我编码的时间不长，但这是迄今为止我见过的最奇特的错误。现在我知道它说 Null Pointer Exception 错误，这是，但它是一个特定的错误，我已经一个多星期没在网上找到了，持续两个。我遇到了一个非常独特的问题，我急需帮助。（我想出了一个解决方法，所以我会按时完成我的项目，但这个错误仍然困扰着我）。（代码将在问题的最底部）。
项目：创建一个交通灯模拟：
- 启动时处于“关闭”状态，必须使用键盘上的“o”键打开。它也必须能够关闭。
- 当按下“f”键时，会导致黄灯闪烁。
- 必须有一个真正的红绿灯进程（绿>黄>红>绿>等）
问题：当我的面板方法中的paintComponent(Graphics pen)发生时，会产生多个错误。（此处的屏幕截图：错误）
我推断它发生在面板内创建对象之后的某个时间（当您阅读代码时更有意义）。这很奇怪，因为解决方法只是从类方法中复制并粘贴到原始面板中。
即，如果我的代码是用 pen.drawOval(insert parameters) 绘制一个椭圆，它将在其中包含paintComponent(Graphics pen) 方法的面板类中工作。但是，如果它在一个 Circle 类中，它就行不通了。
仅当paintComponent 访问另一个类的对象、另一个类的变量或另一个类的方法时，才会生成此错误。
如果使用 Circle 对象，则 object.drawOval(parameters) 将不起作用，但如果面板中使用了另一个公共 void 方法，称为 objectDraw(Graphics pen) 并从 paintComponent 作为 objectDraw(pen) 运行，它将起作用.
我希望我能尽力解释它。
现在是生成区域的代码。
我有一种令人作呕的感觉，它是如此简单以至于我忽略了。这真的是我最后的手段，因为我无法发现为什么会产生错误。
（JComponent 会损坏吗？它可能只是用户特定的吗？）
编码：
将 Lights 视为一个组类，其中包含多个对象作为私有类。（这可能是问题吗，私人课程？）
/******************
* Lights.java
*
* Creates graphic objects.
*
******************/
import java.awt.*; //Import the awt package.
public class Lights{ //Driver class header.
private final int RED = 1; //Creates switch constant.
private final int YELLOW = 2; //Creates switch constant.
private final int GREEN = 3; //Creates switch constant.
private final int ALL_ON = 5; //Creates switch constant.
private final int ALL_OFF = 6; //Creates switch constant.
private final int BACKGROUND = 0; //Background switch constant.
private final int FOREGROUND = 8; //Foreground switch constant.
private final int FLASHING_YELLOW = 4; //Creates Flashing Yellow
private final int DARK_YELLOW = 9; //Case in case.
private int flashingColor = YELLOW; //Case in case.
private boolean flashingYellow = false; //Creates false by default flashingYellow boolean.
private Color lightColor; //Color variable lightcolor.
private int colorSwitch; //Switch integer using constants and lightcolor.
private int typeSwitch; //Also similar to above.
private int x, y, lightswitch; //Other integers.
private Rectangle grayBG, blackBG;
private Circle red, green, yellow, redFore, greenFore, yellowFore;
public Lights(){
Rectangle grayBG = new Rectangle(35, 35, 280, 830, true);
Rectangle blackBG = new Rectangle(50, 50, 250, 800, true);
Circle red = new Circle(80, 75, 200, 200, true);
Circle green = new Circle(80, 575, 200, 200, true);
Circle yellow = new Circle(80, 325, 200, 200, true);
Circle redFore = new Circle(80, 75, 200, 200, false);
Circle greenFore = new Circle(80, 575, 200, 200, false);
Circle yellowFore = new Circle(80, 325, 200, 200, false);
}
//Drawn when flash is on.
public void flashOn(Graphics g){
drawYellowLight(g, true);
drawGreenLight(g, false);
drawRedLight(g, false);
drawFore(g);
}
//Drawn when light is off.
public void flashOff(Graphics g){
drawYellowLight(g, false);
drawGreenLight(g, false);
drawRedLight(g, false);
drawFore(g);
}
//Draws the foreground.
public void drawFore(Graphics pen){
drawRed(pen);
drawYellow(pen);
drawGreen(pen);
}
//Draws the foreground's red, yellow, and green.
public void drawRed(Graphics pen){
pen.setColor(Color.RED);
redFore.draw(pen);
}
public void drawYellow(Graphics pen){
pen.setColor(Color.YELLOW);
yellowFore.draw(pen);
}
public void drawGreen(Graphics pen){
pen.setColor(Color.GREEN);
greenFore.draw(pen);
}
//Draws the background.
public void drawBox(Graphics pen){
//HTML Color Code for 'LightYellow4' (looks gray though); Color GRAY = new Color(139, 139, 122);
pen.setColor(new Color(139, 139, 122));
grayBG.draw(pen);
pen.setColor(Color.BLACK);
blackBG.draw(pen);
}
//Draws the green light whether on or off.
public void drawGreenLight(Graphics pen, boolean isOn){
//Color darkGreen = new Color(105, 139, 105);
//Color lightGreen = new Color(0, 238, 0);
if(!isOn){
pen.setColor(new Color(105, 139, 105));
green.draw(pen);
} else {
pen.setColor(new Color(0, 238, 0));
green.draw(pen);
}
}
//Draws the yellow light whether on or off.
public void drawYellowLight(Graphics pen, boolean isOn){
//Color darkYellow = new Color(139, 69, 0);
//Color lightYellow = new Color(255, 215, 0);
if(!isOn){
pen.setColor(new Color(139, 69, 0));
yellow.draw(pen);
} else {
pen.setColor(new Color(255, 215, 0));
yellow.draw(pen);
}
}
//Draws the red light whether on or off.
public void drawRedLight(Graphics pen, boolean isOn){
//Color darkRed = new Color(139, 26, 26);
//Color lightRed = new Color(255, 48, 48);
if(!isOn){
pen.setColor(new Color(139, 26, 26));
red.draw(pen);
} else {
pen.setColor(new Color(255, 48, 48));
red.draw(pen);
}
}
/******************
* Lights$Rectangle.java
*
* Creates a graphic object named Rectangle.
*
******************/
private class Rectangle{ //Driver class header.
//Define global private integers.
private int x, y, h, w;
private boolean fill;
//Methods
//Object constructor, Definition.
//Defines the info for a rectangle.
public Rectangle(int a, int b, int height, int width, boolean isFilled){
x = a;
y = b;
h = height;
w = width;
fill = isFilled;
}
public void draw(Graphics pen){
if(fill == true){
int xPosition = x - w;
int yPosition = y - h;
pen.fillRect(xPosition, yPosition, w, h);
} else {
int xPosition = x - w;
int yPosition = y - h;
pen.drawRect(xPosition, yPosition, w, h);
}
}
}
/******************
* Lights$Circle.java
*
* Creates a graphic object named Rectangle.
*
******************/
private class Circle{
//Define global private integers.
private int x, y, radius;
private boolean fill;
//Object constructor, Definition.
//Defines the info for a circle.
public Circle(int a, int b, int size, int size_alt, boolean isFilled){
x = a;
y = b;
radius = ((int)size/2);
fill = isFilled;
}
public void draw(Graphics pen){
if(fill == true){
int diameter = (radius*2);
int xPosition = x - radius;
int yPosition = y - radius;
pen.fillOval(xPosition, yPosition, diameter, diameter);
} else {
int diameter = (radius*2);
int xPosition = x - radius;
int yPosition = y - radius;
pen.drawOval(xPosition, yPosition, diameter, diameter);
}
}
}
}
现在为框架类。编码：
Traffic.java 是这个项目的文字 JFrame 部分：
/****************************
* Traffic.java
*
*
******************************/
import javax.swing.*; //Import swing package.
import java.awt.*; //Import awt package.
import java.awt.event.*; //Import events package.
public class Traffic extends JFrame{
public static void main(String[] args){
//Create new JFrame.
JFrame lightbox = new JFrame("Light Box");
//Create new LightboxPanel.
TrafficPanel traffic = new TrafficPanel();
//Set focus to the LightboxPanel in order to allow key inputs.
traffic.setFocusable(true);
//Set the Default Close Operation to exit when the "x" button is selected.
lightbox.setDefaultCloseOperation(EXIT_ON_CLOSE);
//Add the LightboxPanel to the Lightbox JFrame.
lightbox.add(traffic);
//Pack the frame to the warranted preferred size of the LightboxPanel.
lightbox.pack();
//Set the frame's visibility to true.
lightbox.setVisible(true);
}
}
最后是面板类本身，TrafficPanel.java 更多的是调用方法从灯光绘制图形的面板。
编码：
//*****************************************************************************
// TrafficPanel.java
//
// Panel that holds graphics for the traffic light simulation.
//*****************************************************************************
import java.util.Scanner; // import Scanner class from util package
import javax.swing.JPanel; // import JPanel class from swing package
import javax.swing.JButton;
import java.awt.*; // import awt package
import java.awt.event.*; // import event package from awt package
import java.util.Random; // import Random class from the util package
public class TrafficPanel extends JPanel
{
private Lights shapes;
private final int RED = 1; //Creates switch constant.
private final int YELLOW = 2; //Creates switch constant.
private final int GREEN = 3; //Creates switch constant.
private final int ALL_ON = 5; //Creates switch constant.
private final int ALL_OFF = 6; //Creates switch constant.
private final int FLASHING_YELLOW = 4; //Creates Flashing Yellow
private final int DARK_YELLOW = 9; //Case in case.
private int flashingColor = YELLOW; //Case in case.
private boolean flashingYellow = false; //Creates false by default flashingYellow boolean.
private Color lightColor; //Color variable lightcolor.
private int colorSwitch; //Switch integer using constants and lightcolor.
private int typeSwitch; //Also similar to above.
private int x, y, lightswitch; //Other integers.
public TrafficPanel()
{
Lights shapes = new Lights();
//KeyListener trafficListener = new KeyListener();
addKeyListener(new KeyListener());
//Set the size.
//Dimension preferred = new Dimension(800, 920);
setPreferredSize(new Dimension(800, 920));
//Set the background.
//Color bisque = new Color(255, 228, 196);
setBackground(new Color(255, 228, 196));
}
public void paintComponent (Graphics g)
{
super.paintComponent (g);
//Normal state of panel. Initially offline.
Lights shapes = new Lights();
shapes.drawBox(g);
shapes.drawRedLight(g, false);
shapes.drawYellowLight(g, false);
shapes.drawGreenLight(g, false);
shapes.drawFore(g);
//Check state of light.
switch(lightswitch){
case ALL_OFF:
shapes.drawBox(g);
shapes.drawRedLight(g, false);
shapes.drawYellowLight(g, false);
shapes.drawGreenLight(g, false);
shapes.drawFore(g);
break;
case ALL_ON:
shapes.drawBox(g);
shapes.drawRedLight(g, true);
shapes.drawYellowLight(g, true);
shapes.drawGreenLight(g, true);
shapes.drawFore(g);
break;
case RED:
shapes.drawBox(g);
shapes.drawRedLight(g, true);
shapes.drawYellowLight(g, false);
shapes.drawGreenLight(g, false);
shapes.drawFore(g);
break;
case GREEN:
shapes.drawBox(g);
shapes.drawRedLight(g, false);
shapes.drawYellowLight(g, false);
shapes.drawGreenLight(g, true);
shapes.drawFore(g);
break;
case YELLOW:
shapes.drawBox(g);
shapes.drawRedLight(g, false);
shapes.drawYellowLight(g, true);
shapes.drawGreenLight(g, false);
shapes.drawFore(g);
break;
case FLASHING_YELLOW: //If Flashing yellow.
switch(flashingColor){
case DARK_YELLOW:
flashingColor = YELLOW;
break;
case YELLOW:
flashingColor = DARK_YELLOW;
break;
}
try{
if(flashingColor == YELLOW){
shapes.flashOn(g);
}
Thread.sleep(250);
if(flashingColor == DARK_YELLOW){
shapes.flashOff(g);
}
} catch(Exception e){}
break;
}
repaint ();
}
/*********************
* TrafficPanel$KeyListener.java
* Author: Ian Effendi
*
* Extension of KeyAdapter.
* Inputs of 'o', 'f', and the spacebar all return actions.
**********************/
private class KeyListener extends KeyAdapter{
//Occurs when a key is pressed.
public void keyPressed(KeyEvent event){
}
//Occurs, only, when a key is released.
public void keyReleased(KeyEvent event){
//If the key input is 'o'.
if(event.getKeyCode() == KeyEvent.VK_O){
flashingYellow = false;
//Switch on what lightswitch is equal to.
switch(lightswitch){
case ALL_ON:
lightswitch = ALL_OFF;
break;
case ALL_OFF:
lightswitch = ALL_ON;
break;
case RED:
lightswitch = ALL_OFF;
break;
case YELLOW:
lightswitch = ALL_OFF;
break;
case GREEN:
lightswitch = ALL_OFF;
break;
case FLASHING_YELLOW:
lightswitch = ALL_OFF;
break;
}
//If the key input is the spacebar.
} else if(event.getKeyCode() == KeyEvent.VK_SPACE){
flashingYellow = false;
//Switch on what lightswitch is equal to.
switch(lightswitch){
case ALL_ON:
lightswitch = RED;
break;
case ALL_OFF:
lightswitch = ALL_OFF;
break;
case RED:
lightswitch = GREEN;
break;
case YELLOW:
lightswitch = RED;
break;
case GREEN:
lightswitch = YELLOW;
break;
case FLASHING_YELLOW:
lightswitch = YELLOW;
break;
}
//If the key input is 'f'.
} else if(event.getKeyCode() == KeyEvent.VK_F){
flashingYellow = true;
//Switch on what lightswitch is equal to.
switch(lightswitch){
case ALL_ON:
lightswitch = FLASHING_YELLOW;
break;
case ALL_OFF:
lightswitch = ALL_OFF;
break;
case RED:
lightswitch = FLASHING_YELLOW;
break;
case YELLOW:
lightswitch = FLASHING_YELLOW;
break;
case GREEN:
lightswitch = FLASHING_YELLOW;
break;
case FLASHING_YELLOW:
lightswitch = YELLOW;
break;
}
//If nothing is input, lightswitch returns itself.
} else {
int x = lightswitch;
lightswitch = x;
}
}
//Occurs when an input is typed(not held).
public void keyTyped(KeyEvent event){
}
}
}//end of class
这就是所有的代码。请尽你所能提供帮助。任何提示也将不胜感激。