您好，我是一名高中水平的新程序员，因此我对编程知之甚少，并且遇到了一些已经解决的错误，而其他一些我完全不理解。我将制作一个简单的Check Box选择程序，用户可以在各种选择之间进行选择，并根据其动作改变图像。程序本身编译得很完美，但是当我运行它时，它给了我一些复杂性。这是我的计划：
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();
}
});
}
}
这部分运行得很顺利，但是当我继续运行程序时，我得到了这个错误。
> 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
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+"\n");
printStream.println(message);//this line
printStream.flush();
//printStream.close();
messageBox.setText("");
}
}
答案 0 :(得分：23)
NullPointerException是更容易诊断的例外。每当你在Java中获得异常并且你看到堆栈跟踪（顺便说一下，你的第二个引用块是什么）时，你会从上到下阅读。通常，您会看到以Java库代码或本机实现方法开始的异常，对于诊断，您可以跳过这些异常，直到看到您编写的代码文件为止。
然后你喜欢在指定的行上查看该行上的每个对象（实例化的类） - 其中一个没有创建，你试图使用它。您可以首先查看代码，看看是否在该对象上调用了构造函数。如果你没有，那就是你的问题，你需要通过调用新的Classname（参数）来实例化该对象。当存在具有相同名称的实例变量时，NullPointerException s的另一个常见原因是意外地声明了具有局部范围的对象。
在您的情况下，在第75行的Workshop构造函数中发生了异常。<init>表示类的构造函数。如果您在代码中查看该行，则会看到
denimjeansButton.addItemListener(this);
这一行上有两个对象：denimjeansButton和this。 this与您当前所在的类实例同义，并且您在构造函数中，因此它不能是this。 denimjeansButton是你的罪魁祸首。你永远不会实例化那个对象。删除对实例变量denimjeansButton的引用或实例化它。
答案 1 :(得分：0)
在Public Workshop（）的代码顶部附近，我假设这是
suitButton = new JCheckBox("Suit");
suitButton.setMnemonic(KeyEvent.VK_Y);
suitButton = new JCheckBox("Denim Jeans");
suitButton.setMnemonic(KeyEvent.VK_U);
应该是
suitButton = new JCheckBox("Suit");
suitButton.setMnemonic(KeyEvent.VK_Y);
denimjeansButton = new JCheckBox("Denim Jeans");
denimjeansButton.setMnemonic(KeyEvent.VK_U);