用列表填充JList <String> 空指針異常

Question

我試圖使用我創建的文件填充JList，然后再編寫將要添加的內容添加到正在讀取的文件中的代碼，我想確保它將被讀取並正確顯示在屏幕上。 我收到一個nullpointerexception，並且我的GUI正在啟動，但僅顯示“輸入”按鈕。 感謝任何人可以為我提供的有關此問題的幫助。

package movieinfo;

import java.awt.Color;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.List;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.File;
import java.io.IOException;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.Map;
import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JList;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;

import org.apache.commons.io.FileUtils;

import com.json.parsers.JSONParser;
import com.json.parsers.JsonParserFactory;


public class Swinggui {
    private static JButton enter;
    private static JTextField movietext;
    private static JTextArea movieinfo;
    private static JList listofmovies;//converts moviestowatch into gui element.
    private static File textfilemovie; //file which movies marked for watching are saved
    private static java.util.List<String> moviestowatch; //arraylist which is populated by textfilemovie than printed to GUI element.
    public static void main(String[] args) throws IOException
    {
        Gui();
        Json();
        YourMovies();
    }
    public static void Gui()
    {
        JFrame maingui = new JFrame("Gui");
        maingui.setLayout(new GridBagLayout());
        GridBagConstraints c = new GridBagConstraints();
        c.fill = GridBagConstraints.VERTICAL;
        enter = new JButton("Enter");
        c.gridx = 2;
        c.gridy = 1;
        maingui.add(enter, c);
        movieinfo = new JTextArea(5,20);
        movieinfo.setBorder(BorderFactory.createMatteBorder(2,2,2,2,Color.red));
        movietext = new JTextField(18);
        c.gridx = 1;
        c.gridy = 1;
        maingui.add(movietext, c);
        final JScrollPane scrolll = new JScrollPane(movieinfo);
        c.gridx = 1;
        c.gridy = 4;
        c.gridwidth = 2;
        maingui.add(scrolll, c);
        final JLabel titlee = new JLabel("Enter movie name below!");
        c.gridx = 1;
        c.gridy = 0;
        maingui.add(titlee, c);
        maingui.setResizable(false);
        maingui.setVisible(true);
        listofmovies = new JList(moviestowatch.toArray());
        c.gridx = 2;
        c.gridy = 3;
        maingui.add(new JScrollPane(listofmovies), c);
        movieinfo.setLineWrap(true);
        movieinfo.setWrapStyleWord(true);
        movieinfo.setEditable(false);
        scrolll.getPreferredSize();
        //pangui.setPreferredSize(new Dimension(300, 150));
        //pangui.add(scrolll, BorderLayout.CENTER);
        //movieinfo.add(scrolll);
        maingui.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        maingui.pack();

    }
    public static void Json()
    {
        enter.addActionListener(new ActionListener(){
            @SuppressWarnings("rawtypes")
            public void actionPerformed(ActionEvent e)  
            {
                System.out.println(apicall.getMovieInfo(movietext.getText()));
                JsonParserFactory factory=JsonParserFactory.getInstance();
                JSONParser parser=factory.newJsonParser();
                Map jsonData=parser.parseJson(apicall.getMovieInfo(movietext.getText()));
                String Title = (String)jsonData.get("Title");
                String Year = (String)jsonData.get("Year");
                String Plot = (String)jsonData.get("Plot");
                movieinfo.setText("Title: "+Title+"\nYear: "+ Year +"\nPlot: "+Plot);
            }
        });
    }
    public static void YourMovies() throws IOException
    {
        textfilemovie = new File("yourmovies.txt");
        moviestowatch = FileUtils.readLines(textfilemovie);

    }
}

Answer 1

我看到在方法Gui()您使用的是未初始化的列表。 之后會初始化列表初始化的方法（方法YourMovies() ）

listofmovies = new JList(moviestowatch.toArray());
                         ^^^^^^^^^^^^^
                         this is null

附帶說明：

根據Java命名約定，方法以小寫字母開頭，然后以駝峰字母yourMovies() gui() ，例如yourMovies() gui()

Answer 2

在實例化moviestowatch之前，您要調用moviestowatch.toArray() 。 您需要先調用YourMovies()然后再調用Gui() 。

Answer 3

moviestowatch Gui時未分配moviestowatch顛倒這些方法的順序

YourMovies();
Gui();

並遵循方法名稱的Java命名約定，后跟首字母小寫 ，例如loadMovies