java - 如何在java中实现一个鼠标监听器来帮助拖动一个圆圈？

Question

我正在尝试弄清楚如何完成下面的作业

1. 为 Circle 类编写一个方法 void move(Point p)，该方法接受一个 Point 并移动圆，使其中心位于该点。
2. 在 CirclePanel 构造函数中，创建一个 CirclesListener 对象并使其监听鼠标事件和鼠标移动事件。
3. 除了 MouseListener 接口(interface)之外，还使 CirclesListener 类实现 MouseMotionListener 接口(interface)。这需要两个步骤: 请注意 header 中 CirclesListener 实现了 MouseMotionListener。为两个 MouseMotionListener 方法(mouseDragged 和 mouseMoved)添加主体。在 mouseDragged 中，只需将圆移动到 MouseEvent 的 getPoint 方法返回的点并重新绘制即可。为 mouseMoved 提供一个空主体。

我知道我需要做什么(大部分)，但我只是不知道该怎么做。 (编程新手)。谢谢!

public class circlePanel extends JPanel {
private final int WIDTH = 600, HEIGHT = 400;
private Circle circle;

// -------------------------------------------------------------------
// Sets up this panel to listen for mouse events.
// -------------------------------------------------------------------
public circlePanel() {
    addMouseListener(new CirclesListener());
    setPreferredSize(new Dimension(WIDTH, HEIGHT));
}

// -------------------------------------------------------------------
// Draws the current circle, if any.
// -------------------------------------------------------------------
public void paintComponent(Graphics page) {
    super.paintComponent(page);
    if (circle != null)
        circle.draw(page);
}

// ******************************************************************
// Represents the listener for mouse events.
// ******************************************************************
private class CirclesListener implements MouseListener, MouseMotionListener {
    // ---------------------------------------------------------------
    // Creates a new circle at the current location whenever the
    // mouse button is pressed and repaints.
    // ---------------------------------------------------------------
    public void mousePressed(MouseEvent event) {
        if (circle == null) {
            circle = new Circle(event.getPoint());
        } else if (circle.isInside(event.getPoint())) {
            circle = null;
        } else {
            circle.move(getMousePosition());
        }
        repaint();
    }

    // -----------------------------------------------------------------
    // Provide empty definitions for unused event methods.
    // -----------------------------------------------------------------
    public void mouseClicked(MouseEvent event) {
    }

    public void mouseReleased(MouseEvent event) {
    }

    public void mouseEntered(MouseEvent event) {
        setBackground(Color.white);
    }

    public void mouseExited(MouseEvent event) {
        setBackground(Color.blue);
    }
}
}

这是圆圈类

public class Circles {
// ----------------------------------------------------------------
// Creates and displays the application frame.
// ----------------------------------------------------------------
public static void main(String[] args) {
    JFrame circlesFrame = new JFrame("Circles");
    circlesFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    circlesFrame.getContentPane().add(new circlePanel());
    circlesFrame.pack();
    circlesFrame.setVisible(true);
}
}

aannnddddd...这是 Circle 类

public class Circle {
private int centerX, centerY;
private int radius;
private Color color;
static Random generator = new Random();

// ---------------------------------------------------------
// Creates a circle with center at point given, random radius and color
// -- radius 25..74
// -- color RGB value 0..16777215 (24-bit)
// ---------------------------------------------------------
public Circle(Point point) {
    radius = Math.abs(generator.nextInt()) % 50 + 25;
    color = new Color(Math.abs(generator.nextInt()) % 16777216);
    centerX = point.x;
    centerY = point.y;
}

// ---------------------------------------------------------
// Draws circle on the graphics object given
// ---------------------------------------------------------
public void draw(Graphics page) {
    page.setColor(color);
    page.fillOval(centerX - radius, centerY - radius, radius * 2,     
radius * 2);
}

public void move(Point p) {
    centerX = p.x;
    centerY = p.y;
}

public boolean isInside(Point p) {
    if (Math.sqrt(Math.pow(p.x - this.centerX, 2) + Math.pow(p.y - 
this.centerY, 2)) < this.radius) {
        return true;
    } else {
        return false;
    }
}
}

Answer 1

所以，基本上，根据您的代码示例，您需要:

• 实现 MouseMotionListener 的功能，这将包括 mouseDragged 方法
• 您需要将 CirclesListener 注册到 circlePanel ( JPanel#addMouseMotionListener )
• 当调用mouseDragged时，您需要从MouseEvent中获取Point并调用Circle#move并重新绘制组件

如果您遇到困难，最好的起点是 How to Write a Mouse Listener