通过键盘在JPanel之间切换

Question

I have a JFrame class with multiple panels and I do all of my graphics by drawing using the paintComponent(Graphics g) method so there is no use in using a CardLayout. To switch frames, I simple hit a button such as enter but, the problem is that when I switch panels, the left over graphics from the paintCompenent are still there. I also tried making the panels visible using panel.setVisible(false) and that somewhat helped except that the panel I am transitioning to is invisible still even after setting visible to true. The buttons and commands to the new panel work and transition back to the first panel so i know that the transition is working, its just not showing how i want it to. here is the code to the 2 panels that interact and the JFrame class that handles them:

//JFRAME CLASS
package main;

import gameStates.Instruction;
import gameStates.Menu;
import gameStates.State;

import java.awt.Dimension;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.util.ArrayList;

import javax.swing.JFrame;

@SuppressWarnings("serial")
public class GameFrame extends JFrame implements KeyListener{

    //panels for Game States in order:
    static ArrayList<State> panels = new ArrayList<State>();//holds all of the game states that the game will eventually reach

    private static int currentState=0;//STATES: 0 - Menu
                               //1 - Instruction
                               //2 - Level 1 and States 3 through 7 are the levels up one respectively up to level 6

    public GameFrame() {
        panels.add(new Menu(this));
        panels.add(new Instruction(this));

        addKeyListener(this);
        setSize(640,480);
        setPreferredSize(new Dimension(640,480));
        setResizable(false);
        setLocation(20,50);
        setName("FBLA: A Success Story");
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        add(panels.get(1));
        add(panels.get(0));
        panels.get(1).setVisible(false);
        panels.get(0).setVisible(true);

        pack();
        setVisible(true);


    }

    @SuppressWarnings("unused")
    public static void main(String[] args) {
        GameFrame g = new GameFrame();
    }


    public void keyTyped(KeyEvent e) {
    }
    public void keyPressed(KeyEvent e) {
        if(e.getKeyCode() == KeyEvent.VK_ESCAPE)
            System.exit(0);
        else {
            panels.get(currentState).buttons(e);
        }
    }
    public void keyReleased(KeyEvent e) {
    }

    public void nextState(int state) {
        panels.get(currentState).setVisible(false);
        panels.get(state).setVisible(true);
        currentState = state;
    }

}
//////////////////////INITIAL PANEL//////////////////////////
package gameStates;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import java.awt.image.BufferedImage;

import javax.imageio.ImageIO;
import javax.swing.Timer;

import main.GameFrame;

@SuppressWarnings("serial")
public class Menu extends State {

    //general items
    public BufferedImage bg;
    public String[] options = new String[3];
    public double x, y, velX, velY;
    public int selected;
    public Graphics2D g2;

    //time stuff
    Timer t = new Timer(5,this);

    public Menu(GameFrame g) {
        super(g);
        setup();
    }
    public void setup() {
        try {
            bg = ImageIO.read(Menu.class.getResourceAsStream("/backgrounds/FBLAmenubg.gif"));
        }
        catch (Exception e) {
            e.printStackTrace();
        }

        options[0] = "PLAY";
        options[1] = "INSTRUCTIONS";
        options[2] = "QUIT";
        selected = 0;

        x=y=velY=0;
        velX=-.1;

        t.start();
    }

    public void paintComponent(Graphics g) {
        g2 = (Graphics2D)g;
        g2.drawImage(bg, (int)x, (int)y, null);
        g2.drawImage(bg, (int)x + 640, (int)y, null);
        Font f = new Font("FreeSerif", Font.ROMAN_BASELINE, 48);
        g2.setFont(f);
        g2.setColor(Color.WHITE);
        for(int j=0;j<3;j++){
            if(selected == j)
                g2.setColor(Color.RED);
            g2.drawString(options[j], 120, 80 + j*48);
            g2.setColor(Color.WHITE);
        }

    }

    private void processSelection() {

        if(selected == 2)
            System.exit(0);
        else if (selected == 1) {
            parent.nextState(1);
        }
        else {

        }
    }

    public void buttons(KeyEvent e) {
        if(e.getKeyCode() == KeyEvent.VK_DOWN) {
            selected++;
            if(selected == 3)
                selected=0;
        }
        if(e.getKeyCode() == KeyEvent.VK_UP) {
            selected--;
            if(selected == -1)
                selected=2;
        }
        if(e.getKeyCode() == KeyEvent.VK_ENTER) {
            processSelection();
        }
    }

    public void actionPerformed(ActionEvent e) {
        x += velX;
        if(x<=-640)
            x=0;;
        repaint();
    }

}

/////////////////////TRANSITION TO THIS PANEL////////////////////////////
package gameStates;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import java.awt.image.BufferedImage;

import javax.imageio.ImageIO;
import javax.swing.Timer;

import main.GameFrame;

@SuppressWarnings("serial")
public class Instruction extends State{

    public BufferedImage bg;
    public Graphics2D g2;
    public Timer t;

    public Instruction(GameFrame g) {
        super(g);
        setup();
    }


    public void buttons(KeyEvent e) {
        if(e.getKeyCode() == KeyEvent.VK_SPACE)
            parent.nextState(0);
    }

    public void setup() {

        try {
            bg = ImageIO.read(Menu.class.getResourceAsStream("/backgrounds/Instructionsbg.gif"));
        }
        catch (Exception e) {
            e.printStackTrace();
        }

        t = new Timer(5,this);
        t.start();

    }

    public void actionPerformed(ActionEvent e) {
        repaint();
    }

    public void paintComponent(Graphics g) {

        g2 = (Graphics2D) g;
        g2.setColor(Color.WHITE);
        g2.fillRect(0,0,640,480);
        g2.drawImage(bg,0,0,null);
        Font f = new Font("FreeSerif", Font.ROMAN_BASELINE, 20);
        g2.setFont(f);
        g2.setColor(Color.RED);
        g2.drawString("SPACE BAR to go back to menu.", 600, 400);

    }

}

Answer 1

so there is no use in using a CardLayout.

This has nothing to do with custom painting. You should be able to use CardLayout (or in fact, what ever layout manager you want).

You never call super.paintComponent , which is responsible for preparing the Graphics context for painting....

 // Should be protected...
 // Should have @Override annotation
 public void paintComponent(Graphics g) {
    // super.paintComponent(g) goes here...
    //...
}

public Graphics2D g2; also scares me, you should NEVER maintain a reference to any Graphics context you didn't explicitly create yourself (and no g.create doesn't count)

The Graphics context in Swing is generally shared for all the components attached to the same native peer (or window in most cases). This means, unless you clean it, what ever was painted to it last will still be there.

This also means that whatever you might have painted to the Graphics context before will be wiped clean, hence the reason we generally discourage the maintaining references to Graphics or painting to it outside of the a normal paint cycle.

Take a look at Painting in AWT and Swing for more details

You may also find that, overtime, you KeyListener stops responding, even randomly. KeyListener has focus issues which makes it in appropropriate for managing multiple components and you will find the Key Bindings API more suited to your needs