在类的运行上空指针

Question

当我运行这个项目时,我得到一个NullPointer.

"Exception in thread "main" java.lang.NullPointerException
    at movieinfo.Swinggui.gui(Swinggui.java:71)
    at movieinfo.Swinggui.main(Swinggui.java:38)
"

它似乎没有在这两行,我访问尚未被声明.我试图设置断点和调试来自己解决它,但无济于事.谢谢你帮助我,我真的不是要求被舀勺.

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
    {
yourMovies();
        gui();
        json();



    }
    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 = 3;
        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 = 4;
        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(){

            private JsonParserFactory factory;
            private JSONParser parser;
            @SuppressWarnings("rawtypes")
            private Map jsonData;

            public void actionPerformed(ActionEvent e)  
            {
                System.out.println(apicall.getMovieInfo(movietext.getText()));
                factory = JsonParserFactory.getInstance();
                parser = factory.newJsonParser();
                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(org.apache.commons.io.FileUtils.getUserDirectory() + "/yourmovies.txt");

        textfilemovie.createNewFile();
        moviestowatch = FileUtils.readLines(textfilemovie);
        while (listofmovies.getSelectedValue().toString()!= null)
        {
            movietext.setText(listofmovies.getSelectedValue().toString());
            enter.doClick();
        }

    }
}

Answer 1

您在调用yourMovies ()之前调用gui(),因此在使用之前不要初始化ArrayList.

但这不是重要的事情.不,重要的是您认识到如何自行调试NPE的过程:

• 检查抛出NPE的线
• 在该行上找到null的变量
• 然后搜索回代码,看看当你认为它不应该是为什么它是null.

通常这些错误很容易修复,一旦你知道如何.

您的其他问题包括:

• 你过度使用静态了.您的代码没有" 状态 ",没有真正的OOP类,这将限制您扩展和改进代码的能力.唯一的静态方法应该是main方法,这应该只包含设置类并启动它们的代码,就是这样.
• 您的json代码应该在它自己的类中,与GUI类分开.这将允许您独立于完全不相关的代码进行调试和增强.寻找凝聚力和耦合.你希望你的阶级凝聚力很高并且耦合度很低:将相关的东西放在一起,将不相关的东西分开.
• 你的while循环while (listofmovies.getSelectedValue().toString()!= null)...在当前位置没有任何意义.doClick()在创建,构建和渲染之前,不应该调用GUI的按钮.
• 您的代码在没有计划的情况下被抛在一起 更好的方法是在提交任何代码之前先在纸上写出程序结构.