不清楚课堂声明范围

Question

我正在尝试为我的高中开始编程课程完成我的代码，而我很困惑java告诉我它找不到Listener（当我已经导入它吗？）。在使用import java.awt.event.*;尝试再次尝试之后，它仍然行不通。有人可以帮助我解决这个加重问题吗？

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

您的Listener类在Key类内部，因此将其移到Key类的范围之外，因为如果没有外部类的实例，就无法使用非静态内类。

    // 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 {

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

JIC：如果您想使用当前实现，那么如我所说，使用外类实例c = new Timer(10, new Key().new Listener());