问为什么白线在区域加减后是可见的？

EN

package Tools;

import java.awt.Color;
import java.io.Serializable;

/**
 *
 * @author DELL-PC
 */
public interface ToolsModel extends Serializable {

    public void setColor(Color color);

    public Color getColor();

    public void setVisible(boolean b);

    public boolean isVisible();

    public void setID(String id);

    public String getID();

    public ToolsModel getShape();

}

public void trimShade(ToolsModel toolsModel) {
        System.out.println("I am at TrimShade");
        Area area = new Area();
        for (ToolsModel model : getShadeList()) {
            if (model.equals(toolsModel) || model.getID().equals(toolsModel.getID())) {
                continue;
            }
            area.add(new Area((Shape)model));
            if (!area.isEmpty()) {
                ((Area) model).subtract(new Area((Shape)toolsModel));

            }
        }
    }