java启动：小程序未初始化错误。我如何解决这个问题？

Question

我试图制作一个小程序作为学校的一个项目，但遇到了这个我以前从未见过的错误。任何建议都是非常受欢迎的，我已经挣扎了几个小时也没有用。另外，这是我在Java中的第一个大项目，所以任何关于编码和风格的其他建议都是非常受欢迎的。我的IDE是blueJ，如果有必要，我会在applet查看器中运行它。干杯

import java.awt.*;
import javax.swing.*;
import java.lang.*;
import java.util.*;
import java.awt.event.*;

public abstract class Renderer extends JApplet implements KeyListener
{
    public PlayerShip playerShip;
    public static final int CANVAS_SIZE=500;
    public static final int FRAMES_PER_SECOND = 20;
    public static final int SKIP_TICKS = 1000 / FRAMES_PER_SECOND;
    public void initLevel(){
        playerShip= new PlayerShip(CANVAS_SIZE);
    }
    @Override 
    public void keyPressed(KeyEvent e){
        if (e.getKeyCode()== KeyEvent.VK_RIGHT){
            playerShip.moveRight();
        }
        else if (e.getKeyCode()== KeyEvent.VK_LEFT){
            playerShip.moveLeft();
        }
        repaint();
    }
    public void paint(Graphics g){
        int sleep_time = 0;
        int next_game_tick = 0;
        long sleepTime;
        boolean Game= true;
        long startTime= System.currentTimeMillis();
        setSize(CANVAS_SIZE, CANVAS_SIZE);
        while(Game== true){
            initLevel();
            int leftSide=playerShip.getLeftBound();
            int width=playerShip.getWidth();
            int topSide=playerShip.getTopBound();
            int height=playerShip.getHeight();


            g.setColor(Color.blue);
            g.fillRect(0, 0, CANVAS_SIZE, CANVAS_SIZE);
            g.setColor(Color.orange);
            g.fillRect (leftSide, topSide, width, height);
            long timeElapsed = System.currentTimeMillis() - startTime;
            next_game_tick += SKIP_TICKS;

            sleepTime= next_game_tick - timeElapsed;
            try{
                Thread.sleep(sleepTime);
            }
            catch(InterruptedException ex)
            {
                Thread.currentThread().interrupt();
            }

        }

    }

}

编辑：这也是玩家的职业

public class PlayerShip
{
    // instance variables - replace the example below with your own
    public static final int SIZE= 20;
    public int shipX;
    public int shipY;
    public int shipLeft;
    public int shipRight;
    public PlayerShip(int canvasSize)
    {
        shipX= canvasSize/2;
        shipY= canvasSize*3/4;
    }
    public void moveLeft(){
        shipX -=1;
    }
    public void moveRight(){
        shipX+=1;
    }
    public int getLeftBound(){
        int leftSide = Math.round(shipX - (SIZE/2));
        return (leftSide);
    }
    public int getWidth(){
        return SIZE;
    }
    public int getTopBound(){
        int topSide = Math.round(shipY - (SIZE/2));
        return (topSide);
    }
    public int getHeight(){
        return SIZE;
    }

}

Answer 1

实现KeyListener

一个问题可能是，您只实现了^{}接口所需的三种方法中的一种。你做了keyPressed，但省略了keyTyped和keyReleased

@Override
public void keyTyped ( KeyEvent e )
{
    …
}

@Override
public void keyPressed ( KeyEvent e )
{
    …
}

@Override
public void keyReleased ( KeyEvent e )
{
    …
}

进口

您可能需要为Color类添加一个显式的import来识别其常量

import java.awt.Color;

按照惯例，Java中的常量都是用大写字母命名的。所以，Color.BLUE

命名约定

这不是编译器或小程序的问题，但为了让其他程序员阅读您的代码，您应该遵循Java命名约定。所以boolean Game应该以小写字母开头。此外，布尔值通常以is前缀命名，所以boolean isGame。我想你可以想出一个更具描述性的措辞

顺便说一下，为了简洁起见，把while ( isGame == true )缩短为while ( isGame )

一个更严重的问题是：在while循环中使用的这个isGame变量永远不会改变其状态。所以你的while循环是无限的

运行

在做了上述更改后，我看到您的小程序在macOS Mojave的MacBook Pro上通过AppletViewerapp version 1.0，使用IntelliJIDE和Java 8（Zul系统的ZuluJVM）启动

顺便说一句，在我看来，你正在用你的行Thread.sleep(sleepTime);睡主GUI线程。我不是applet或AWT方面的专家，但正如我所记得的，你应该永远不要睡在主GUI线程上

package work.basil.example;

public class PlayerShip
{
    // instance variables - replace the example below with your own
    public static final int SIZE = 20;
    public int shipX;
    public int shipY;
    public int shipLeft;
    public int shipRight;

    public PlayerShip ( int canvasSize )
    {
        shipX = canvasSize / 2;
        shipY = canvasSize * 3 / 4;
    }

    public void moveLeft ()
    {
        shipX -= 1;
    }

    public void moveRight ()
    {
        shipX += 1;
    }

    public int getLeftBound ()
    {
        int leftSide = Math.round( shipX - ( SIZE / 2 ) );
        return ( leftSide );
    }

    public int getWidth ()
    {
        return SIZE;
    }

    public int getTopBound ()
    {
        int topSide = Math.round( shipY - ( SIZE / 2 ) );
        return ( topSide );
    }

    public int getHeight ()
    {
        return SIZE;
    }

}

…还有

package work.basil.example;

import javax.swing.*;
import java.awt.*;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

public class Renderer extends JApplet implements KeyListener
{
    public PlayerShip playerShip;
    public static final int CANVAS_SIZE = 500;
    public static final int FRAMES_PER_SECOND = 20;
    public static final int SKIP_TICKS = 1000 / FRAMES_PER_SECOND;

    public void initLevel ()
    {
        playerShip = new PlayerShip( CANVAS_SIZE );
    }

    public void paint ( Graphics g )
    {
        int sleep_time = 0;
        int next_game_tick = 0;
        long sleepTime;
        boolean isGame = true;
        long startTime = System.currentTimeMillis();
        setSize( CANVAS_SIZE , CANVAS_SIZE );
        while ( isGame  )
        {
            initLevel();
            int leftSide = playerShip.getLeftBound();
            int width = playerShip.getWidth();
            int topSide = playerShip.getTopBound();
            int height = playerShip.getHeight();

            g.setColor( Color.BLUE );
            g.fillRect( 0 , 0 , CANVAS_SIZE , CANVAS_SIZE );
            g.setColor( Color.ORANGE );
            g.fillRect( leftSide , topSide , width , height );
            long timeElapsed = System.currentTimeMillis() - startTime;
            next_game_tick += SKIP_TICKS;

            sleepTime = next_game_tick - timeElapsed;
            try
            {
                Thread.sleep( sleepTime );
            } catch ( InterruptedException ex )
            {
                Thread.currentThread().interrupt();
            }

        }

    }

    //     |  Override `java.awt.event.KeyListener`  |         

    @Override
    public void keyPressed ( KeyEvent e )
    {
        if ( e.getKeyCode() == KeyEvent.VK_RIGHT )
        {
            playerShip.moveRight();
        } else if ( e.getKeyCode() == KeyEvent.VK_LEFT )
        {
            playerShip.moveLeft();
        }
        repaint();
    }

    @Override
    public void keyTyped ( KeyEvent e )
    {
        // No code needed here.
    }

    @Override
    public void keyReleased ( KeyEvent e )
    {
        // No code needed here.
    }

}

我还要补充一点，即Java Applet技术正在被网络浏览器制造商、Oracle和Java社区逐步淘汰。这个项目很适合学习。但在实际工作中，您可能会想了解OpenJFX，并使用jlink绑定Java运行时