Java 对象中的空指针异常错误

Question

我编写了一个包含对象的程序，但在尝试运行时出现 NullPointerException 错误，但我无法发现我的代码的确切问题。在这里（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

而不是赋值给这两个成员变量：

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

你用这两行声明了新的局部变量：

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

修复它们以初始化您的 Rooms 和 Movies class 成员：

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

Answer 2

这两个数组应该是class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;
}

为什么不使用列表来存储电影（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 3

问题是您的构造函数隐藏了您的 class 成员。

如果你声明 private Movie Movies[]; 那么在你的构造函数中你应该写：Movies = new Movie[MAX_NUM_MOVIES];

当你写：Movie Movies[] = new Movie[MAX_NUM_MOVIES]; 那么你实际上声明了一个新变量，它是构造函数的局部变量，这就是为什么 class 成员 Movies 为空，因为它不是谁被初始化了

Answer 4

而不是定义这个：

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

为什么你只定义一个集合！？喜欢：

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

然后，在您的构造函数中，您必须初始化您的集合：

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

我会说这是 Java 方式。还要记住，属性名称应该是小写的。

Answer 5

我认为错误出在您的 Cinema class 构造函数中。它应该看起来像这样：

   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;
}

Class 属性将被初始化并且 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，如果您想在构造函数中分配内存，您需要：

Rooms = new Room[MAX_NUM_ROOMS];

Java 对象中的空指针异常错误

Java null pointer Exception Error in Objects

java

methods

error-handling

object

nullpointerexception