Array null pointer exception error

2020-04-08 14:04发布

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问题:

I am continuing with my project for school and have seemed to encounter another error. So what is happening is basically I am receiving a null pointer exception even though the code looks fine. I believe something is wrong with my array, and even after hours of searching I can't seem to find the error. Once again, any help/solution would be greatly appreciated. 

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;
}

}

I have a student class with a constructor, and a method that states

public String getFirstName() {
        return fName;
    }

but still I am getting an error. I know it is a lot of code to go through, so once again any kind of help would be greatly appreciated.

回答1:

As your code is currently written,

System.out.println(studentArray[0].getFirstName());

Will throw an NPE since you're never initializing the array.

private static students[] studentArray;

Just declares it, doesn't initialize it.

Maybe you should refactor your code as follows

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"));
//...

If you're committed to using an array and not a List,

private static students[] studentArray;

needs to be initialized with something like

private static students[] studentArray = new students[10];

and then you need to assign its elements

studentArray[0] = stu1;

or

studentArray[0] = new students(65435, "Bob", "Ted");


回答2:

You should always create a object before trying to access it. You have declared a reference pointing to a array :

private static students[] studentArray;

but never assigned any object to the reference. Hence you get a NullPointerException.

Use this :

private static students[] studentArray = new students[size];

Above code will create a new object (array of students), pointed by a reference variable (studentArray). OR

You can use List or Set which are more efficient then array.



标签： java arrays class nullpointerexception
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