public class CheckOSFrame extends JFrame {

    private JPanel contentPane;
    JLabel OSNameLabel = new JLabel("OS Name");  <-ここに移動

    ....

        btnCheck.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                // 以下2行を追記
                String osName = System.getProperty("os.name");
                OSNameLabel.setText(osName);
            }
        });

package net.hobnote;

public class DefineHeader {

}

package net.HobNote;

public class DefineHeader {
    private DefineHeader () {}
    ////
    public static final int diZERO   = (0);
    public static final int diONE    = (1);
    public static final int diTWO    = (2);
    public static final int diTHREE  = (3);
    public static final int diFOUR   = (4);
    public static final int diFIVE   = (5);
    public static final int diSIX    = (6);
    public static final int diSEVEN  = (7);
    public static final int diEIGHT  = (8);
    public static final int diNINE   = (9);
    ////
    public static final String dsZERO   = "Zero";
    public static final String dsONE    = "One";
    public static final String dsTWO    = "Two";
    public static final String dsTHREE  = "Three";
    public static final String dsFOUR   = "Four";
    public static final String dsFIVE   = "Five";
    public static final String dsSIX    = "Six";
    public static final String dsSEVEN  = "Seven";
    public static final String dsEIGHT  = "Eight";
    public static final String dsNINE   = "Nine";
}

package net.hobnote;

public class DefineProg {

}

package net.hobnote;

import static net.hobnote.DefineHeader.*;

public class DefineProg {
    public static void main (String [] args) {
        System.out.println("Test substitute for #define on Java.\n");
        System.out.println(diZERO+diONE+dsZERO+"\n"+diTWO);
        ////
        String str = String.valueOf(diTHREE)+String.valueOf(diFOUR)+String.valueOf(diFIVE)
                        +String.valueOf(diSIX)+String.valueOf(diSEVEN)+String.valueOf(diEIGHT)
                        +String.valueOf(diNINE);
        System.out.println(str);
        ////
        System.out.println(dsNINE+" "+dsEIGHT+" "+dsSEVEN+" "+dsSIX+" "
                        +dsFIVE+" "+dsFOUR+" "+dsTHREE+" "+dsTWO+" "+dsONE);
}

Test substitute for #define on Java.

1Zero
2
3456789
Nine Eight Seven Six Five Four Three Two One

....
        JMenuItem mntmNewMenuItem = new JMenuItem("Exit");
        mntmNewMenuItem.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {
                // ↓この行を追記
                System.exit(0);
            }
        });
        mnNewMenu.add(mntmNewMenuItem);
....

....
public class FrameMenubar extends JFrame {

    private JPanel contentPane;
    // 以下2行が各々の宣言場所から移動してここに置き直したもの
    JLabel label1 = new JLabel("Label 1");
    JLabel label2 = new JLabel("Label 2");
....

....
        mntmLabel1.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                // 以下2行を追記
                label1.setText("Label 1 Selected");
                label2.setText("Label 2");
            }
        });
....

....
        mntmLabel2.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                // 以下2行を追記
                label1.setText("Label 1");
                label2.setText("Label 2 Selected");
            }
        });
....