我正在制作一个基本的乌龟类型程序，您可以在其中输入命令并绘制一条线。
我的问题是每次尝试画线时程序都会崩溃。我已经尝试了一些东西，但是一旦命令＆＃34; penup＆＃34;它崩溃了。输入。
我有这个错误：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at GUI.GraphicalInterface$2.actionPerformed(GraphicalInterface.java:161)
at javax.swing.JTextField.fireActionPerformed(Unknown Source)
at javax.swing.JTextField.postActionEvent(Unknown Source)
at javax.swing.JTextField$NotifyAction.actionPerformed(Unknown Source)
at javax.swing.SwingUtilities.notifyAction(Unknown Source)
at javax.swing.JComponent.processKeyBinding(Unknown Source)
at javax.swing.JComponent.processKeyBindings(Unknown Source)
at javax.swing.JComponent.processKeyEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.KeyboardFocusManager.redispatchEvent(Unknown Source)
at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(Unknown Source)
at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(Unknown Source)
at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(Unknown Source)
at java.awt.DefaultKeyboardFocusManager.dispatchEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$500(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
#GraphicalInterface class
package GUI;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.Graphics;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;
import javax.swing.*;
import GUI.Dialog;
import java.util.Scanner;
public class GraphicalInterface extends JFrame implements ActionListener{
public static BufferedImage image;
private final static Color BACKGROUND_COL = Color.DARK_GRAY;
public void paint(Graphics g) {
// render the image on the panel.
g.drawImage(image, 0, 0, null);
}
public void drawLine(Color color, int x1, int y1, int x2, int y2) {
Graphics g = image.getGraphics();
g.setColor(color);
g.drawLine(0, 10, 0, 0);
}
public void paintComponent(Graphics g)
{
// Draws the image to the canvas
g.drawImage(image, 0, 0, null);
}
public static void clear() {
Graphics g = image.getGraphics();
g.setColor(BACKGROUND_COL);
g.fillRect(0, 0, image.getWidth(), image.getHeight());
}
public static void main(String[] args) {
//Frame Start
JFrame frame = new JFrame("My Title");
LayoutManager layout = new BorderLayout();
frame.setLayout(layout);
frame.setSize(1000, 800);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setResizable(true);
//Panel
//GraphicalInterface panel = new GraphicalInterface();
JPanel frame2 = new JPanel();
Graphics g = frame2.getGraphics();
frame2.setSize(500, 500);
frame2.setVisible(true);
frame2.setBackground(BACKGROUND_COL);
//JText input START
JTextField Field = new JTextField();
//adds Field to frame and sets JTextField to bottom of app
frame.add(Field, BorderLayout.PAGE_END);
frame.add(frame2, BorderLayout.CENTER);
//Sets welcome text within JText
Field.setText("Enter command here:");
//Calls aml from event lib, assigns to app
Field.addMouseListener(new MouseAdapter() {
//Clears text on click
public void mouseClicked(MouseEvent e) {
Field.setText("");
}
});
//assign listener to field
Field.addActionListener(new ActionListener() {
//if statement for commands
public void actionPerformed(ActionEvent e) {
//assign variable to hold commands
String command = Field.getText();
if (command.equals("close")) {
System.exit(0);
}
else if (command.equals("penup"))
{
g.drawLine(0, 20, 0, 0);
}
else if (command.equals("pendown"))
{
}
else if (command.equals("turnleft"))
{
}
else if (command.equals("turnright"))
{
}
else if (command.equals("forward"))
{
}
else if (command.equals("backward"))
{
}
else if (command.equals("black"))
{
}
else if (command.equals("green"))
{
}
else if (command.equals("red"))
{
}
else if (command.equals("reset"))
{
clear();
}
}
});
//JText input END
//Menu
//Creates Menu bar
JMenuBar mbar = new JMenuBar();
//Creates the actual drop down
JMenu File = new JMenu("File");
//Creates the second drop down
JMenu Help = new JMenu("Help");
//Creates button within Help drop down
JButton About = new JButton("About");
About.addActionListener( new ActionListener() {
//Creates an event
public void actionPerformed( ActionEvent e ) {
//Calls the class Dialog.About
Dialog.About(args);
}
});
//Creates a sub menu button from File
JButton New = new JButton("New");
New.addActionListener( new ActionListener() {
//Creates an event
public void actionPerformed( ActionEvent e ) {
//Calls the class Dialog.New
Dialog.New(args);
}
});
//Creates a sub menu button from File
JButton Load = new JButton("Load");
Load.addActionListener( new ActionListener() {
//Creates an event
public void actionPerformed( ActionEvent e ) {
//Calls the class Dialog.Load and runs code
Dialog.Load(args);
}
});
//Creates a sub menu button from File
JButton Save = new JButton("Save");
//Listens for Save
Save.addActionListener( new ActionListener() {
//Creates an event
public void actionPerformed( ActionEvent e ) {
//Calls the class Dialog.Save and runs code
Dialog.Save(args);
}
});
//asigns exit to a button
JButton Exit = new JButton("Exit");
//Listens for Save
Exit.addActionListener( new ActionListener() {
//Creates an event
public void actionPerformed( ActionEvent e ) {
//Calls the class Dialog.Save and runs code
Diag.Exit(args);
}
});
//Assigns buttons to File and Help
File.add(New);
File.add(Load);
File.add(Save);
File.add(Exit);
Help.add(About);
mbar.add(File);
mbar.add(Help);
frame.setJMenuBar(mbar);
File.setVisible(true);
Help.setVisible(true);
mbar.setVisible(true);
frame.setVisible(true);
//Menu end
}
GraphicalInterface() {
setPreferredSize(new Dimension(500, 300));
image = new BufferedImage(800, 400, BufferedImage.TYPE_INT_RGB);
// Set max size of the panel, so that is matches the max size of the image.
setMaximumSize(new Dimension(image.getWidth(), image.getHeight()));
clear();
}
@Override
public void actionPerformed(ActionEvent e) {
// TODO Auto-generated method stub
}
}