java - 类声明范围不明确

Question

我正在尝试完成我的高中初级编程类(class)的代码，我很困惑 Java 告诉我它找不到 Listener(当我已经导入它时？)。用 import java.awt.event.*; 再次尝试后，它仍然不起作用。有人可以帮我解决这个棘手的问题吗？

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;
import javax.swing.JPanel;
import javax.swing.Timer;
public class PrizePanelA extends JPanel {
    private static final int FRAME = 400;
    private static final Color BACKGROUND = new Color(204, 204, 204);
    private BufferedImage myImage;
    private Graphics myBuffer;
    private Ball ball;
    private Polkadot prize;
    private int hits;
    private Timer c;
    private Bumper bumper1;
    private Bumper bumper2;

    public PrizePanelA() {
        myImage = new BufferedImage(400, 400, 1);
        myBuffer = this.myImage.getGraphics();

        ball = new Ball(200, 200, 50, Color.black);
        ball.jump(400, 400);
        bumper1 = new Bumper(50, 170, 50, 50, Color.yellow);
        prize = new Polkadot(200, 200, 25, Color.red);
        prize.jump(400, 400);
        hits = 0;
        addKeyListener(new Key());
        setFocusable(true);
        c = new Timer(10, new Listener());
        c.start();
    }
    private class Key extends KeyAdapter {

        public void keyPressed(KeyEvent e) {
            if ((e.getKeyCode() == 38)) {
                prize.setY(prize.getY() + 10);
            }
        }

        public void paintComponent(Graphics g) {
            g.drawImage(myImage, 0, 0, getWidth(), getHeight(), null);
        }

        private class Listener implements ActionListener {
            private Listener() {}

            public void actionPerformed(ActionEvent e) {
                myBuffer.setColor(BACKGROUND);
                myBuffer.fillRect(0, 0, 400, 400);

                ball.move(400.0, 400.0);
                collide(ball, prize);

                ball.draw(myBuffer);
                prize.draw(myBuffer);

                myBuffer.setColor(Color.BLACK);
                myBuffer.setFont(new Font("Monospaced", 1, 24));
                myBuffer.drawString("Count: " + hits, 250, 25);
                repaint();
            }
        }

        public void collide(Ball b, Polkadot pd) {
            double d = distance(b.getX(), b.getY(), pd.getX(), pd.getY());
            if (d <= b.getDiameter() / 2 + pd.getDiameter() / 2) {
                pd.jump(400, 400);
                hits += 1;
            }
        }

        private double distance(double x1, double y1, double x2, double y2) {
            return Math.sqrt(Math.pow(x1 - x2, 2.0) + Math.pow(y1 - y2, 2.0));
        }
    }
  }

错误:

PrizePanelA.java:37: error: cannot find symbol
        c = new Timer(10, new Listener());
                              ^
  symbol:   class Listener
  location: class PrizePanelA

Answer 1

你的 Listener 类在 Key 类里面，所以把它移到 Key 类的范围之外，因为你不能使用非静态内部类没有外部类的实例化。

    // move it outside of class Key
    private class Listener implements ActionListener {
        private Listener() {}

        public void actionPerformed(ActionEvent e) {
            myBuffer.setColor(BACKGROUND);
            myBuffer.fillRect(0, 0, 400, 400);

            ball.move(400.0, 400.0);
            collide(ball, prize);

            ball.draw(myBuffer);
            prize.draw(myBuffer);

            myBuffer.setColor(Color.BLACK);
            myBuffer.setFont(new Font("Monospaced", 1, 24));
            myBuffer.drawString("Count: " + hits, 250, 25);
            repaint();
        }
    }

    private class Key extends KeyAdapter {

引用: What is the difference between static and non static inner class?

JIC:如果你想继续当前的实现，那么正如我所说，使用外部类实例 c = new Timer(10, new Key().new Listener());