如何从JTextField获取用户输入？

Question

我正在尝试一个非常简单的表单,旨在将用户输入带入JTextField并通过弹出对话框显示相同的输入.

我可以使用setText()将JTextField硬编码为具有预设编号.如果我这样做,我的程序完美无瑕.

但是,当我将该字段留空并尝试使用getText()在弹出对话框中显示文本时,我要么得到一个空的弹出框架,要么我得到一个'空字符串'异常(我试图将String解析为Double) .)

package buttontest;

import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.ComponentEvent;
import javax.swing.*;
import java.awt.event.ActionEvent;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;



public class ButtonTest
{
    public static void main(String[] args)
    {
        ButtonFrame frame = new ButtonFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}
class ButtonFrame extends JFrame
{
    @SuppressWarnings("LeakingThisInConstructor")
    public ButtonFrame()
    {


                setTitle("SunStream Loan Calculator v2.0");
        setSize(900,900);
        ButtonPanel panel = new ButtonPanel();
        panel.add(new JLabel("Enter your loan amount:"));
                loanAmt = new JTextField(40);
                panel.add(loanAmt);

                add(panel,BorderLayout.CENTER);
    }


        public JTextField loanAmt;

class ButtonPanel extends JPanel implements ActionListener
{
    private Component frame;
    public ButtonPanel()
    {

                final JButton b2 = new JButton("Calculate");
        add(b2, BorderLayout.SOUTH);
                b2.setActionCommand("calculate");
                b2.addActionListener(new ActionListener(){
                    public void actionPerformed(ActionEvent e)
                        {
                              ButtonFrame bf = new ButtonFrame();
                              if("calculate".equals(e.getActionCommand()))
                                {
                                     JOptionPane.showMessageDialog(frame, bf.loanAmt.getText());
                                }
                        }

    });

        }

        @Override
        public void actionPerformed(ActionEvent ae) {
            throw new UnsupportedOperationException("Not supported yet.");
        }


  }
}

任何帮助将不胜感激.我正在使用KeyListener或KeyEvent进行研究,但我不太了解它.

Answer 1

你正在b2的ActionListener中创建一个"阴影"ButtonFrame变量.是的,bf变量引用的ButtonFrame对象与显示的ButtonFrame对象具有相同的类,但它引用了一个完全不同且不可视化的对象.解决方案的关键是从实际显示的ButtonFrame对象中获取文本,这可以通过ButtonFrame.this构造从内部类中获取:

     b2.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
           //!! ButtonFrame bf = new ButtonFrame();
           if ("calculate".equals(e.getActionCommand())) {

              //!! note use of ButtonFrame.this:
              JOptionPane.showMessageDialog(frame, ButtonFrame.this.loanAmt.getText());
           }
        }

     });

接下来考虑使用公共getter,而不是直接访问JTextField等字段.这减少了代码导致副作用的可能性,例如无意中更改了JTextField对象的属性.

例如(由// !!评论表示的变化):

import java.awt.*;
import java.awt.event.ActionListener;
import javax.swing.*;
import java.awt.event.ActionEvent;

public class ButtonTest {
   public static void main(String[] args) {
      ButtonFrame frame = new ButtonFrame();
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setVisible(true);
   }
}

class ButtonFrame extends JFrame {

   private JTextField loanAmt; // !! Make field private

   @SuppressWarnings("LeakingThisInConstructor")
   public ButtonFrame() {

      setTitle("SunStream Loan Calculator v2.0");
      setSize(900, 900);
      ButtonPanel panel = new ButtonPanel();
      panel.add(new JLabel("Enter your loan amount:"));
      loanAmt = new JTextField(40);
      panel.add(loanAmt);

      add(panel, BorderLayout.CENTER);
   }

   // !! create a public method to get JTextField's text
   // !! without exposing the JTextField itself.
   public String getLoanAmtText() {
      return loanAmt.getText();
   }

   class ButtonPanel extends JPanel implements ActionListener {
      private Component frame;

      public ButtonPanel() {

         final JButton b2 = new JButton("Calculate");
         add(b2, BorderLayout.SOUTH);
         b2.setActionCommand("calculate");
         b2.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
               // !! ButtonFrame bf = new ButtonFrame();
               if ("calculate".equals(e.getActionCommand())) {

                  //!! call public method on ButtonFrame object
                  JOptionPane.showMessageDialog(frame,
                        ButtonFrame.this.getLoanAmtText());
               }
            }

         });

      }

      @Override
      public void actionPerformed(ActionEvent ae) {
         throw new UnsupportedOperationException("Not supported yet.");
      }

   }
}