如何让 JTextField 只接受一个字母？

Question

我有一个具有多个文本字段的应用程序，我希望文本字段仅采用一个字母并使用另一个字母。我试图通过 ActionListener 和 KeyAdapter 并为所有文本字段仅创建一个侦听器来完成此操作。

唯一的问题是，当它在一个文本字段中接受输入时，它不会在其他文本字段中接受输入。那么请问我的代码有什么问题吗？

到目前为止我的代码：

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class OnlyAlphabet extends JFrame 
{ 
int  checker=0;
private final KeyAdapter listener = new KeyAdapter() {
public void keyTyped(KeyEvent e) {
  char c = e.getKeyChar();
  if(c==KeyEvent.VK_BACK_SPACE )
          { checker =0 ; return;  }  
  if(checker!=0)
          { e.consume();return;}
  if(!(Character.isAlphabetic(c) || (c==KeyEvent.VK_BACK_SPACE) || c==KeyEvent.VK_DELETE)) 
         {  e.consume(); return; }
  if(Character.isAlphabetic(c) && checker==0)
         {  checker =1; return; }
     }
  };

public void initComponent() {
  setLayout(new FlowLayout());
  JLabel lbl = new JLabel("Enter a Letter: ");
  JTextField textField = new JTextField(15);
  JTextField textField2 = new JTextField(15);
  add(lbl);
  add(textField);
  add(textField2);
  textField.addKeyListener(listener);
  textField2.addKeyListener(listener);
  setSize(300,300);
  setLocationRelativeTo(null);
  setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  setVisible(true);
}

public static void main(String[]args){
  new OnlyAlphabet().initComponent();
}
}

请检查一下，我想将相同的侦听器添加到多个文本字段，代码现在只包含 2 个，但会有更多，例如 25-30

Answer 1

当您希望在文本字段中的文本发生更改时收到通知。当您想要过滤底层文档中发生更改的内容

import java.awt.EventQueue;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.border.EmptyBorder;
import javax.swing.text.AbstractDocument;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.DocumentFilter;

public class Main {
    public static void main(String[] args) {
        new Main();
    }

    public Main() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame();
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {
        public TestPane() {
            setBorder(new EmptyBorder(32, 32, 32, 32));
            setLayout(new GridBagLayout());
            GridBagConstraints gbc = new GridBagConstraints();
            gbc.gridwidth = GridBagConstraints.REMAINDER;

            JTextField[] fields = new JTextField[2];
            for (int index = 0; index < fields.length; index++) {
                fields[index] = createTextField();
                add(fields[index], gbc);
            }
        }

        protected JTextField createTextField() {
            JTextField textField = new JTextField(4);
            ((AbstractDocument) textField.getDocument()).setDocumentFilter(new LimitDocumentFilter(1));
            return textField;
        }
    }

    public class LimitDocumentFilter extends DocumentFilter {

        private int maxCharacters;

        public LimitDocumentFilter(int maxChars) {
            maxCharacters = maxChars;
        }

        protected String filter(FilterBypass fb, String str) {
            StringBuilder buffer = new StringBuilder(str);
            for (int i = buffer.length() - 1; i >= 0; i--) {
                char ch = buffer.charAt(i);
                if (!Character.isAlphabetic(ch)) {
                    buffer.deleteCharAt(i);
                }
            }
            int limit = maxCharacters - fb.getDocument().getLength();
            if (limit == 0) {
                return "";
            }
            return buffer.substring(0, Math.min(limit, buffer.length()));
        }

        public void insertString(FilterBypass fb, int offs, String str, AttributeSet a) throws BadLocationException {
            String filtered = filter(fb, str);
            if (!filtered.isEmpty()) {
                super.insertString(fb, offs, filtered, a);
            }
        }

        public void replace(FilterBypass fb, int offs, int length, String str, AttributeSet a) throws BadLocationException {
            String filtered = filter(fb, str);
            if (!filtered.isEmpty()) {
                super.replace(fb, offs, length, filtered, a);
            }
        }

    }
}