Java初学者：nullpointerexception错误

Question

我正在开发一个图书馆管理程序（这是一个在线课程的作业），但我无法解决运行时出现的空指针异常错误。

这是目前为止的程序：

//file Library.java
public class Library {

String address;
Book[] collection;
int collectionCounter = 0;

    // Add the missing implementation to this class
    public static void main(String[] args)
    {
        // Create two libraries
        Library firstLibrary = new Library("10 Main St.");
        Library secondLibrary = new Library("228 Liberty St.");
        // Add four books to the first library
        firstLibrary.addBook(new Book("The Da Vinci Code"));
        firstLibrary.addBook(new Book("Le Petit Prince"));
        firstLibrary.addBook(new Book("A Tale of Two Cities"));
        firstLibrary.addBook(new Book("The Lord of the Rings"));
    }

    //Constructor
    public Library(String libraryName)
    {
        address = libraryName;
        collectionCounter = 0;
    }

    //Methods
    public void addBook(Book newBook)
    {
        System.out.println(this.collectionCounter);
        this.collection[this.collectionCounter] = newBook;
        this.collectionCounter += 1;
    }

还有另一个 .java 文件，用于 Book 类：

public class Book {
String title;
boolean borrowed;

    // Creates a new Book
    public Book(String bookTitle) {
        // Implement this method
        title = bookTitle;
        borrowed = false;
    }
    // Marks the book as rented
    public void rented() {
        // Implement this method
        this.borrowed = true;
    }

    // Marks the book as not rented
    public void returned() {
        // Implement this method
        this.borrowed = false;
    }

    // Returns true if the book is rented, false otherwise
    public boolean isBorrowed() {
        // Implement this method
        return this.borrowed;
    }

    // Returns the title of the book
    public String getTitle() {
        return this.title;
    }

    public static void main(String[] arguments) {

        // Small test of the Book class
        Book example = new Book("The Da Vinci Code");
        System.out.println("Title (should be The Da Vinci Code): " + example.getTitle());
        System.out.println("Borrowed? (should be false): " + example.isBorrowed());
        example.rented();
        System.out.println("Borrowed? (should be true): " + example.isBorrowed());
        example.returned();
        System.out.println("Borrowed? (should be false): " + example.isBorrowed());
    }

}

这是程序输出：

0
Exception in thread "main" java.lang.NullPointerException
    at Library.addBook(Library.java:59)
    at Library.main(Library.java:14)

我知道错误是由书籍数组引起的，但我真的不知道该怎么做，我从未见过将对象实例化作为方法的参数。提前致谢！

Answer 1

您必须创建数组collection 才能将任何内容放入其中。替换

Book[] collection;

通过

Book[] collection = new Book[BOOK_COLLECTION_SIZE];

BOOK_COLLECTION_SIZE 是一个相当高的数字。

更好的是，使用List 而不是数组。这样您就不必事先猜测库的大小：

List<Book> collection = new LinkedList<Book>();

那么addBook可以是这样的：

public void addBook(Book newBook)
{
    this.collection.add(newBook);
}

你可以摆脱collectionCounter。如果您真的需要书的数量，可以使用this.collection.size() 获取。

Answer 2

问题在于您的Library 课程。 它是addBook 方法使用this.collection，但是这只是声明，因此仍然是null。

public void addBook(Book newBook)
{
    System.out.println(this.collectionCounter);
    this.collection[this.collectionCounter] = newBook;
    this.collectionCounter += 1;
}