编译 Java 程序'Cannot find symbol'时出错

Question

尝试在终端中使用javac -g命令编译"PongMain.java"时出现以下错误：

错误：

    tests-iMac:~ finnfallowfield$ javac -g /Users/finnfallowfield/Desktop/Developer/Java:Javascript/Game Development/Java Pong/src/main/pong/PongMain.java 
/Users/finnfallowfield/Desktop/Developer/Java:Javascript/Game Development/Java Pong/src/main/pong/PongMain.java:9: error: cannot find symbol
import main.pong.Ball;
                ^
  symbol:   class Ball
  location: package main.pong
/Users/finnfallowfield/Desktop/Developer/Java:Javascript/Game Development/Java Pong/src/main/pong/PongMain.java:10: error: cannot find symbol
import main.pong.PaddleLeft;
                ^
  symbol:   class PaddleLeft
  location: package main.pong
/Users/finnfallowfield/Desktop/Developer/Java:Javascript/Game Development/Java Pong/src/main/pong/PongMain.java:11: error: cannot find symbol
import main.pong.PaddleRight;
                ^
  symbol:   class PaddleRight
  location: package main.pong
/Users/finnfallowfield/Desktop/Developer/Java:Javascript/Game Development/Java Pong/src/main/pong/PongMain.java:15: error: cannot find symbol
    Ball ball;
    ^
  symbol:   class Ball
  location: class PongMain
/Users/finnfallowfield/Desktop/Developer/Java:Javascript/Game Development/Java Pong/src/main/pong/PongMain.java:16: error: cannot find symbol
    PaddleLeft pLeft;
    ^
  symbol:   class PaddleLeft
  location: class PongMain
/Users/finnfallowfield/Desktop/Developer/Java:Javascript/Game Development/Java Pong/src/main/pong/PongMain.java:17: error: cannot find symbol
    PaddleRight pRight;
    ^
  symbol:   class PaddleRight
  location: class PongMain
/Users/finnfallowfield/Desktop/Developer/Java:Javascript/Game Development/Java Pong/src/main/pong/PongMain.java:29: error: cannot find symbol
        ball = new Ball();
                   ^
  symbol:   class Ball
  location: class PongMain
/Users/finnfallowfield/Desktop/Developer/Java:Javascript/Game Development/Java Pong/src/main/pong/PongMain.java:30: error: cannot find symbol
        pLeft = new PaddleLeft();
                    ^
  symbol:   class PaddleLeft
  location: class PongMain
/Users/finnfallowfield/Desktop/Developer/Java:Javascript/Game Development/Java Pong/src/main/pong/PongMain.java:31: error: cannot find symbol
        pRight = new PaddleRight(ball.getY() - 35);
                     ^
  symbol:   class PaddleRight
  location: class PongMain
9 errors

我正在尝试编译一个乒乓球游戏，所有其他 java 文件都编译得很好，但这个没有。这是我尝试编译的文件的代码：

文件源代码：

package main.pong.main;
import java.applet.*;
import java.awt.event.*;
import java.awt.*;
import javax.swing.Timer;
import main.pong.Ball;
import main.pong.PaddleLeft;
import main.pong.PaddleRight;
public class PongMain extends Applet implements MouseMotionListener, ActionListener
{
        Ball ball;
        PaddleLeft pLeft;
        PaddleRight pRight;
        Font newFont = new Font("sansserif", Font.BOLD, 20);
        Graphics bufferGraphics;
        Image offscreen;
        final int WIDTH = 500, HEIGHT = 300;
        long currentTime;
        public void init()
        {
                //Sets the applet to be 500 * 300
                setSize(500, 300);
                //Initiate ball and two paddles
                ball = new Ball();
                pLeft = new PaddleLeft();
                pRight = new PaddleRight(ball.getY() - 35);
                //Add mousMotionListener
                addMouseMotionListener(this);
                setBackground(Color.blue);
                offscreen = createImage(WIDTH, HEIGHT);
                bufferGraphics = offscreen.getGraphics();
        }
        public void start(){
                currentTime = System.currentTimeMillis();
                //Set up frame-rate
                Timer time = new Timer(15, this);
                time.start();
                while(pRight.getScore() < 10){
                }
                time.stop();
                currentTime = System.currentTimeMillis() - currentTime;
                repaint();
        }
        public void stop(){
        }
        public void paint(Graphics g)
        {
                bufferGraphics.clearRect(0,0,WIDTH,HEIGHT);
                bufferGraphics.setColor(Color.green);
                //Left side
                bufferGraphics.fillRect(pLeft.XPOS,pLeft.getPos(),10,70);
                //Right side
                bufferGraphics.fillRect(pRight.XPOS, pRight.getPos(), 10, 70);
                //White lines
                bufferGraphics.setColor(Color.white);
                bufferGraphics.setFont(newFont);
                bufferGraphics.drawString("Futile", 150, 15);
                bufferGraphics.drawString(""+ pRight.getScore(),300,15);
                bufferGraphics.fillRect(240,0,20,300);
                if(pRight.getScore() == 10){
                        //Display for how long game lasted
                        bufferGraphics.drawString("You Lasted: " + (currentTime/ 1000) + "sec.", 40, 150);
                }
                //We draw the ball
                bufferGraphics.setColor(Color.red);
                bufferGraphics.fillRect(ball.getX(), ball.getY(),10, 10);
                g.drawImage(offscreen,0,0,this);
                Toolkit.getDefaultToolkit().sync();
    }

    // STUFF
        public void update(Graphics g)
        {
                paint(g);
        }
        public void mouseMoved(MouseEvent evt)
        {
                pLeft.setPos(evt.getY()- 35);
        }
        public void mouseDragged(MouseEvent evt)
        {
        }
        public void checkCollision(){
                if(ball.getY() == 0 || ball.getY() == 290){
                        ball.dy = (ball.dy * -1);
                }
                if((ball.getX() == 40) && hitPaddle()){
                        ball.dx = (ball.dx * -1);
                }
                if(ball.getX() == 460){
                        ball.dx = (ball.dx * -1);
                }
                if(ball.getX() == 0){
                         pRight.setScore(pRight.getScore() + 1);
                         ball.reset();
                }
        }
        public boolean hitPaddle(){
                boolean didHit = false;
                if((pLeft.getPos() - 10) <= ball.getY() && (pLeft.getPos() + 70) > ball.getY()){
                        didHit = true;
                }
                return didHit;
        }
        @Override
        public void actionPerformed(ActionEvent arg0) {
                ball.move();
                pRight.setPos(ball.getY() - 35);
                checkCollision();
                repaint();
        }
}

感谢您的阅读，请回复并注意我是一个完全的 java 初学者和堆栈交换，所以我需要很多帮助来解决这个问题！

Answer 1

为了正确编译，需要满足一些外部条件：

1. 必须有一个名为 main.pong.Ball 的类
必须
2. Ball.java或Ball.class可用
3. 如果是源文件，则它必须位于编译器在编译时类路径上可用的目录中main/pong或者必须在javac命令行上命名
4. 如果是类文件，则它需要位于编译时类路径main/pong目录中。

其中一个条件没有得到满足;满足所有条件，这些问题应该会消失（当然，也许会被新的问题所取代）。通常，鉴于您似乎拥有的设置，实现此目的的最简单方法是使用" cd"命令更改为目录/Users/finnfallowfield/Desktop/Developer/Java:Javascript/Game Development/Java Pong/src/，然后运行javac main/pong/PongMain.java。