BattlePanel.java

/**
 * import部分は省略
 */

public class BattlePanel extends JPanel implements MainInterface
{
/**
 * メンバ変数が横に長くなってしまったので以下のように省略
 * 
 * RoundRectangle2D.Double status_rect ステータス表示部分
 * RoundRectangle2D.Double command_rect コマンド選択表示部分
 * BasicStroke border_stroke 枠線の太さ指定用
 */

public BattlePanel(){
/* 省略 */
}

@Override
public void paintComponent( Graphics g ){
super.paintComponent( g );
requestFocusInWindow();
Graphics2D g2 = (Graphics2D) g;
g2.setRenderingHint( RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON );

g2.setColor( Color.BLACK );
g2.fill( status_rect );
g2.fill( command_rect );

g2.setColor( Color.WHITE );
g2.setStroke( border_stroke );
g2.draw( status_rect );
g2.draw( command_rect );

}
}

MainPanel.java

/**
 * import部分は省略
 */

public class MainPanel extends JPanel implements MainInterface, KeyListener
{
private final BattlePanel battle_panel;
private int game_mode;

public MainPanel(){
setBackground( Color.BLACK );
setPreferredSize( FRAMESIZE );
addKeyListener( this );

battle_panel = new BattlePanel();
add( battle_panel );

setGameMode( MAIN );
}

@Override
public void paintComponent( Graphics g ){
super.paintComponent( g );
requestFocusInWindow();
}

@Override
public void keyPressed( KeyEvent e ){
int key_code = e.getKeyCode();
if( key_code == KeyEvent.VK_SPACE ){
setGameMode( BATTLE );
}
}

@Override
public void keyReleased( KeyEvent e ){

}

@Override
public void keyTyped( KeyEvent e ){

}

public void setGameMode( int mode ){
this.game_mode = mode;
SwingUtilities.invokeLater(() -> {
switch( game_mode ){
case MAIN :
setVisible( true );
battle_panel.setVisible( false );
System.out.println("Load Main");
break;
case BATTLE :
battle_panel.setVisible( true );
System.out.println("GameMode Battle");
break;
default:
battle_panel.setVisible( false );
System.out.println("ERROR GameModeが正しくありません。");
break;
}
});
}

public int getGameMode(){
return game_mode;
}
}

MainFrame.java
まずは、インターフェースの実装を
implements MainInterface

次に、メンバ変数に以下を追加
private int game_mode;
private final OpeningPanel opening;

そして、コンストラクタに以下を追加
opening = new OpeningPanel();
setGameMode( OPENING );

最後に、メンバ関数を追加
public void setGameMode( int mode ){
this.game_mode = mode;
if( getContentPane().getComponentCount() != 0 )
getContentPane().removeAll();

switch( game_mode ){
case OPENING :
getContentPane().add( opening );
revalidate();
System.out.println("GameMode Opening");
break;
default:
System.out.println("ERROR GameModeが正しくありません。");
}
}

public int getGameMode(){
return game_mode;
}

OpeningPanel.java

/**
 * import部分は長くなっちゃうので省略します
 */

public class OpeningPanel extends JPanel implements MainInterface
{
private final JButton game_start;
private final JLabel title;

public OpeningPanel(){
setBackground( Color.BLACK );
setPreferredSize( FRAMESIZE );
setLayout( null );

title = new JLabel("RPG Game");
title.setBounds( 0, 0, FRAMESIZE.width, FRAMESIZE.height / 3 );
title.setForeground( Color.RED );
title.setFont( new Font( Font.SERIF, Font.PLAIN, 100 ) );
title.setHorizontalAlignment( JLabel.CENTER );

game_start = new JButton("Game Load!");
settingButton();

add( title );
add( game_start );
}

private void settingButton(){
game_start.setActionCommand("start");
game_start.setBounds( FRAMESIZE.width / 3, FRAMESIZE.height / 3, FRAMESIZE.width / 3, FRAMESIZE.height / 8 );
}
}

import java.awt.Dimension;

public interface MainInterface {
public static final Dimension FRAMESIZE = new Dimension( 640, 480 );

/**
 * ゲームモード
 */
public static final int OPENING = 0;
public static final int MAIN = 1;
public static final int INIT = 2;
public static final int BATTLE = 3;
}

import java.awt.Dimension;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;

public class MainFrame extends JFrame
{
private final Dimension FRAMESIZE = new Dimension( 640, 480 );

public static void main(String[] args) {
SwingUtilities.invokeLater(() -> {
MainFrame frame = new MainFrame();
frame.setVisible( true );
});
}

public MainFrame(){
setSize( FRAMESIZE );
setTitle("RPGGame!");
setLocationRelativeTo( null );
setResizable( false );
setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
}
}

pre{
white-space:nowrap;
}

Test.java
public class Test{
public static void main( String args[] ){
System.out.println("HellowWorld");
}
}