Question

一些上下文 - 我正在创建Scrabble的基本实现，GUI依赖于Java Swing和AWT。下面的代码摘录包含Cell类的构造函数（Scrabble板上的单个空间）。我处于概念验证阶段，正在测试添加和删除单个单元格的硬编码字母图标。每个单元格都是一个带有JLabel的单个JPanel（其中包含一个字母的ImageIcon）。代码看起来好像没有错误，但每5-6次添加/删除（通过鼠标单击）会导致类强制转换异常。具体例外是：

线程“AWT-EventQueue-0”中的异常java.lang.ClassCastException：无法将单元格强制转换为javax.swing.JLabel

我无法看到导致此异常的位置，更具体地说，为什么它只会在多次成功添加和删除后才会出现。任何见解都非常感激;我是Java GUI的初学者。

public class Cell extends JPanel {

/*Tile Colors*/
public static Color twColor = new Color(255, 0, 0);
public static Color dwColor = new Color(255, 153, 255);
public static Color tlColor = new Color(0, 51, 255);
public static Color dlColor = new Color(102, 204, 255);
public static Color defaultColor = new Color(255, 255, 255);

private JLabel selected = null;
private JLabel clicked = null;
private JLabel letterIcon;
private ImageIcon defaultIcon;
private ImageIcon testImg;

public Cell(int xPos, int yPos, int premiumStatus) {

    defaultIcon = new ImageIcon ("img/transparent.png");
    testImg = new ImageIcon ("img/test.jpg"); // Letter image hard-coded for testing
    letterIcon = new JLabel("", defaultIcon, JLabel.CENTER);
    add(letterIcon);

    letterIcon.addMouseListener(new MouseAdapter() {
      public void mouseClicked(MouseEvent e) {
        JLabel clicked = (JLabel) getComponentAt(e.getPoint());
        System.out.println(clicked);
        if (clicked == null) {
          return;
        }
        if (selected != null) {
          selected.removeAll();
          selected.revalidate();
          selected.setIcon(defaultIcon);
          selected.repaint();
          System.out.println("Test");
          selected = null;
          return;
        }
        if (selected == null) {
          selected = clicked;
          selected.setIcon(testImg);
          selected.revalidate();
          selected.repaint();
        }
      }
    });
}

Answer 1

当鼠标坐标已经转换为getComponentAt(e.getPoint());的坐标空间时，通过调用Cell上的letterIcon来解决问题。

单击某个组件时，MouseEvent的点将自动转换为侦听器注册到的组件的坐标空间。

在您的情况下，那是letterIcon。这意味着0x0处的一个点是letterIcon的左上角（尽管它可能位于物理位置）。

因此，调用getComponentAt(e.getPoint())会要求Cell返回与实际上仅与letterIcon相关的位置对应的组件，这将（在大多数情况下）返回Cell本身。

相反，您应该只是使用MouseEvent#getComponent来返回触发事件的组件，即letterIcon

使用简单示例进行更新

这是一个将JLabel设置为鼠标目标的简单示例。单击鼠标时，标签及其父容器都将根据鼠标单击的坐标绘制一个小点。

还有一个额外的好处，即父容器还会将点击点转换为它的坐标空间并绘制第二个点，该点应与标签位于同一点。

enter image description here

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.GridBagLayout;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TestMouseClicked {

    public static void main(String[] args) {
        new TestMouseClicked();
    }

    public TestMouseClicked() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private JLabel clickMe;
        private Point clickPoint;

        public TestPane() {
            setLayout(new GridBagLayout());
            clickMe = new JLabel("Click me") {
                @Override
                protected void paintComponent(Graphics g) {
                    super.paintComponent(g);
                    g.setColor(Color.MAGENTA);
//                    paintPoint(g, clickPoint);
                }
            };
            add(clickMe);
            clickMe.addMouseListener(new MouseAdapter() {
                @Override
                public void mouseClicked(MouseEvent e) {
                    clickPoint = e.getPoint();
                    repaint();
                }
            });
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.RED);
            paintPoint(g, clickPoint);
            if (clickPoint != null) {
                g.setColor(Color.BLUE);
                // Convert the point from clickMe coordinate space to local coordinate space
                paintPoint(g, SwingUtilities.convertPoint(clickMe, clickPoint, this));
            }
        }

        protected void paintPoint(Graphics g, Point clickPoint) {
            if (clickPoint != null) {
                int size = 4;
                g.fillOval(clickPoint.x - size, clickPoint.y - size, size * 2, size * 2);
            }
        }
    }
}

类Cast Exception - 鼠标事件

1 个答案: