PaintComponent（）绘制图形，但即使在调用repaint（）时也不使用更新的值

Question

这里我的程序旨在创建一个GUI，允许用户通过在JPanel上单击和释放鼠标来绘制图形。其他选项包括更改颜色和是否填充形状。一旦绘制了形状，其他选项应该使用任何新的修饰符重新绘制相同的形状。例如，如果我画一个红色矩形，如果我选择蓝色，矩形应该只是改变颜色。到目前为止，我只实现了“矩形”选项。

public class WardA4 extends JFrame implements ActionListener{
     private String[] chooseShapeOptions = {"Rectangle", "Square", "Oval", "Circle", "Line",
                                "Rounded Rectangle", "3D Rectangle"};
     private JCheckBox chooseFill;
     private JComboBox chooseShape;
     private JButton chooseColor;
     private JPanel userInterface, displayPanel;
     private JLabel chooseFillLabel, chooseColorLabel;
     private Color color = Color.WHITE;
     private int shapeIndex = 0;
     private double xStart = 100, yStart = 100, xEnd = 200, yEnd = 200;
     private boolean isFilled;

public WardA4(){
    super("Sandbox");
    chooseShape = new JComboBox(chooseShapeOptions);
    chooseFill  = new JCheckBox();
    chooseColor = new JButton();
    chooseFillLabel = new JLabel("Fill");
    chooseColorLabel = new JLabel ("Color");

    userInterface = new JPanel(new FlowLayout());
    userInterface.add(chooseShape);
    userInterface.add(chooseFillLabel);
    userInterface.add(chooseFill);
    userInterface.add(chooseColorLabel);
    userInterface.add(chooseColor);

    displayPanel = new JPanel(){        
        public void paintComponent (Graphics g){
            super.paintComponent(g);
            System.out.println("Entering paint component");
            System.out.println("starting coordinates are (" + xStart + "," + yStart + ")\n width is " + (int)Math.abs(xStart-xEnd) + "\n height is " + (int)Math.abs(yStart-yEnd));
            g.setColor(color);
            //System.out.println("" + (int)xStart + " " + (int)yStart + " " + (int)Math.abs(xStart-xEnd) + " " + (int)Math.abs(yStart-yEnd));
            switch(shapeIndex){
                case 0:
                    if(isFilled){
                        System.out.println("Entering is filled");
                        g.fillRect((int)xStart, (int)yStart, (int)Math.abs(xStart-xEnd), (int)Math.abs(yStart-yEnd));
                        //g.fillRect(100,100,100,100);
                    }
                    else{
                        System.out.println("Entering is not filled");
                        g.drawRect((int)xStart, (int)yStart, (int)Math.abs(xStart-xEnd), (int)Math.abs(yStart-yEnd));
                        //g.drawRect(100,100,100,100);
                    }
                    break;
                case 1:
                    break;
                case 2:
                    break;
                case 3:
                    break;
                case 4:
                    break;
                case 5:
                    break;
                case 6:
                    break;
            }
        }
    };

    displayPanel.setBackground(Color.BLACK);

    add(displayPanel, BorderLayout.CENTER);
    add(userInterface, BorderLayout.SOUTH);

    chooseShape.addActionListener(this);
    chooseFill.addActionListener(this);
    chooseColor.addActionListener(this);

    displayPanel.addMouseListener(new MouseAdapter(){
        public void mousePressed (MouseEvent me){
            System.out.println("Entering mouse pressed");
            xStart = MouseInfo.getPointerInfo().getLocation().getX();
            yStart = MouseInfo.getPointerInfo().getLocation().getY();
            System.out.println("mouse pressed at (" + xStart + "," + yStart + ")");
        }
        public void mouseReleased (MouseEvent me){
            System.out.println("Entering mouse released");
            xEnd = MouseInfo.getPointerInfo().getLocation().getX();
            yEnd = MouseInfo.getPointerInfo().getLocation().getY();
            System.out.println("mouse released at (" + xEnd + "," + yEnd + ")");
            repaint();
        }
    });
}

public void actionPerformed(ActionEvent e){
    if (e.getSource() == chooseShape){
        shapeIndex = chooseShape.getSelectedIndex();
    }
    else if (e.getSource() == chooseFill){
        isFilled = chooseFill.isSelected();
    }
    else if (e.getSource() == chooseColor){
        color = JColorChooser.showDialog(null, "Choose color", color);
        if (color == null)
            color = Color.WHITE;
    }
    repaint();
}

public static void main(String[] args) {
    WardA4 frame = new WardA4();
    frame.setSize(400,300);
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setLocationRelativeTo(null);
}

Entering paint component
starting coordinates are (100.0,100.0)
 width is 100
 height is 100
Entering is not filled
Entering paint component
starting coordinates are (100.0,100.0)
 width is 100
 height is 100
Entering is not filled

Entering mouse pressed
mouse pressed at (906.0,449.0)
Entering mouse released
mouse released at (1092.0,612.0)
Entering paint component
starting coordinates are (906.0,449.0)
 width is 186
 height is 163
Entering is not filled

Entering mouse pressed
mouse pressed at (1092.0,612.0)
Entering mouse released
mouse released at (1092.0,612.0)
Entering paint component
starting coordinates are (1092.0,612.0)
 width is 0
 height is 0
Entering is not filled

Answer 1

被查询的事件是错误的（使用错误的坐标）。例如。

xStart = MouseInfo.getPointerInfo().getLocation().getX(); // gets location ON SCREEN
yStart = MouseInfo.getPointerInfo().getLocation().getY();

应该是：

xStart = me.getX(); // gets location relative TO PANEL
yStart = me.getY();