❗️ 먼저 직접 Properties 파일을 만든 후 진행했다.

• myframe.properties
  : 규칙을 지켜가며 짜야지 작동된다.
  

✏️ MyFrame(본 창)

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.*;
import java.util.Properties;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JOptionPane;

public class MyFrame extends JFrame {

    private JLabel lblText;

    private Properties prop = new Properties();

    private JMenuBar mBar;
    private JMenu mSettings;
    private JMenuItem miProperties;
    private File file = new File("myframe.properties");

    public MyFrame() {
        FileInputStream fis = null;
        try {
            fis = new FileInputStream(file);
            prop.load(fis);

        } catch (FileNotFoundException fe) {
            if(!file.exists()) {
                try{
                    file.createNewFile();
                } catch (IOException e1) {
                    fe.printStackTrace();
                }
            }
        } catch(IOException e) {
            e.printStackTrace();
        } finally {
            IOUtil.closeAll(fis);
        }

        mBar = new JMenuBar();
        mSettings = new JMenu("settings");
        miProperties = new JMenuItem("set properties");

        mSettings.add(miProperties);
        mBar.add(mSettings);
        setJMenuBar(mBar);

        miProperties.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                new SettingDlg(MyFrame.this);
            }
        });

        lblText = new JLabel(prop.getProperty("myframe.text"), JLabel.CENTER);
        add(lblText, BorderLayout.CENTER);

        setTitle(prop.getProperty("myframe.title"));
        setSize(getIntValue("myframe.width"), getIntValue("myframe.height"));
        setLocation(getIntValue("myframe.x"), getIntValue("myframe.y"));
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
    }

    // String -> int 변환 메서드
    private int getIntValue(String key) {
        return Integer.parseInt(prop.getProperty(key));
    }
    // prop 저장
    public void setProp(String key, String value) {
        prop.setProperty(key, value);
    }
    // value 값 불러오기
    public String getValue(String key) {
        return prop.getProperty(key);
    }
    // key값을 String[]로 변환
    public String[] getKeys() {
        return prop.keySet().toArray(new String[0]);
    }

    public void save() {
        FileOutputStream fos = null;

        try {
            fos = new FileOutputStream(file);
            prop.store(fos, "myframe.properties");
            int choice = JOptionPane.showConfirmDialog(
                    MyFrame.this,
                    "저장완료! 변경된 내용을 위해 다시 시작해주세요",
                    "저장알림",
                    JOptionPane.OK_CANCEL_OPTION,
                    JOptionPane.INFORMATION_MESSAGE
            );
            if(choice == JOptionPane.OK_OPTION) {
                JOptionPane.showMessageDialog(
                        MyFrame.this,
                        "MyFrame을 종료합니다."
                );
                System.exit(0);
            }

        } catch (IOException e) {
            JOptionPane.showConfirmDialog(
                    MyFrame.this,
                    "저장에 실패하였습니다. 다시 시도해주세요.",
                    "저장에러",
                    JOptionPane.ERROR_MESSAGE,
                    JOptionPane.OK_OPTION
            );
        } finally {
            IOUtil.closeAll(fos);
        }
    }

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

✏️ SettingDlg(보조 창)

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JDialog;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class SettingDlg extends JDialog {
    private MyFrame owner;
    private InputPanel[] pnls; // Jpanel[]
    private JButton btnSave;
    private JButton btnClose;

    public SettingDlg(MyFrame owner) {
        super(owner, "settings", true);
        this.owner = owner;
        String[] info = owner.getKeys(); // info: key값 []

        pnls = new InputPanel[info.length];
        for(int i = 0; i < pnls.length; i++) {
            pnls[i] = new InputPanel(info[i]);
        }

        JPanel pnlCenter = new JPanel(new GridLayout(0, 1));
        for(InputPanel pnl : pnls) {
            pnlCenter.add(pnl);
        }

        btnSave = new JButton("save");
        btnClose = new JButton("close");

        JPanel pnlSouth = new JPanel(new GridLayout(1, 2));
        pnlSouth.add(btnSave);
        pnlSouth.add(btnClose);

        add(pnlCenter, BorderLayout.CENTER);
        add(pnlSouth, BorderLayout.SOUTH);

        ActionListener aListener = new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                if(e.getSource() == btnSave) {
                    for(InputPanel pnl : pnls) {
                        owner.setProp(pnl.getName(), pnl.getValue());
                    }
                    owner.save();
                }
                if(e.getSource() == btnClose) {
                    dispose();
                }
            }
        };
        btnSave.addActionListener(aListener);
        btnClose.addActionListener(aListener);

        setTitle("settings");
        pack();
        setLocationRelativeTo(owner);
        setDefaultCloseOperation(DISPOSE_ON_CLOSE);
        setVisible(true);
    }
    // Inner class: InputPanel(JPanel)
    private class InputPanel extends JPanel {
        private JLabel lbl;
        private JTextField tfInput;
        private String name;

        private InputPanel(String text) {
            this.name = text; // text == name == owner.key
            lbl = new JLabel(text, JLabel.LEFT);
            lbl.setPreferredSize(new Dimension(150, 20));

            tfInput = new JTextField(20);
            tfInput.setText(owner.getValue(text)); // 원래 값 갖고옴

            add(lbl);
            add(tfInput);
        }
        // key값
        public String getName() {
            return name;
        }
        public String getValue() {
            return tfInput.getText();
        }
    }

}