线程“AWT-EventQueue-0"中的异常java.lang.NullPointerException 错误 [英] Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException Error
问题描述
您好，我是高中级别的新程序员，因此我对编程知之甚少，并且遇到了很多已解决的错误，而其他错误我完全不明白.我将制作一个简单的 Check Box 选择程序，用户可以在其中选择各种选项，并根据他们的操作改变图像.程序本身编译完美，但是当我运行它时，它给了我一些复杂性.这是我的程序:
Hello I'm a new programmer at an high school level as a result I do not know much about programming and am getting quite a few errors which have been resolved while others I completely do not understand. I am to make a simple Check Box selection program where the user gets to pick between a variety of choices and depending on their action the image changes. The program itself compiles perfectly but when I run it however it gives me some complications. Here is my program:
package components;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Workshop extends JPanel
implements ItemListener {
JCheckBox winterhatButton;
JCheckBox sportshatButton;
JCheckBox santahatButton;
JCheckBox redshirtButton;
JCheckBox brownshirtButton;
JCheckBox suitButton;
JCheckBox denimjeansButton;
JCheckBox blackpantsButton;
JCheckBox khakipantsButton;
StringBuffer choices;
JLabel pictureLabel;
public Workshop() {
super(new BorderLayout());
//Create the check boxes.
winterhatButton = new JCheckBox("Winter Hat");
winterhatButton.setMnemonic(KeyEvent.VK_Q);
sportshatButton = new JCheckBox("Sports Hat");
sportshatButton.setMnemonic(KeyEvent.VK_W);
santahatButton = new JCheckBox("Santa hat");
santahatButton.setMnemonic(KeyEvent.VK_E);
redshirtButton = new JCheckBox("Red Shirt");
redshirtButton.setMnemonic(KeyEvent.VK_R);
brownshirtButton = new JCheckBox("Brown Shirt");
brownshirtButton.setMnemonic(KeyEvent.VK_T);
suitButton = new JCheckBox("Suit");
suitButton.setMnemonic(KeyEvent.VK_Y);
suitButton = new JCheckBox("Denim Jeans");
suitButton.setMnemonic(KeyEvent.VK_U);
blackpantsButton = new JCheckBox("Black Pants");
blackpantsButton.setMnemonic(KeyEvent.VK_I);
khakipantsButton = new JCheckBox("Khaki Pants");
khakipantsButton.setMnemonic(KeyEvent.VK_O);
//Register a listener for the check boxes.
winterhatButton.addItemListener(this);
sportshatButton.addItemListener(this);
santahatButton.addItemListener(this);
redshirtButton.addItemListener(this);
brownshirtButton.addItemListener(this);
suitButton.addItemListener(this);
denimjeansButton.addItemListener(this);
blackpantsButton.addItemListener(this);
khakipantsButton.addItemListener(this);
//Indicates
choices = new StringBuffer("---------");
//Set up the picture label
pictureLabel = new JLabel();
pictureLabel.setFont(pictureLabel.getFont().deriveFont(Font.ITALIC));
updatePicture();
//Put the check boxes in a column in a panel
JPanel checkPanel = new JPanel(new GridLayout(0, 1));
checkPanel.add(winterhatButton);
checkPanel.add(sportshatButton);
checkPanel.add(santahatButton);
checkPanel.add(redshirtButton);
checkPanel.add(brownshirtButton);
checkPanel.add(suitButton);
checkPanel.add(denimjeansButton);
checkPanel.add(blackpantsButton);
checkPanel.add(khakipantsButton);
add(checkPanel, BorderLayout.LINE_START);
add(pictureLabel, BorderLayout.CENTER);
setBorder(BorderFactory.createEmptyBorder(20,20,20,20));
}
/** Listens to the check boxes. */
public void itemStateChanged(ItemEvent e) {
int index = 0;
char c = '-';
Object source = e.getItemSelectable();
if (source == winterhatButton) {
index = 0;
c = 'q';
} else if (source == sportshatButton) {
index = 1;
c = 'w';
} else if (source == santahatButton) {
index = 2;
c = 'e';
} else if (source == redshirtButton) {
index = 3;
c = 'r';
} else if (source == brownshirtButton) {
index = 4;
c = 't';
} else if (source == suitButton) {
index = 5;
c = 'y';
} else if (source == denimjeansButton) {
index = 6;
c = 'u';
} else if (source == blackpantsButton) {
index = 7;
c = 'i';
} else if (source == khakipantsButton) {
index = 8;
c = 'o';
}
if (e.getStateChange() == ItemEvent.DESELECTED) {
c = '-';
}
//Apply the change to the string.
choices.setCharAt(index, c);
updatePicture();
}
protected void updatePicture() {
//Get the icon corresponding to the image.
ImageIcon icon = createImageIcon(
"images/bear/bear-"
+ choices.toString()
+ ".gif");
pictureLabel.setIcon(icon);
pictureLabel.setToolTipText(choices.toString());
if (icon == null) {
pictureLabel.setText("Missing Image");
} else {
pictureLabel.setText(null);
}
}
/** Returns an ImageIcon, or null if the path was invalid. */
protected static ImageIcon createImageIcon(String path) {
java.net.URL imgURL = Workshop.class.getResource(path);
if (imgURL != null) {
return new ImageIcon(imgURL);
} else {
System.err.println("Couldn't find file: " + path);
return null;
}
}
private static void createAndShowGUI() {
//Create and set up the window.
JFrame frame = new JFrame("Build a Bear at Safeer's Workshop!");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
//Create and set up the content pane.
JComponent newContentPane = new Workshop();
newContentPane.setOpaque(true); //content panes must be opaque
frame.setContentPane(newContentPane);
//Display the window.
frame.pack();
frame.setVisible(true);
}
public static void main(String[] args) {
//Schedule a job for the event-dispatching thread:
//creating and showing this application's GUI.
javax.swing.SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGUI();
}
});
}
}
到目前为止，它运行顺利并符合要求，但是当我继续运行该程序时，我收到此错误.
Well up to this part it runs smoothly and complies but when I proceed to run the program I get this error.
> run components.Workshop
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at components.Workshop.<init>(Workshop.java:75)
at components.Workshop.createAndShowGUI(Workshop.java:195)
at components.Workshop.access$0(Workshop.java:189)
at components.Workshop$1.run(Workshop.java:209)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$000(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$1.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)
可能是一个愚蠢的错误，但我似乎无法弄清楚这一点.请帮忙，谢谢
Might be a silly mistake however I cant seem to figure this out. Please Help and thank you
Here is the line that generates that error
private void jButtonSendActionPerformed(java.awt.event.ActionEvent evt) {
// TODO add your handling code here:
String message;
if(messageBox.getText().length() > 0){
message = messageBox.getText();
chatBox.append(message+"
");
printStream.println(message);//this line
printStream.flush();
//printStream.close();
messageBox.setText("");
}
}
推荐答案
NullPointerException 是更容易诊断的异常之一，而且经常发生.每当您在 Java 中遇到异常并看到堆栈跟踪时(顺便说一下，这就是您调用的第二个引号块)，您会从上到下阅读.通常，您会看到以 Java 库代码或本机实现方法开始的异常，为了进行诊断，您可以跳过这些异常，直到看到您编写的代码文件.
NullPointerExceptions are among the easier exceptions to diagnose, frequently. Whenever you get an exception in Java and you see the stack trace ( that's what your second quote-block is called, by the way ), you read from top to bottom. Often, you will see exceptions that start in Java library code or in native implementations methods, for diagnosis you can just skip past those until you see a code file that you wrote.
然后您喜欢在指示的行上查看该行上的每个对象(实例化的类)——其中一个没有被创建，您试图使用它.您可以首先查看您的代码，看看您是否在该对象上调用了构造函数.如果没有，那就是您的问题，您需要通过调用 new Classname(arguments) 来实例化该对象.NullPointerException 的另一个常见原因是当存在同名的实例变量时意外声明了具有局部作用域的对象.
Then you like at the line indicated and look at each of the objects ( instantiated classes ) on that line -- one of them was not created and you tried to use it. You can start by looking up in your code to see if you called the constructor on that object. If you didn't, then that's your problem, you need to instantiate that object by calling new Classname( arguments ). Another frequent cause of NullPointerExceptions is accidentally declaring an object with local scope when there is an instance variable with the same name.
在您的情况下，异常发生在第 75 行的 Workshop 构造函数中.<init> 表示类的构造函数.如果您查看代码中的那一行，您将看到该行
In your case, the exception occurred in your constructor for Workshop on line 75. <init> means the constructor for a class. If you look on that line in your code, you'll see the line
denimjeansButton.addItemListener(this);
这一行有两个对象很明显:denimjeansButton 和 this.this 与您当前所在的类实例和构造函数中的类实例同义，因此它不能是 this.denimjeansButton 是你的罪魁祸首.您从未实例化该对象.删除对实例变量 denimjeansButton 的引用或实例化它.
There are fairly clearly two objects on this line: denimjeansButton and this. this is synonymous with the class instance you are currently in and you're in the constructor, so it can't be this. denimjeansButton is your culprit. You never instantiated that object. Either remove the reference to the instance variable denimjeansButton or instantiate it.
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