java - JFormattedTextField は別のオブジェクトがフォーカスを受け取るとクリアされる

Question

2 つの JFormattedTextField、2 つの JComboBox、2 つの JLabel、および 1 つの JButton を含むプログラムがあります。1 つの JFormattedTextField にテキストを入力し、前述のコントロールのいずれかをクリックすると、テキストを入力した JFormattedTextField がクリアされます。何故ですか？

私のコード：

    import java.awt.*;
import java.awt.event.*;

import javax.swing.*;
import javax.swing.event.*;
import javax.swing.text.*;

import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeEvent;

import java.text.*;
/*'Current' refers to the Current XP/Level controls
 *'Desired' refers to the desired XP/Level controls*/
public class RSXPCalc
{
    //Variables to use in multiple controls
    static private String[] choices = {"Level", "XP"};

    /*Global controls*/
    //Current XP/Level Controls
    static private JFormattedTextField currentBox = new JFormattedTextField(createFormatter("#########"));
    static private JLabel currentLabel = new JLabel("Value according to above :"); 
    static private JComboBox<String> currentChoice = new JComboBox<String>(choices);

    //Desired XP/Level Controls
    static private JFormattedTextField desiredBox = new JFormattedTextField(createFormatter("#########"));
    static private JLabel desiredLabel = new JLabel("Value according to above :"); 
    static private JComboBox<String> desiredChoice = new JComboBox<String>(choices);

    //Other controls
    static private JButton calc = new JButton("Calculate XP");  //Button
    static private JLabel CXPL = new JLabel("");                //Current XP Label

    //Vairiables used in calculation
    static private double CXP, DXP, CLevel, DLevel;
    static String temp;

    public static void main(String[] args) 

    {
        //Creating the main window
        JFrame main = new JFrame("Test Frame");

        //Configuring the frame
        main.setSize(400,200);                                  //Size
        main.setLayout(null);                                   //Allows for free-placement on a coordinate grid of objects
        main.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);    //Exitcode
        main.setResizable(false);                               //making it a fixed size

        //Configuring the 'current' controls
        currentBox.setBounds(165,43,70,30);     //Setting the (x,y) location and size of the current level/XP textbox
        currentLabel.setBounds(10,43,155,30);   //Setting the (x,y) location and size of the current level/XP label
        currentChoice.setBounds(10,10,225,30);  //Setting the (x,y) location and size of the current level/XP combobox

        //Configuring the 'desired' controls
        desiredBox.setBounds(165,109,70,30);    //Setting the (x,y) location and size of the desired level/XP textbox
        desiredLabel.setBounds(10,109,155,30);  //Setting the (x,y) location and size of the desired level/XP label
        desiredChoice.setBounds(10,76,225,30);  //Setting the (x,y) location and size of the desired level/XP combobox

        //Configuring the button
        calc.setBounds(245, 10, 142, 62);
        //Placing the CurrentXP Label
        CXPL.setBounds(245, 65, 142, 62);

        //Adding the controls to the window and making the window visible\
        //'Current' Controls
        main.add(currentBox);
        main.add(currentLabel);
        main.add(currentChoice);
        //Desired Controls
        main.add(desiredBox);
        main.add(desiredLabel);
        main.add(desiredChoice);

        //Other controls
        main.add(calc); //Button
        main.add(CXPL); //CurrentXP label

        main.setVisible(true);

    }

    //Format method. Taken from oracle.com
    static protected MaskFormatter createFormatter(String s) 
    {
       MaskFormatter formatter = null;
        try {
            formatter = new MaskFormatter(s);
        } catch (java.text.ParseException exc) {
            System.err.println("formatter is bad: " + exc.getMessage());
            System.exit(-1);
        }
        return formatter;
    }
}