无法重绘我的JFrame/JPanel

让我们从显而易见的…开始。

这是有问题的…

@Override
public void paintComponent(Graphics g) {
g.setColor(Color.GREEN);
g.fillOval(x, y, 15, 15);
}
@Override
public void paint(Graphics g) {
dbImage = createImage(getWidth(), getHeight());
dbg = dbImage.getGraphics();
paintComponent(dbg);
g.drawImage(dbImage, 0, 0, this);
}


现在已经不需要在Swing组件中实现双缓冲了。另外，通过不将paint方法称为super方法，您正在破坏绘画合同

整个事情应该是…

@Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(Color.GREEN);
g.fillOval(x, y, 15, 15);
}


有关更多详细信息，请参见AWT中的绘画和摇摆和执行自定义绘画。

KeyListener因存在问题而众所周知。仅当注册到的组件可操作并且具有键盘焦点时，它才会引发键事件。JPanel默认情况下，A
不可聚焦。在尝试并使其具有针对性（让您非常失望）之前，您应该改用Key Bindings
API，该API旨在克服以下限制：KeyListener

作为一个基本的例子…

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.event.actionEvent;
import java.awt.event.KeyEvent;
import javax.swing.Abstractaction;
import javax.swing.action;
import javax.swing.actionmap;
import javax.swing.InputMap;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.KeyStroke;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
public class Main {
public static void main(String[] args) {
new Main();
}
public Main() {
EventQueue.invokeLater(new Runnable() {
@Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassname());
} catch (ClassnotFoundException | InstantiationException | IllegalaccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(new Game());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
public class Game extends JPanel {
int x, y, xCoord, yCoord;
public Game() {
x = 250;
y = 250;
addKeyBinding(KeyEvent.VK_LEFT, "move.left", new Moveaction(this, -1, 0));
addKeyBinding(KeyEvent.VK_RIGHT, "move.right", new Moveaction(this, 1, 0));
addKeyBinding(KeyEvent.VK_UP, "move.up", new Moveaction(this, 0, -1));
addKeyBinding(KeyEvent.VK_DOWN, "move.down", new Moveaction(this, 0, 1));
}
protected void addKeyBinding(int keyCode, String name, action action) {
addKeyBinding(KeyStroke.getKeyStroke(keyCode, 0), name, action);
}
protected void addKeyBinding(KeyStroke keyStroke, String name, action action) {
InputMap inputMap = getInputMap(WHEN_IN_FOCUSED_WINDOW);
actionmap actiOnmap= getactionmap();
inputMap.put(keyStroke, name);
actionmap.put(name, action);
}
public void changeCoord() {
x += xCoord;
y += yCoord;
if (x <= 20) {
x = 20;
}
if (x >= 480) {
x = 480;
}
if (y <= 40) {
y = 40;
}
if (y >= 480) {
y = 480;
}
repaint();
}
public void setXCoord(int xcoord) {
xCoord = xcoord;
changeCoord();
}
public void setYCoord(int ycoord) {
yCoord = ycoord;
changeCoord();
}
@Override
public Dimension getPreferredSize() {
return new Dimension(480, 480);
}
@Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(Color.GREEN);
g.fillOval(x, y, 15, 15);
}
}
public class Moveaction extends Abstractaction {
private int xDelta;
private int yDelta;
// I'd prefer an interface with just the "move" methods, but
// that's more time I don't have
private Game game;
public Moveaction(Game game, int xDelta, int yDelta) {
this.xDelta = xDelta;
this.yDelta = yDelta;
this.game = game;
}
@Override
public void actionPerformed(actionEvent e) {
game.setXCoord(xDelta);
game.setYCoord(yDelta);
}
}
}


但是，等等，那不完全是您想要的（相信我，我是互联网上的糊涂人；）），一个更好的例子可能是…

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.event.actionEvent;
import java.awt.event.actionListener;
import java.awt.event.KeyEvent;
import java.util.HashSet;
import java.util.Set;
import javax.swing.Abstractaction;
import javax.swing.action;
import javax.swing.actionmap;
import javax.swing.InputMap;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.KeyStroke;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
public class Main {
public static void main(String[] args) {
new Main();
}
public Main() {
EventQueue.invokeLater(new Runnable() {
@Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassname());
} catch (ClassnotFoundException | InstantiationException | IllegalaccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(new Game());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
public enum Direction {
UP,
LEFT,
DOWN,
RIGHT;
}
public class Game extends JPanel {
int x, y, xCoord, yCoord;
private Set movement;
public Game() {
x = 250;
y = 250;
movement = new HashSet<>(4);
addKeypressedBinding(KeyEvent.VK_LEFT, "left.pressed", new Moveaction(movement, Direction.LEFT, true));
addKeyReleasedBinding(KeyEvent.VK_LEFT, "left.released", new Moveaction(movement, Direction.LEFT, false));
addKeypressedBinding(KeyEvent.VK_RIGHT, "right.pressed", new Moveaction(movement, Direction.RIGHT, true));
addKeyReleasedBinding(KeyEvent.VK_RIGHT, "right.released", new Moveaction(movement, Direction.RIGHT, false));
addKeypressedBinding(KeyEvent.VK_UP, "up.pressed", new Moveaction(movement, Direction.UP, true));
addKeyReleasedBinding(KeyEvent.VK_UP, "up.released", new Moveaction(movement, Direction.UP, false));
addKeypressedBinding(KeyEvent.VK_DOWN, "down.pressed", new Moveaction(movement, Direction.DOWN, true));
addKeyReleasedBinding(KeyEvent.VK_DOWN, "down.released", new Moveaction(movement, Direction.DOWN, false));
Timer timer = new Timer(40, new actionListener() {
@Override
public void actionPerformed(actionEvent e) {
changeCoord();
}
});
timer.start();
}
protected void addKeyBinding(int keyCode, String name, action action) {
addKeyBinding(KeyStroke.getKeyStroke(keyCode, 0), name, action);
}
protected void addKeypressedBinding(int keyCode, String name, action action) {
addKeyBinding(KeyStroke.getKeyStroke(keyCode, 0, false), name, action);
}
protected void addKeyReleasedBinding(int keyCode, String name, action action) {
addKeyBinding(KeyStroke.getKeyStroke(keyCode, 0, true), name, action);
}
protected void addKeyBinding(KeyStroke keyStroke, String name, action action) {
InputMap inputMap = getInputMap(WHEN_IN_FOCUSED_WINDOW);
actionmap actiOnmap= getactionmap();
inputMap.put(keyStroke, name);
actionmap.put(name, action);
}
public void changeCoord() {
if (movement.contains(Direction.UP)) {
y--;
} else if (movement.contains(Direction.DOWN)) {
y++;
}
if (movement.contains(Direction.LEFT)) {
x--;
} else if (movement.contains(Direction.RIGHT)) {
x++;
}
x += xCoord;
y += yCoord;
if (x <= 20) {
x = 20;
}
if (x >= 480) {
x = 480;
}
if (y <= 40) {
y = 40;
}
if (y >= 480) {
y = 480;
}
repaint();
}
public void setXCoord(int xcoord) {
xCoord = xcoord;
changeCoord();
}
public void setYCoord(int ycoord) {
yCoord = ycoord;
changeCoord();
}
@Override
public Dimension getPreferredSize() {
return new Dimension(480, 480);
}
@Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(Color.GREEN);
g.fillOval(x, y, 15, 15);
}
}
public class Moveaction extends Abstractaction {
private Set movement;
private Direction direction;
private boolean pressed;
public Moveaction(Set movement, Direction direction, boolean pressed) {
this.movement = movement;
this.direction = direction;
this.pressed = pressed;
}
@Override
public void actionPerformed(actionEvent e) {
if (pressed) {
movement.add(direction);
} else {
movement.remove(direction);
}
}
}
}


这样做只是在按下某个键时激活一个标志（并在释放它时将其禁用），然后在Swing中Timer，我们检查哪些键是“活动的”并更新球的位置。

这样做是消除了第一次按下并按住某个键时由操作系统引起的键“口吃”。在第一个键和重复的键事件之间插入一个延迟。相反，我们只是根据需要打开和关闭该标志。

它还允许您同时在两个方向（水平和垂直）上移动

了解Swing中的并发性以及如何使用Swing计时器以了解更多详细信息