这是我的代码：
public static List<String> matched_file_names=null;
//Constructor
public compareTwoTextualFiles() {
}
public String compareFiles(File File1, File File2) throws IOException {
try {
// Create FileReader & Writer Objects.
FileReader File1Reader = new FileReader(File1.getPath());
FileReader File2Reader = new FileReader(File2.getPath());
// Create Buffered Object.
BufferedReader File1BufRdr = new BufferedReader(File1Reader);
BufferedReader File2BufRdr = new BufferedReader(File2Reader);
// Get the file contents into String Variables.
String File1Content = File1BufRdr.readLine();
String File2Content = File2BufRdr.readLine();
//New String Builder
StringBuilder buffer = new StringBuilder();
// Compare the Contents of the files.
String startOfComparision = "---------START----------";
buffer.append(startOfComparision).append("\n");
boolean isDifferent = false;
int lineNumber = 1;
if (File1Content != null || File2Content != null) {
// Check whether file1 contains data or not
while ((File1Content != null)) {
// Check whether file2 contains data or not
if (((File2Content) != null)) {
// Check whether both the files have same data in the lines
if (!File1Content.equals(File2Content)) {
buffer.append("Difference in Line " + lineNumber + ":- " + File1.getName() + " contains :" + File1Content + " " + File2.getName() + " Contains : " + File2Content).append("\n");
isDifferent = true;
}
lineNumber = lineNumber + 1;
File2Content = File2BufRdr.readLine();
} else {
buffer.append("Difference in Line " + lineNumber + ":- " + File1.getName() + " contains :" + File1Content + " " + File2.getName() + " Contains : " + File2Content).append("\n");
isDifferent = true;
lineNumber = lineNumber + 1;
}
File1Content = File1BufRdr.readLine();
}
// Check for the condition : if File2 has Data & File1 doesn't contain data.
while ((File2Content != null) && (File1Content == null)) {
buffer.append("Difference in Line " + lineNumber + " :- " + File1.getName() + " contains :" + File1Content + " " + File2.getName() + " Contains : " + File2Content).append("\n");
isDifferent = true;
lineNumber = lineNumber + 1;
File2Content = File2BufRdr.readLine();
}
} else {
// Mention that both the files don't have Data.
buffer.append(File1.getName() + " and " + File2.getName() + " do not contain any data.");
isDifferent = true;
}
// Check is there any difference or not.
String endOfComparision = "-----------END----------";
if (isDifferent) {
buffer.append(endOfComparision).append("\n");
} else {
buffer.append("No Difference Found \nThe Contents Of The Files Are Identical.").append("\n");
buffer.append(endOfComparision).append("\n");
问题出在以下这一行：
matched_file_names.add("Path: " + File2.getPath() + "\nFile Name: " + File2.getName());
}
这是stacktrace：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at compareTwoTextualFiles.compareFiles(compareTwoTextualFiles.java:93)
at comparisonForm.btntxtFilesCompareActionPerformed(comparisonForm.java:625)
at comparisonForm.access$400(comparisonForm.java:24)
at comparisonForm$4.actionPerformed(comparisonForm.java:176)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6525)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6290)
at java.awt.Container.processEvent(Container.java:2234)
at java.awt.Component.dispatchEventImpl(Component.java:4881)
at java.awt.Container.dispatchEventImpl(Container.java:2292)
at java.awt.Component.dispatchEvent(Component.java:4703)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
at java.awt.Container.dispatchEventImpl(Container.java:2278)
at java.awt.Window.dispatchEventImpl(Window.java:2739)
at java.awt.Component.dispatchEvent(Component.java:4703)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:746)
at java.awt.EventQueue.access$400(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:697)
at java.awt.EventQueue$3.run(EventQueue.java:691)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:719)
at java.awt.EventQueue$4.run(EventQueue.java:717)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:716)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
当选择不同的文件时它工作正常但是当同一个文件作为file1和file2传递时它会给出异常。请纠正它。
答案 0 :(得分：3)
确保在使用前初始化matched_file_names列表。
这可以解决您的问题。
答案 1 :(得分：0)
什么时候初始化matched_file_names？
它为空，因此您将获得NullPoniterException
用
替换public static List<String> matched_file_names=null;
public static List<String> matched_file_names = new ArrayList<String>();
或List接口的任何其他实现
答案 2 :(得分：0)
Matched_file名称尚未发布任何内容，它仍然是null！