使用摇摆代码获得意外的输出

Question

我正在尝试构建一个简单的应用程序，它有一个半透明的框架，它会在用户想要的地方画线。我还添加了侦听器来捕获鼠标事件并相应地显示这些事件。一切正常，但问题是： 1）窗口不透明 2）它是完全黑色的，线条呈白色。

任何人都可以 代码如下：

import java.awt.Color;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Paint;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class MouseListen2 extends JFrame implements MouseListener {

    String str = "Nothing";
    int x[][] = new int[100][2];
    int count = 0;
    int flag = 1;
    boolean draw = false;

    MouseListen2() {
        super("Line Draw App");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(400, 300);
        this.addMouseListener(this);
        setBackground(new Color(0, 0, 0, 0));
        JPanel jp = new JPanel() {

            public void paintComponent(Graphics g) {
                super.paintComponent(g);
                Graphics2D g2 = (Graphics2D) g;
                Paint gp = new GradientPaint(300, 700, new Color(20, 20, 210, 0), 100, 00, new Color(10, 20, 40, 255));
                g2.setPaint(gp);
                g2.fillRect(0, 0, getWidth(), getHeight());
            }
        };
        setContentPane(jp);
        setVisible(true);
        //c.setOpaque(true);
    }

    public void paint(Graphics g) {
        //Graphics g=this.getGraphics();
        //super.paint(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.clearRect(0, 0, getWidth(), getHeight());
        g2.drawString(str, 50, 50);
        //initially count=0 hence i<-1 so loop will not automatically run in the beginning
        for (int i = 0; i < count - 1; i = i + 2) {
            g2.drawLine(x[i][0], x[i][1], x[i + 1][0], x[i + 1][1]);
        }
        //repaint(); using this here creates an infinite loop as after mouse event paint is called and  at the end
        //this method is again called using the repaint() and so on the loop continues.
    }

    public static void main(String[] args) {
        JFrame.setDefaultLookAndFeelDecorated(true);
        SwingUtilities.invokeLater(new Runnable() {

            public void run() {
                new MouseListen2();
            }
        });
    }

    @Override
    public void mouseClicked(MouseEvent e) {
        str = "clicked";
        repaint();

    }

    @Override
    public void mousePressed(MouseEvent e) {
        str = "pressed";
        repaint();
        x[count][0] = e.getX();
        x[count][1] = e.getY();
        count++;
    }

    @Override
    public void mouseReleased(MouseEvent e) {
        str = "released";
        draw = true;
        x[count][0] = e.getX();
        x[count][1] = e.getY();
        count++;
        //draw();
        repaint();
    }

    @Override
    public void mouseEntered(MouseEvent e) {
        str = "entered";
        repaint();
    }

    @Override
    public void mouseExited(MouseEvent e) {
        str = "exited";
        repaint();
    }
}

Answer 1

1. 必须将MouseListener 添加到JPanel，因为您将MouseListener 添加到JFrame (this.addMouseListener(this);)

2. 此代码行setContentPane(jp); 将JPanel 放到BorderLayout.CENTER 位置到JFrame，在这种情况下（没有其他JComponent 添加到JFrame）填充整个/所有可用空间到JFrame

3. 那么对于mouse event只能访问JPanel，鼠标无法访问JFrame的RootPane或ContentPane

4. 删除/注释public void paint(Graphics g) 并将相关代码块移至paintComponent 以获取JPanel

Answer 2

见How to Create Translucent and Shaped Windows。