想法#1，存储Graphics 对象根本行不通。 Graphics 不应被视为“持有”某些显示内存，而应被视为访问显示内存区域的句柄。在BufferedImage 的情况下，每个Graphics 对象将始终是相同给定图像内存缓冲区的句柄，因此它们都将代表相同的图像。更重要的是，您实际上无法对存储的 Graphics 执行任何操作：由于它们不存储任何内容，因此他们无法“重新存储”任何内容。
想法#2，克隆BufferedImages 是一个更好的想法，但你确实会浪费内存，并且很快就会用完。它仅有助于存储受绘图影响的图像部分，例如使用矩形区域，但它仍然需要大量内存。将这些撤消图像缓冲到磁盘可能会有所帮助，但它会使您的 UI 变慢且无响应，这不好；此外，它使您的应用程序更加复杂且容易出错。
我的替代方法是将图像修改存储在一个列表中，在图像顶部从头到尾呈现。然后，撤消操作只需从列表中删除修改。
这需要您“具体化”图像修改，即通过提供执行实际绘图的void draw(Graphics gfx) 方法来创建一个实现单个修改的类。
正如您所说，随机修改会带来额外的问题。但是，关键问题是您使用Math.random() 创建随机数。相反，使用从固定种子值创建的Random 执行每个随机修改，以便（伪）随机数序列在每次调用draw() 时都相同，即每次抽奖具有完全相同的效果。 （这就是为什么它们被称为“伪随机”——生成的数字看起来是随机的，但它们与任何其他函数一样具有确定性。）
与存在内存问题的图像存储技术相比，这种技术的问题是许多修改可能会使 GUI 变慢，尤其是在修改计算量很大的情况下。为防止这种情况，最简单的方法是修复一个适当的可撤消修改列表的最大大小。如果添加新修改会超出此限制，请删除列表中最旧的修改并将其应用于支持 BufferedImage 本身。
以下简单的演示应用程序展示了这一切（以及如何）协同工作。它还包括一个不错的“重做”功能，用于重做已撤消的操作。
package stackoverflow;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.util.LinkedList;
import java.util.Random;
import javax.swing.*;
public final class UndoableDrawDemo
implements Runnable
{
public static void main(String[] args) {
EventQueue.invokeLater(new UndoableDrawDemo()); // execute on EDT
}
// holds the list of drawn modifications, rendered back to front
private final LinkedList<ImageModification> undoable = new LinkedList<>();
// holds the list of undone modifications for redo, last undone at end
private final LinkedList<ImageModification> undone = new LinkedList<>();
// maximum # of undoable modifications
private static final int MAX_UNDO_COUNT = 4;
private BufferedImage image;
public UndoableDrawDemo() {
image = new BufferedImage(600, 600, BufferedImage.TYPE_INT_RGB);
}
public void run() {
// create display area
final JPanel drawPanel = new JPanel() {
@Override
public void paintComponent(Graphics gfx) {
super.paintComponent(gfx);
// display backing image
gfx.drawImage(image, 0, 0, null);
// and render all undoable modification
for (ImageModification action: undoable) {
action.draw(gfx, image.getWidth(), image.getHeight());
}
}
@Override
public Dimension getPreferredSize() {
return new Dimension(image.getWidth(), image.getHeight());
}
};
// create buttons for drawing new stuff, undoing and redoing it
JButton drawButton = new JButton("Draw");
JButton undoButton = new JButton("Undo");
JButton redoButton = new JButton("Redo");
drawButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
// maximum number of undo's reached?
if (undoable.size() == MAX_UNDO_COUNT) {
// remove oldest undoable action and apply it to backing image
ImageModification first = undoable.removeFirst();
Graphics imageGfx = image.getGraphics();
first.draw(imageGfx, image.getWidth(), image.getHeight());
imageGfx.dispose();
}
// add new modification
undoable.addLast(new ExampleRandomModification());
// we shouldn't "redo" the undone actions
undone.clear();
drawPanel.repaint();
}
});
undoButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
if (!undoable.isEmpty()) {
// remove last drawn modification, and append it to undone list
ImageModification lastDrawn = undoable.removeLast();
undone.addLast(lastDrawn);
drawPanel.repaint();
}
}
});
redoButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
if (!undone.isEmpty()) {
// remove last undone modification, and append it to drawn list again
ImageModification lastUndone = undone.removeLast();
undoable.addLast(lastUndone);
drawPanel.repaint();
}
}
});
JPanel buttonPanel = new JPanel(new FlowLayout());
buttonPanel.add(drawButton);
buttonPanel.add(undoButton);
buttonPanel.add(redoButton);
// create frame, add all content, and open it
JFrame frame = new JFrame("Undoable Draw Demo");
frame.getContentPane().add(drawPanel);
frame.getContentPane().add(buttonPanel, BorderLayout.NORTH);
frame.pack();
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
//--- draw actions ---
// provides the seeds for the random modifications -- not for drawing itself
private static final Random SEEDS = new Random();
// interface for draw modifications
private interface ImageModification
{
void draw(Graphics gfx, int width, int height);
}
// example random modification, draws bunch of random lines in random color
private static class ExampleRandomModification implements ImageModification
{
private final long seed;
public ExampleRandomModification() {
// create some random seed for this modification
this.seed = SEEDS.nextLong();
}
@Override
public void draw(Graphics gfx, int width, int height) {
// create a new pseudo-random number generator with our seed...
Random random = new Random(seed);
// so that the random numbers generated are the same each time.
gfx.setColor(new Color(
random.nextInt(256), random.nextInt(256), random.nextInt(256)));
for (int i = 0; i < 16; i++) {
gfx.drawLine(
random.nextInt(width), random.nextInt(height),
random.nextInt(width), random.nextInt(height));
}
}
}
}