java - Java空指针异常-不明白为什么

MovieList.java中主要方法上的运行时错误。

我不确定我的程序设计在本质上是否很好，但是我想知道为什么它崩溃。提前致谢。

package javaPractical.week3;

import javax.swing.*;

public class Movie {
    //private attributes
    private String title;
    private String movieURL;
    private String year;
    private String genre;
    private String actor;

    // constructor
    Movie(String t, String u, String y, String g, String a) {
        this.title = t;
        this.movieURL = u;
        this.year = y;
        this.genre = g;
        this.actor = a;

    }
    //getters and setters
    public void setTitle(String t) {
        this.title = t;
    }

    public String getTitle() {
        return this.title;
    }

    public void set_url(String a) {
        this.movieURL = a;
    }

    public String get_url() {
        return this.movieURL;
    }

    public void setYear(String y) {
        this.year = y;
    }

    public String getYear() {
        return this.year;
    }

    public void setGenre(String g) {
        this.genre = g;
    }

    public String getGenre() {
        return this.genre;
    }

    public void setActor(String a) {
        this.actor = a;
    }

    public String getActor() {
        return this.actor;
    }


    //output movie details
    public String toString() {
        return ("Title: " + this.title + "\nURL: " + this.movieURL + "\nYear: "
            + this.year + "\nGenre: " + this.genre + "\nActor: "
            + this.actor);
    }

    public static void main(String[] args) {
        //testing Movie class
        Movie Movie1 = new Movie("Spiderman", "www.", "2002", "Action",
            "Tobey M");

        JOptionPane.showMessageDialog(null, Movie1.toString());
        //testing MovieList class
    }
}

package javaPractical.week3;

import javax.swing.*;

import java.util.ArrayList;

public class MovieList1 {

    private static ArrayList myFavouriteMovies = new ArrayList();
    private static int NUM_OF_MOVIES = 10;
    private int numberOfMovies = 0;
    private int index = 0;

    public MovieList1() {
        this.myFavouriteMovies = null;
        this.numberOfMovies = 0;
        this.index = 0;
    }

    public int getNumberOfMovies() {
        return this.myFavouriteMovies.size();
    }

    public boolean isEmpty() {
        if (this.myFavouriteMovies.isEmpty()) {
            return true;

        } else
        return false;

    }

    public static void main(String[] args) {
        MovieList1 List = new MovieList1();
        String titleADD;
        String movieURLADD;
        String yearADD;
        String genreADD;
        String actorADD;

        titleADD = JOptionPane.showInputDialog(null, "Enter title:");
        movieURLADD = JOptionPane.showInputDialog(null, "Enter URL:");
        yearADD = JOptionPane.showInputDialog(null, "Enter year:");
        genreADD = JOptionPane.showInputDialog(null, "Enter genre:");
        actorADD = JOptionPane.showInputDialog(null, "Enter actor:");

        Movie TempMovie = new Movie(titleADD, movieURLADD, yearADD, genreADD,
            actorADD);

        myFavouriteMovies.add(TempMovie);
    }
}

最佳答案

尝试将新的Movie添加到myFavouriteMovies时，程序崩溃，因为myFavouriteMovies是null。

尽管myFavouriteMovies初始化为一个新的空ArrayList，但随后在null构造函数中将其设置为MovieList1。

目前，myFavouriteMovies是static，因此每个MovieList1实例之间仅共享此变量的一个副本。您可能想从static声明中删除myFavouriteMovies修饰符。然后，每个MovieList1对象将具有其自己的myFavouriteMovies字段。但是，您随后将向MovieList1类添加新方法，以允许您的main方法将影片添加到影片列表中，也许是这样的：

List.add(TempMovie);

另外，您还需要删除

this.myFavouriteMovies = null;

从构造函数中获取，因为已将其初始化为空的ArrayList，所以您不想将其设置回null。

关于java - Java空指针异常-不明白为什么，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/4061075/