Question

当我尝试运行此方法时，我得到一个空指针异常，目标是填充booklist对象数组但不填充3个对象。当我设置booklist [0] = b

时会发生错误

private Book [] booklist;
public boolean borrowBook(Book b)
{
    if(booklist == null)
    {
        booklist[0] = b;
        System.out.println(this.name+" has successfully borrowed "+b);
        return true;
    }
    if(booklist.length < 3)
    {
        booklist[booklist.length] = b;
        System.out.println(this.name+" has successfully borrowed "+b);
        return true;
    }
    System.out.println(this.name+" has reached the borrowing limit! Return those books "+this.name);
    return false;

Answer 1

您需要ArrayList而不是array

ArrayList<Book> booklist = new ArrayList<Book>();

public boolean borrowBook(Book b){
    if(booklist.size() == 0){
        booklist.add(b);
        System.out.println(this.name+" has successfully borrowed "+b);
        return true;
    }
    if(booklist.size() < 3){ //I'm not sure what you are trying to achieve here
        booklist.add(booklist.size(), b);
        System.out.println(this.name+" has successfully borrowed "+b);
        return true;
    }

    System.out.println(this.name+" has reached the borrowing limit! Return those books "+this.name);
    return false;
}

Answer 2

试试这个。

private Book [] booklist;

public boolean borrowBook(Book b) {
    if (booklist == null) {
        booklist = new Book[3];
        booklist[0] = b;
        return true;
    }
    for (int i = 0; i < booklist.length; i++) {
        if (booklist[i] == null) {
            booklist[i] = b;
            return true;
        }
    }
    return false;
}

public static void main(String[] args) {
    Book caller = new Book();
    System.out.println(caller.borrowBook(new Book()));
    System.out.println(caller.borrowBook(new Book()));
    System.out.println(caller.borrowBook(new Book()));
    System.out.println(caller.borrowBook(new Book()));
}

Answer 3

同样，@ August说。在行booklist[0] = b;

之前

您必须先使用booklist = new Book[3];

进行初始化

Answer 4

Java数组的大小不是动态的，可以编写一个方法来实现它。例如，

private static Book[] addBook(Book[] arr, Book b) {
    int newLen = (arr == null ? 1 : arr.length + 1);
    Book[] dest = new Book[newLen];
    System.arraycopy(arr, 0, dest, 0, arr == null ? 0 : arr.length);
    dest[newLen - 1] = b;
    return dest;
}

然后你可以用它像

这样的东西

public boolean borrowBook(Book b) {
    if (booklist == null || booklist.length < 3) {
        booklist = addBook(booklist, b);
        System.out.println(this.name + " has successfully borrowed " + b);
        return true;
    }
    System.out.println(this.name + " has reached the borrowing limit! "
            + "Return those books " + this.name);
    return false;
}

或者，您可以只分配大小为n的数组，或使用类似ArrayList的集合。

Answer 5

初始化booklist会修复NullPointerException，但您方法的逻辑仍然存在缺陷。您似乎想要将追加一本新书附加到已检出的图书数组中，而您的方法只会设置数组中的最后一个元素。

要解决此问题，您可以使用存储插入的最后一本书的索引的计数器。或者，我建议使用ArrayList之类的可增长数组，它支持使用ArrayList#add附加新元素。

对象数组的麻烦

5 个答案: