Adding $ sign into the textfield when the user enter in the field

Question

当用户在字段中输入$并禁止用户取消$符号时，任何人都知道如何自动将$符号添加到文本字段中。

Answer 1

Not "strictly" what you're asking, but start by taking How to use for attend text fields , for example...

paymentField = new JFormattedTextField(NumberFormat.getCurrencyInstance());
paymentField.setValue(new Double(payment));
paymentField.setColumns(10);
paymentField.setEditable(false);

The problem with this is it's possible for the user to remove the $ sign and the validation is very strict, meaning that the text entered into the field MUST start with a $

Another possibility is to use the BuddySupport API from Swing Labs, SwingX library

NumberFormat nf = NumberFormat.getNumberInstance();
nf.setMinimumFractionDigits(2);
paymentField = new JFormattedTextField(nf);
paymentField.setValue(100d);
paymentField.setColumns(10);
paymentField.setEditable(false);
BuddySupport.addLeft(new JLabel("$"), paymentField);

This means that that $ is a separate component from the actual field and can't be removed by the user (but is contained within the field so it is not affected by the parent container's layout manager)

Answer 2

The java.awt.TextField could be monitored for changes by adding a TextListener for TextEvent's. In the JTextComponent based components, changes are broadcasted from the model via a DocumentEvent to DocumentListeners. The DocumentEvent gives the location of the change and the kind of change if desired.

You need to use the DocumentListener and combine it with some Regex to do the magic. If the text at a given point does not match the format you want, do not update the JTextField or simply using the .charAt() method would do but then it is up to you

SSCCE without DocumentListener

package stack;

import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class OhMyDollar {

    static JFrame frame;
    static JTextField field;

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable(){
            @Override
            public void run(){
                frame = new JFrame("Useless Title");
                field = new JTextField("$", 30);
                frame.getContentPane().add(field);
                frame.pack();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setVisible(true);

                field.addKeyListener(new KeyListener(){

                    @Override
                    public void keyPressed(KeyEvent event) {

                    }

                    @Override
                    public void keyReleased(KeyEvent event) {

                    }

                    @Override
                    public void keyTyped(KeyEvent event) {
                        StringBuffer text = new StringBuffer(field.getText());
                        StringBuffer dollar = new StringBuffer("$");
                        if(field.getText().isEmpty() || text.charAt(0)!='$'){
                            field.setText(dollar.append(text).toString());
                        }
                    }
                });
            }
        });
    }
}