java - 将数据从后端的 ArrayList 传递到 JComboBox GUI 前端 - Java Swing

Question

这是我的 FileIOManagement 类，我想处理从文本文件等读取数据以在 GUI 中显示的所有读取操作。

这是我当前的 FileIOManagement 类的代码:

包裹 Swing ；

import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashSet;
import java.util.Scanner;

public class FileIOManagement {

     private ArrayList<String> nameList = new ArrayList<String>(); 
     private ArrayList<String> courseList = new ArrayList<String>(); 
     private ArrayList<String> semesterList = new ArrayList<String>();
     private ArrayList<String> moderatorList = new ArrayList<String>();
     private ArrayList<String> programList = new ArrayList<String>();
     private ArrayList<String> majorList = new ArrayList<String>();


     public FileIOManagement(){
         readTextFile();
     }

      private void readTextFile(){
         try{
             Scanner scan = new Scanner(new File("Course.txt"));

             while(scan.hasNextLine()){
             String line = scan.nextLine();            
             String[] tokens = line.split("~");
             String course = tokens[0].trim();
             String examiner = tokens[1].trim();
             String moderator = tokens[2].trim();
             String semester = tokens[3].trim();
             String program = tokens[4].trim();
             String major = tokens[5].trim();


             courseList.add(course);
             semesterList.add(semester);
             nameList.add(examiner);
             moderatorList.add(moderator);
             programList.add(program);
             majorList.add(major);
             HashSet hs = new HashSet();
             hs.addAll(nameList);
             nameList.clear();
             nameList.addAll(hs);
             Collections.sort(nameList);

         }
             scan.close();
         }
         catch (FileNotFoundException e){
             e.printStackTrace();
         }
     }



}

这是我需要将 ArrayList 数据传递到的类。正如您所看到的，我正在尝试使用我试图通过 ArrayList 获取的数据来填充comboBox1和comboBox2:

package swinging;

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.util.*;
import javax.swing.border.EmptyBorder;


public class ReportGUI extends JFrame{
    //Fields
    private JButton viewAllReports = new JButton("View All Program Details");
    private JButton viewPrograms = new JButton("View Programs and Majors Associated with this course"); 
    private JButton viewTaughtCourses = new JButton("View Courses this Examiner Teaches"); 
    private JLabel courseLabel = new JLabel("Select a Course: ");
    private JLabel examinerLabel = new JLabel("Select an Examiner: "); 
    private JPanel panel = new JPanel(new GridLayout(6,2,4,4));  
    FileIOManagement fileName;
    ArrayList<String> names = new ArrayList<String>(fileName.getNameList());
    ArrayList<String> courses = new ArrayList<String>(fileName.getCourseList());








     public ReportGUI(){   
            reportInterface();

            allReportsBtn();     
//            fileRead();

            comboBoxes();
     }        




    private void reportInterface(){         
          setTitle("Choose Report Specifications");                   
          setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
          JPanel panel = new JPanel(new FlowLayout());        
          add(panel, BorderLayout.CENTER);
          setSize(650,200);
          setVisible(true);    
          setResizable(false);
          setLocationRelativeTo(null);
}    
    private void allReportsBtn(){
        JPanel panel = new JPanel(new GridLayout(1,1)); 
        panel.setBorder(new EmptyBorder(70, 50, 70, 25));
        panel.add(viewAllReports);        
        viewAllReports.addActionListener(new ActionListener(){
            @Override
            public void actionPerformed(ActionEvent e){               
                new AllDataGUI();
            }
        });         
        add(panel, BorderLayout.LINE_END);
    }       
   private void comboBoxes(){     

        panel.setBorder(new EmptyBorder(0, 5, 5, 10));
       String[] comboBox1Array = names.toArray (new String[names.size()]);
        JComboBox comboBox1 = new JComboBox(comboBox1Array);          
        panel.add(examinerLabel);
        panel.add(comboBox1);          
        panel.add(viewTaughtCourses);
         viewTaughtCourses.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {                 
                 new ViewCourseGUI();
            }
        });  
         String[] comboBox2Array = courses.toArray(new String[courses.size()]);
         JComboBox comboBox2 = new JComboBox(comboBox2Array);
         panel.add(courseLabel);         
         panel.add(comboBox2); 
         panel.add(viewPrograms);
         viewPrograms.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {                 
                 new ViewProgramGUI();             
            }
        });  
         add(panel, BorderLayout.LINE_START); 

    }   

}

但是，当我尝试编译时，我得到一个 NullPointerException ，如下图所示:

我在这里做错了什么？

Answer 1

您所做的是尝试获取 fileName.getNameList()哪里fileName从未被实例化，因此它将返回 null .

问题:

FileIOManagement fileName; //was not instantiated
ArrayList<String> names = new ArrayList<String>(fileName.getNameList()); //fileName.getNameList() is null
ArrayList<String> courses = new ArrayList<String>(fileName.getCourseList()); //fileName.getCourseList() is null

解决方案:

实例化您的FileIOManagement fileName在获取它的列表之前。