为ArrayList实现撤消重做时出错

Question

类ScreenShot：它包含两个ArrayLists（将从旧的ArrayLists创建一个对象并保存在其中然后添加到撤消堆栈）

import java.util.ArrayList;


public class ScreenShot {
private ArrayList<Shapes> shapes;
private ArrayList<Intersection> intersections;

public ArrayList<Shapes> getShapes() {
    return shapes;
}

public void setShapes(ArrayList<Shapes> shapes) {
    this.shapes = shapes;
}

public ArrayList<Intersection> getIntersections() {
    return intersections;
}

public void setIntersections(ArrayList<Intersection> intersections) {
    this.intersections = intersections;
}

}

包含撤消重做堆栈的类管理器

public class Manager {

public static Stack<ScreenShot> undoStack = new Stack();
public static Stack<ScreenShot> redoStack = new Stack(); 


}

包含撤消重做按钮的JFrame：

public class Canvas extends javax.swing.JFrame {

public static ArrayList<Shapes> shapes;
public Canvas() {
    initComponents();
}                       

private void undoActionPerformed(java.awt.event.ActionEvent evt) {                                     
  if(Manager.undoStack.isEmpty()==false)
 {
     ScreenShot temp = Manager.undoStack.pop();
     System.out.println(Manager.undoStack.size());
     Manager.redoStack.push(temp);
     redo.setEnabled(true);

     Canvas.prevShapes=(temp.getShapes());

    Canvas.intersects.clear();
     Canvas.intersects = temp.getIntersections();
     if(Manager.undoStack.isEmpty())
         undo.setEnabled(false);
 }
 cc.repaint();  
}                                    

private void redoActionPerformed(java.awt.event.ActionEvent evt) {                                     
       if(Manager.redoStack.isEmpty()==false)
 {undo.setEnabled(true);
     ScreenShot temp = Manager.redoStack.pop();
     Manager.undoStack.push(temp);
     Canvas.prevShapes= temp.getShapes();
     Canvas.intersects = temp.getIntersections();
     if(Manager.redoStack.isEmpty())
         redo.setEnabled(false);
 }
 cc.repaint();
}                                    

我试图尽可能地缩短守则。所以这里是我遵循Memento设计模式进行撤销和重做技术的基本思路。它正在使用旧的数组列表并正确地将它们保存在堆栈中。但是正在发生的问题是旧的arraylist无论发生什么都不会被清除（如果重要的话，ArrayList是静态的）