Ich mache mit meinem Projekt für die Schule weiter und bin anscheinend auf einen anderen Fehler gestoßen. Was also passiert, ist, dass ich eine Nullzeiger-Ausnahme erhalte, obwohl der Code gut aussieht. Ich glaube, dass etwas mit meinem Array nicht stimmt, und selbst nach stundenlangem Suchen kann ich den Fehler scheinbar nicht finden. Auch hier wäre jede Hilfe / Lösung sehr dankbar.

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;
}

}

Ich habe eine Studentenklasse mit einem Konstruktor und einer Methode, die besagt

public String getFirstName() {
        return fName;
    }

aber trotzdem erhalte ich einen fehler. Ich weiß, dass es eine Menge Code ist, der durchlaufen werden muss, daher wäre jede Art von Hilfe sehr dankbar.