新しくページを追加する場合にはどの文が必要なのでしょうか？

１．MainFrame.java
package panels;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class MainFrame extends JFrame{
//mainではなくmpが正しいので修正 201509
//public String[] PanelNames = {"main","Sub1","Sub2","Sub3"};
public String[] PanelNames = {"mp","Sub1","Sub2","Sub3"};
MainPanel mp = new MainPanel(this,PanelNames[0]);
SubPanel sp = new SubPanel(this,PanelNames[1]);
SubPanel sp2 = new SubPanel(this,PanelNames[2]);
SubSubPanel sp3 = new SubSubPanel(this,PanelNames[3]);

public MainFrame(){
    this.add(mp);mp.setVisible(true);
    this.add(sp);sp.setVisible(false);
    this.add(sp2);sp2.setVisible(false);
    this.add(sp3);sp3.setVisible(false);
    this.setBounds(100, 100, 400, 200);
}
public static void main(String[] args) {
    MainFrame mf = new MainFrame();
    mf.setDefaultCloseOperation(EXIT_ON_CLOSE);
    mf.setVisible(true);
}
public void PanelChange(JPanel jp, String str){
    //コメントで指摘頂いた部分を修正 201509
    /*
    String name = jp.getName();
    if(name==PanelNames[0])
        mp = (MainPanel)jp; mp.setVisible(false);
    if(name==PanelNames[1])
        sp = (SubPanel)jp; sp.setVisible(false);
    if(name==PanelNames[2])
        sp2 = (SubPanel)jp; sp2.setVisible(false);
    if(name==PanelNames[3])
        sp3 = (SubSubPanel)jp; sp3.setVisible(false);
     
    if(str==PanelNames[0])
        mp.setVisible(true);
    if(str==PanelNames[1])
        sp.setVisible(true);
    if(str==PanelNames[2])
        sp2.setVisible(true);
    if(str==PanelNames[3])
        sp3.setVisible(true);
    */
    System.out.println(jp.getName());
    String name = jp.getName();
    if(name==PanelNames[0]){
        mp = (MainPanel)jp;
        mp.setVisible(false);
    }else if(name==PanelNames[1]){
        sp = (SubPanel)jp;
        sp.setVisible(false);
    }else if(name==PanelNames[2]){
        sp2 = (SubPanel)jp;
        sp2.setVisible(false);
    }else if(name==PanelNames[3]){
        sp3 = (SubSubPanel)jp;
        sp3.setVisible(false);
    }
    if(str==PanelNames[0]){
        mp.setVisible(true);
    }else if(str==PanelNames[1]){
        sp.setVisible(true);
    }else if(str==PanelNames[2]){
        sp2.setVisible(true);
    }else if(str==PanelNames[3]){
        sp3.setVisible(true);
    }
}

}
ここではルート、サブ画面を登録し、それらの画面から別の画面に移動する際にPanelChangeメソッドで、移動元の画面を移動先の画面を判別し、それぞれ非表示、表示しています。
////////////////////////////////////////////////////////////
２．MainPanel.java
package panels;

import java.awt.Color;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

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

public class MainPanel extends JPanel {
JButton btn,btn2,btn3;
JLabel paneltitle;
MainFrame mf;
String str;
public MainPanel(MainFrame m,String s){
mf = m;
str = s;
this.setName("mp");
this.setLayout(null);
this.setSize(400, 200);
paneltitle = new JLabel("これは"
+getClass().getCanonicalName()+"クラスのパネルです");
paneltitle.setBounds(0, 5, 400, 40);
this.add(paneltitle);
btn = new JButton("SubPanelに移動");
btn.setBounds(20, 50, 150, 40);
btn.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
pc(mf.PanelNames[1]);
}
});
this.add(btn);
btn2 = new JButton("SubPanel2に移動");
btn2.setBounds(20, 100, 150, 40);
btn2.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
pc(mf.PanelNames[2]);
}
});
this.add(btn2);
btn3 = new JButton("SubPanel3に移動");
btn3.setBounds(200, 100, 150, 40);
btn3.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
pc(mf.PanelNames[3]);
}
});
this.add(btn3);
this.setBackground(Color.getHSBColor(65, 0.7f, 0.9f));
}
public void pc(String str){
mf.PanelChange((JPanel)this, str);
}
}
ルート画面には画面移動ボタンが３つあります。いずれを押した場合もMainFrameクラスのPanelChangeメソッドを呼び出しますが、それぞれ押したボタンによって第2引数（移動先画面の名前）が変わります。
///////////////////////////////////////////////////////////
３．SubPanel.java
package panels;

import java.awt.Color;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

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

public class SubPanel extends JPanel {
JButton btn = new JButton("MainPanelに移動");
MainFrame mf;
String str;
public SubPanel(MainFrame m,String s){
mf = m;
str = s;
this.setName(s);
this.setLayout(null);
this.setSize(400, 200);
JLabel paneltitle = new JLabel("これは"
+getClass().getCanonicalName()+"クラスのパネルです");
paneltitle.setBounds(0, 5, 400, 40);
this.add(paneltitle);
btn.setBounds(150, 50, 200, 40);
btn.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
pc();
}
});
this.add(btn);
Float f = (float) Math.random();
this.setBackground(Color.getHSBColor(f, 0.5f, 0.8f));
}
public void pc(){
mf.PanelChange((JPanel)this, mf.PanelNames[0]);
}
}
ルート画面に戻るためのボタンを持ったクラスです。
///////////////////////////////////////////////////////////
４．SubSubPanel.java
package panels;

import javax.swing.JPanel;

public class SubSubPanel extends SubPanel {
public SubSubPanel(MainFrame m,String s){
super(m,s);
btn.setText("SubPanel2に移動");
}
@Override
public void pc(){
mf.PanelChange((JPanel)this, mf.PanelNames[2]);
}
}
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