Question

我用对象编写了一个程序，当我尝试运行它时出现NullPointerException错误，但我无法发现代码的确切问题。这是（粗线是日食指向错误的地方）：

public class Cinema {
    private String cinemaName;
    private String Address;
    private Room Rooms[];
    private Movie Movies[];
    private final int MAX_NUM_MOVIES = 20;
    private final int MAX_NUM_ROOMS = 5;
    private int movieCounter;
    private int roomCounter;

    public Cinema(String cinemaName, String Address) {
        this.cinemaName = cinemaName;
        this.Address = Address;
        Room Rooms[] = new Room[MAX_NUM_ROOMS];
        @SuppressWarnings("unused")
        Movie Movies[] = new Movie[MAX_NUM_MOVIES];
        movieCounter = 0;
        roomCounter = 0;
    }

    public boolean addMovieToCinema (Movie theMovie) {
        if (theMovie == null) {
            return false;
        }
        if (movieCounter < MAX_NUM_MOVIES) {
            Movies[movieCounter] = new Movie(theMovie);
            movieCounter++;
            return true;
        } else
            return false;
    }
}

有什么想法吗？

Answer 1

而不是分配给这两个成员变量：

private Room Rooms[];
private Movie Movies[];

使用以下两行声明新的局部变量：

Room Rooms[] = new Room[MAX_NUM_ROOMS];
Movie Movies[] = new Movie[MAX_NUM_MOVIES];

修复此问题以初始化您的Rooms和Movies班级成员：

Rooms = new Room[MAX_NUM_ROOMS];
Movies = new Movie[MAX_NUM_MOVIES];

Answer 2

而不是定义：

private Room Rooms[];
private Movie Movies[];

为什么你要定义一个集合！？像：

private Collection<Room> rooms;
private Collection<Movie> movies;

然后，在构造函数中，您必须初始化集合：

this.rooms = new ArrayList<>();
this.movies = new ArrayList<>();

我会说这是Java方式。另请注意，属性名称应为小写。

Answer 3

这两个数组应该是类字段

private int movieCounter;
private int roomCounter;
private Room[] Rooms = new Room[MAX_NUM_ROOMS];
private Movie[] Movies = new Movie[MAX_NUM_MOVIES];

public Cinema(String cinemaName, String Address) {
  this.cinemaName = cinemaName;
  this.Address = Address;
  movieCounter = 0;
  roomCounter = 0;
}

为什么不使用列表存储电影（和房间）？您不必提供硬编码限制。

List<Movie> movies = new ArrayList<>();

public boolean addMovieToCinema(Movie theMovie) {
    if (theMovie == null) {
        return false;
    }
    movies.add( theMovie );
    return true;
}

一切都变得简单。

Answer 4

问题是你的构造函数隐藏了你的类成员。

如果你在构造函数中声明private Movie Movies[];，那么你应该写：Movies = new Movie[MAX_NUM_MOVIES];

当您编写：Movie Movies[] = new Movie[MAX_NUM_MOVIES];时，您实际上声明了一个构造函数本地的新变量，这就是类成员Movies为空的原因，因为它不是初始化的。

Answer 5

我认为错误发生在Cinema类的构造函数中。它应该是这样的：

   public Cinema(String cinemaName, String Address) {
    this.cinemaName = cinemaName;
    this.Address = Address;
// Remove type before variable to initialize class attribute instead of declaring a new varialble
    Rooms = new Room[MAX_NUM_ROOMS];
    @SuppressWarnings("unused")
// Remove type before variable to initialize class attribute instead of declaring a new varialble
    Movies = new Movie[MAX_NUM_MOVIES];
    movieCounter = 0;
    roomCounter = 0;
}

将初始化Class属性，并且NullpointerException应该消失。

干杯，塞巴斯蒂安。

Answer 6

一种选择是在声明数组时分配内存：

private Room[] Rooms = new Room[MAX_NUM_ROOMS];

或者，如果要在构造函数中分配内存，则需要：

Rooms = new Room[MAX_NUM_ROOMS];

Java空指针对象中的异常错误

6 个答案: