java - JComboBox 空指针异常。需要从框中获取 Int 并将 int 设置为从框中选择的 int

Question

我的程序现在需要加载两个文本文件，我正在按字符串对它们进行排序。 JcomboxBox 应该允许您在 1 和 4 之间选择 int(要比较的字符串的大小)所以 2 会返回“我是”，而 1 会返回“我” 我得到空点异常，我以前从未使用过组合框。请帮忙/

import java.awt.GridLayout;

import javax.swing.JFrame;
import java.awt.event.*;
import javax.swing.*;
import java.awt.*;
import java.io.*;
import java.util.Scanner;
import java.util.Arrays;
import javax.swing.JComboBox;


public class Lab8 extends JPanel
{
  private JPanel text;
  private JComboBox input;
  private JLabel label;
  private JButton load, go,go2;
  private CountList<SuperString> words;
  private String filename;
  private int width = 400;
  private int height = 600;
  private TextArea textarea,textarea2;
  Scanner scan;

  public Lab8()
  {
    Integer [] select = {1,2,3,4};
    JComboBox input = new JComboBox(select);
    text = new JPanel(new GridLayout(1,2));
    go = new JButton("Select Text File");
    go2 = new JButton("Select 2nd Text File");
    label = new JLabel("How many sequenced words would you like to analyze? (Must be => 1)" );
    input.setSelectedIndex(0);

    ButtonListener listener = new ButtonListener();
    go.addActionListener(listener);
    go2.addActionListener(listener);
    input.addActionListener(listener);

    textarea = new TextArea("",0,0,TextArea.SCROLLBARS_VERTICAL_ONLY);
    textarea2 = new TextArea("",0,0,TextArea.SCROLLBARS_VERTICAL_ONLY);
    textarea.setFont(new Font("Helvetica",Font.PLAIN,24));
    textarea2.setFont(new Font("Helvetica",Font.PLAIN,24));
    textarea.setPreferredSize(new Dimension(width,height));
    textarea2.setPreferredSize(new Dimension(width,height));
    setPreferredSize(new Dimension(900,600));
    text.add(textarea);
    text.add(textarea2);
    add(input);
    add(go);
    add(go2);
    add(text);

    textarea.setText("No File Selected");
    textarea2.setText("No File Selected");


  }
    public class ButtonListener implements ActionListener //makes buttons do things
    {
      JFileChooser chooser = new JFileChooser("../Text");


      public void actionPerformed(ActionEvent event)
      {
       Integer N = input.getSelectedIndex();

        if(event.getSource() == go)
      {
        int returnvalue = chooser.showOpenDialog(null);

    if(returnvalue == JFileChooser.APPROVE_OPTION)
    {
      try
      {
        File file = chooser.getSelectedFile();
        filename = file.getName();
        System.err.println(filename);
        scan = new Scanner(file);
      }
      catch (IOException e)
      {
        System.err.println("IO EXCEPTION");
        return;
      }       
    }
    else
    {
      return;
    }

     String[] storage = new String[N];
    words = new CountLinkedList<SuperString>();
   for(int i=1;i<N;i++)
     storage[i] = scan.next().toLowerCase().replace(",","").replace(".","");

    while(scan.hasNext())
    {
      for(int i=0;i<=N-2;i++)
        storage[i] = storage[i+1];
      storage[N-1] = scan.next().toLowerCase();
      storage[N-1] = storage[N-1].replace(",","").replace(".","");
      SuperString ss = new SuperString(storage);
    //  System.out.println(ss);
      words.add(ss );
    }
    scan.close();
    textarea.append("    "+filename+" has wordcount: "+words.size()+
      "\n-------------------------\n\n");

    SuperString[] ss = new SuperString[words.size()];
    int i=0;
    for(SuperString word: words)
    {
      ss[i] = word;
      i++;
    }
    Arrays.sort(ss, new SuperStringCountOrder());
    for(SuperString word : ss)
    {
        textarea.append("  "+word+"\n");     
    }
  }


         if(event.getSource() == go2)
      {
        int returnvalue = chooser.showOpenDialog(null);

    if(returnvalue == JFileChooser.APPROVE_OPTION)
    {
      try
      {
        File file = chooser.getSelectedFile();
        filename = file.getName();
        System.err.println(filename);
        scan = new Scanner(file);
      }
      catch (IOException e)
      {
        System.err.println("IO EXCEPTION");
        return;
      }       
    }
    else
    {
      return;
    }
     String[] storage = new String[N];
    words = new CountLinkedList<SuperString>();
   for(int i=1;i<N;i++)
     storage[i] = scan.next().toLowerCase().replace(",","").replace(".","");

    while(scan.hasNext())
    {
      for(int i=0;i<=N-2;i++)
        storage[i] = storage[i+1];
      storage[N-1] = scan.next().toLowerCase();
      storage[N-1] = storage[N-1].replace(",","").replace(".","");
      SuperString ss = new SuperString(storage);
      words.add(ss );
    }
    scan.close();
    textarea2.append("    "+filename+" has wordcount: "+words.size()+
      "\n-------------------------\n\n");

    SuperString[] ss = new SuperString[words.size()];
    int i=0;
    for(SuperString word: words)
    {
      ss[i] = word;
      i++;
    }
    Arrays.sort(ss, new SuperStringCountOrder());
    for(SuperString word : ss)
    {
        textarea2.append("  "+word+"\n");     
    }
  }

    }
    }


   public static void main(String[] arg)
  {
    JFrame frame = new JFrame("Lab 8");
    frame.getContentPane().add(new Lab8());
    frame.pack();
    frame.setVisible(true);
  }
}

Answer 1

您要在构造函数中重新声明 JComboBox 变量，从而隐藏在类中找到的字段。通过这样做，该字段保持为空:

public class Lab8 extends JPanel
{
  private JPanel text;
  private JComboBox input;  // this guy remains null

  // .... etc ....

  public Lab8()
  {
    Integer [] select = {1,2,3,4};

    // the line below initializes a local input variable.
    // this variable is visible only inside of the constructor
    JComboBox input = new JComboBox(select); // ***** here ****

不要重新声明变量:

public class Lab8 extends JPanel
{
  private JPanel text;
  private JComboBox input;

  // .... etc ....

  public Lab8()
  {
    Integer [] select = {1,2,3,4};
    // JComboBox input = new JComboBox(select); // ***** here ****
    input = new JComboBox(select); // ***** note difference? *****