java - 对象数组的问题

Question

当我尝试运行此方法时出现空指针异常，目标是填充书目对象数组但不超过 3 个对象。当我设置 booklist[0] = b

时发生错误

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

你需要 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;
}