点击JTree时出现异常（空指针异常）
Exception when clicking JTree (null pointer exception)
所以当我单击包含 Jtree 的 Messages tabPane 时，这是我的 java swing 中的预览，看起来不错。
pict 1 (loading the message)
pict 2. (done)
当我单击 JTree 中的任何 checkboxes 时 它应该是 加载（检查）或卸载（取消选中）消息列表中的消息swingworker 运行 查看进度。但是在我点击复选框（任何条件）之后发生了什么，是的 swingworker 运行 并给出了 loading/unloading 进度，但在那之后，我得到了这个：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException:
Cannot invoke "model.Message.getContents()" because "message" is null
并使 message lists 不可点击，在我尝试点击 JTree 中的复选框之前它是可点击的。
目前我不需要 JTree 来学习 swing，所以我并没有真正考虑这个 JTree 课程，但我需要修复它以便我可以继续学习教程。这就是为什么我不太确定哪些代码有问题并且需要放入此线程中。所以如果我的问题不清楚，我很抱歉。如果我还有什么要放在这个线程上的，请问我，我很乐意把它放在这里。
这是异常中提到的class
public class MessagePanel extends JPanel implements ProgressDialogListener{
public MessagePanel(JFrame parent) {
messageListModel = new DefaultListModel();
messageList = new JList(messageListModel);
messageList.setCellRenderer(new MessageListRenderer());
messageList.addListSelectionListener(new ListSelectionListener() {
@Override
public void valueChanged(ListSelectionEvent e) {
Message message = (Message)messageList.getSelectedValue();
textPanel.setText(message.getContents());
}
});
}
这是class与上面class
相关的方法
public class MessageListRenderer implements ListCellRenderer {
private JPanel panel;
private JLabel label;
private Color selectedColor,normalColor;
public MessageListRenderer() {
//some ui settings
}
@Override
public Component getListCellRendererComponent(JList list, Object value,
int index, boolean isSelected, boolean cellHasFocus) {
Message message = (Message)value;
label.setText(message.getTitle());
panel.setBackground(cellHasFocus ? selectedColor: normalColor);
return panel;
}
}
===================
public class TextPanel extends JPanel{
public void setText(String text) {
textArea.setText(text);
}
}
===================
public class Message {
private String title,contents;
public Message(String title, String contents) {
super();
this.title = title;
this.contents = contents;
}
public String getTitle() {return title;}
public void setTitle(String title) {this.title = title;}
public String getContents() {return contents;}
public void setContents(String contents) {this.contents = contents;}
}
您的 Message class constructor 需要两个参数（属于：String，String) 以创建 Message 的实例。我不知道您当前使用什么来创建 Message 实例，也不知道存储这些实例的是什么。您确实需要跟踪它们，否则您会将它们丢给 JVM 垃圾收集器。
我想也许您可能想要稍微修改一下 Message Class 以便您可以在内部（或外部）存储 Message 实例并在需要时轻松访问这些实例中的任何一个，例如：
public class Message {
// A List Interface object to hold Message instances.
private static java.util.List<Message> messageInstances = new java.util.ArrayList<>();
// The OS System's New-Line character to use for console writing.
private final static String ls = System.lineSeparator();
// Instance member variables
private String title;
private String contents;
/**
* Constructor #1
* Does Nothing but adds the instance to the messageInstances List!
* Relies on Setters to fill instance member variables.
*/
public Message() {
messageInstances.add((this));
}
/**
* Constructor #2
* Contains parameters of which the arguments will fill instance member
* variables listed within the Parameters list below.
*
* @param title (String) The Message Title.<br>
*
* @param contents (String) The message content related to the above title.
*/
public Message(String title, String contents) {
super();
this.title = title;
this.contents = contents;
messageInstances.add((this));
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getContents() {
return contents;
}
public void setContents(String contents) {
this.contents = contents;
}
public static java.util.List<Message> getMessageInstances() {
return messageInstances;
}
/**
* Removes one (or more) Message instances from the messageInstances List.
* This method must be supplied at least one integer index value of the
* Message instance to remove otherwise a Warning is displayed within the
* console window. Several index values can be supplied providing they are
* delimited with a comma or all desired Message Instance index values to
* remove are supplied within a Single Dimensional int[] Array.<br><br>
*
* <b>Valid uses of this class method:</b><pre>
*
* removeMessageInstance(0, 4, 2, 16);
*
* OR
*
* int[] indexes = {0, 4, 2, 16};
* removeMessageInstance(indexes);</pre>
*
* @param instanceIndexes
*/
public static void removeMessageInstance(int... instanceIndexes) {
int[] iIndex = null;
if (instanceIndexes.length == 0) {
System.err.println("Message.removeMessageInstance() method Warning!" + ls
+ "Require an index value of the Message Instance to remove!" + ls
+ "Ignoring Removal call!" );
return;
}
iIndex = new int[instanceIndexes.length];
System.arraycopy(instanceIndexes, 0, iIndex, 0, instanceIndexes.length);
for (int i = 0; i < iIndex.length; i++) {
if(iIndex[i] < 0 || iIndex[i] > messageInstances.size()) {
System.err.println("Message.removeMessageInstance() method Warning!" + ls
+ "The supplied Message Instance index value (" + iIndex[i] + ") is invalid!" + ls
+ "Ignoring Removal call for Message Instance at Index " + iIndex[i] + "!");
continue;
}
messageInstances.remove(iIndex[i]);
}
}
@Override
public String toString() {
return new StringBuilder("").append(title).append(" | ")
.append(contents).toString();
}
}
尽你所能创建 Message 实例。
现在，在你的 MessagePanel class 内 ListSelectionListener:
public void valueChanged(ListSelectionEvent e) {
String title = messageList.getSelectedValue().toString(); // toString() may not be required.
List<Message> messages = Message.getMessageInstances();
for (Message msg : messages) {
if (msg.getTitle().equalsIgnoreCase(title)) {
textPanel.setText(msg.getContents());
break;
}
}
}
所以当我单击包含 Jtree 的 Messages tabPane 时，这是我的 java swing 中的预览，看起来不错。
pict 1 (loading the message)
pict 2. (done)
当我单击 JTree 中的任何 checkboxes 时 它应该是 加载（检查）或卸载（取消选中）消息列表中的消息swingworker 运行 查看进度。但是在我点击复选框（任何条件）之后发生了什么，是的 swingworker 运行 并给出了 loading/unloading 进度，但在那之后，我得到了这个：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: Cannot invoke "model.Message.getContents()" because "message" is null
并使 message lists 不可点击，在我尝试点击 JTree 中的复选框之前它是可点击的。
目前我不需要 JTree 来学习 swing，所以我并没有真正考虑这个 JTree 课程，但我需要修复它以便我可以继续学习教程。这就是为什么我不太确定哪些代码有问题并且需要放入此线程中。所以如果我的问题不清楚，我很抱歉。如果我还有什么要放在这个线程上的，请问我，我很乐意把它放在这里。
这是异常中提到的class
public class MessagePanel extends JPanel implements ProgressDialogListener{
public MessagePanel(JFrame parent) {
messageListModel = new DefaultListModel();
messageList = new JList(messageListModel);
messageList.setCellRenderer(new MessageListRenderer());
messageList.addListSelectionListener(new ListSelectionListener() {
@Override
public void valueChanged(ListSelectionEvent e) {
Message message = (Message)messageList.getSelectedValue();
textPanel.setText(message.getContents());
}
});
}
这是class与上面class
相关的方法public class MessageListRenderer implements ListCellRenderer {
private JPanel panel;
private JLabel label;
private Color selectedColor,normalColor;
public MessageListRenderer() {
//some ui settings
}
@Override
public Component getListCellRendererComponent(JList list, Object value,
int index, boolean isSelected, boolean cellHasFocus) {
Message message = (Message)value;
label.setText(message.getTitle());
panel.setBackground(cellHasFocus ? selectedColor: normalColor);
return panel;
}
}
===================
public class TextPanel extends JPanel{
public void setText(String text) {
textArea.setText(text);
}
}
===================
public class Message {
private String title,contents;
public Message(String title, String contents) {
super();
this.title = title;
this.contents = contents;
}
public String getTitle() {return title;}
public void setTitle(String title) {this.title = title;}
public String getContents() {return contents;}
public void setContents(String contents) {this.contents = contents;}
}
您的 Message class constructor 需要两个参数（属于：String，String) 以创建 Message 的实例。我不知道您当前使用什么来创建 Message 实例，也不知道存储这些实例的是什么。您确实需要跟踪它们，否则您会将它们丢给 JVM 垃圾收集器。
我想也许您可能想要稍微修改一下 Message Class 以便您可以在内部（或外部）存储 Message 实例并在需要时轻松访问这些实例中的任何一个，例如：
public class Message {
// A List Interface object to hold Message instances.
private static java.util.List<Message> messageInstances = new java.util.ArrayList<>();
// The OS System's New-Line character to use for console writing.
private final static String ls = System.lineSeparator();
// Instance member variables
private String title;
private String contents;
/**
* Constructor #1
* Does Nothing but adds the instance to the messageInstances List!
* Relies on Setters to fill instance member variables.
*/
public Message() {
messageInstances.add((this));
}
/**
* Constructor #2
* Contains parameters of which the arguments will fill instance member
* variables listed within the Parameters list below.
*
* @param title (String) The Message Title.<br>
*
* @param contents (String) The message content related to the above title.
*/
public Message(String title, String contents) {
super();
this.title = title;
this.contents = contents;
messageInstances.add((this));
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getContents() {
return contents;
}
public void setContents(String contents) {
this.contents = contents;
}
public static java.util.List<Message> getMessageInstances() {
return messageInstances;
}
/**
* Removes one (or more) Message instances from the messageInstances List.
* This method must be supplied at least one integer index value of the
* Message instance to remove otherwise a Warning is displayed within the
* console window. Several index values can be supplied providing they are
* delimited with a comma or all desired Message Instance index values to
* remove are supplied within a Single Dimensional int[] Array.<br><br>
*
* <b>Valid uses of this class method:</b><pre>
*
* removeMessageInstance(0, 4, 2, 16);
*
* OR
*
* int[] indexes = {0, 4, 2, 16};
* removeMessageInstance(indexes);</pre>
*
* @param instanceIndexes
*/
public static void removeMessageInstance(int... instanceIndexes) {
int[] iIndex = null;
if (instanceIndexes.length == 0) {
System.err.println("Message.removeMessageInstance() method Warning!" + ls
+ "Require an index value of the Message Instance to remove!" + ls
+ "Ignoring Removal call!" );
return;
}
iIndex = new int[instanceIndexes.length];
System.arraycopy(instanceIndexes, 0, iIndex, 0, instanceIndexes.length);
for (int i = 0; i < iIndex.length; i++) {
if(iIndex[i] < 0 || iIndex[i] > messageInstances.size()) {
System.err.println("Message.removeMessageInstance() method Warning!" + ls
+ "The supplied Message Instance index value (" + iIndex[i] + ") is invalid!" + ls
+ "Ignoring Removal call for Message Instance at Index " + iIndex[i] + "!");
continue;
}
messageInstances.remove(iIndex[i]);
}
}
@Override
public String toString() {
return new StringBuilder("").append(title).append(" | ")
.append(contents).toString();
}
}
尽你所能创建 Message 实例。
现在，在你的 MessagePanel class 内 ListSelectionListener:
public void valueChanged(ListSelectionEvent e) {
String title = messageList.getSelectedValue().toString(); // toString() may not be required.
List<Message> messages = Message.getMessageInstances();
for (Message msg : messages) {
if (msg.getTitle().equalsIgnoreCase(title)) {
textPanel.setText(msg.getContents());
break;
}
}
}