isbn, cost, sRating);
firstArrayBook = false;
public static boolean deleteStudent(String lName, int stuNum)
students[] arrayLookedUp = lookUpLastName(lName);
boolean deleted = false;
for (int i = 0; i < arrayLookedUp.length; i++)
if ((arrayLookedUp[i].getStuNum()) == stuNum)
System.out.println("checker");
if (firstArrayStu == true)
for (int j = 0; j < studentArray.length; j++)
if (arrayLookedUp[i] == studentArray[j])
studentArray[j] = null;
deleted = true;
break;
break;
else if (firstArrayStu == false)
for (int j = 0; j < stuArrayAlt.length; j++)
if (arrayLookedUp[i] == stuArrayAlt[j])
stuArrayAlt[j] = null;
deleted = true;
break;
else if (i == (arrayLookedUp.length - 1))
deleted = false;
return deleted;
public static boolean checkStuNum(int stuNum)
// private static students[] studentArray;
// private static students[] stuArrayAlt;
// private static boolean firstArrayStu=true;
boolean stuNumNew = false;
System.out.println("test");
if (firstArrayStu == true)
for (int i = 0; i < studentArray.length; i++)
System.out.println("false");
if ((studentArray[i].getStuNum()) == stuNum)
System.out.println("true");
stuNumNew = true;
break;
else if (firstArrayStu == false)
for (int i = 0; i < stuArrayAlt.length; i++)
if ((stuArrayAlt[i].getStuNum()) == stuNum)
stuNumNew = true;
break;
return stuNumNew;
我有一个带有构造函数的学生类，以及一个声明方法
public String getFirstName()
return fName;
但我仍然收到错误消息。我知道要处理很多代码，因此再次感谢任何形式的帮助。
【问题讨论】：
错误出现在哪里？也就是说，堆栈跟踪指向哪一行？ （粘贴错误消息也会很有帮助。） 查看日志cat中空指针异常所在的行号。 在这里过去异常堆栈跟踪！ @iamnotmaynard 第 110 行，library.main(library.java:110) 处的线程“main”java.lang.NullPointerException 中的异常 我们不知道第 110 行是哪一行。 【参考方案1】：由于您当前编写的代码，
System.out.println(studentArray[0].getFirstName());
将抛出 NPE，因为您从未初始化数组。
private static students[] studentArray;
只声明它，不初始化它。
也许你应该重构你的代码如下
private static ArrayList<students> studentArray = new ArrayList<students>();
// Inside the constructor or some method called before you access studentArray:
studentArray.add(stu1);
studentArray.add(stu2);
//...
// or, more concisely, since you're not using the temporary `students` variables:
studentArray.add(new students(65435, "Bob", "Ted"));
studentArray.add(new students(45546, "Guy", "Sid"));
//...
如果您致力于使用数组而不是列表，
private static students[] studentArray;
需要用类似的东西初始化
private static students[] studentArray = new students[10];
然后你需要分配它的元素
studentArray[0] = stu1;
或
studentArray[0] = new students(65435, "Bob", "Ted");
【讨论】：
我用您建议使用的完全相同的代码行初始化了我的数组，但由于某种原因它仍然无法正常工作。 初始化它是不够的，您仍然需要在引用之前为元素分配一个值。请参阅我添加的最后两行。【参考方案2】：您应该始终在尝试访问它之前创建一个对象。 你已经声明了一个指向数组的引用：
private static students[] studentArray;
但从未将任何对象分配给引用。因此你会得到一个 NullPointerException。
使用这个：
private static students[] studentArray = new students[size];
上面的代码将创建一个由引用变量（studentArray）指向的新对象（学生数组）。 或
您可以使用List 或Set，它们比数组更有效。
【讨论】：
你对 bookArray 和 bookArrayAlt 犯了同样的错误。也为他们做同样的事情。检查它给你异常的行。以上是关于数组空指针异常错误的主要内容，如果未能解决你的问题，请参考以下文章