我的代码中出现 NullPointerException 错误，我已将其缩小为导致异常的这段代码
AttributedString mStyledText = new AttributedString(mText);
我曾认为 mText 的值将使用以下代码从代码中重新检索：
BufferedReader br = null;
try {
br = new BufferedReader(new FileReader(path));
List<String> list = new ArrayList<String>();
while ((mText = br.readLine()) != null) {
//Displays the contents of the file in terminal
System.out.println(mText);
list.add(mText);
}
但是我仍然得到null错误，我认为值mText会来自上面的代码并被使用但是这似乎没有发生，为什么会这样，整个代码在下面
import java.awt.*;
import java.awt.font.*;
import java.awt.geom.*;
import java.awt.print.*;
import java.text.*;
import java.io.*;
import javax.swing.*;
import java.util.List;
import java.util.ArrayList;
public class PrintText implements Printable {
private List<String> list;
// Below the code will allow the user to select a file and then print out the contents of the file
public static void main(String[] args) throws IOException {
new PrintText();
}
public PrintText() {
EventQueue.invokeLater(new Runnable() {
@Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
}
//selects the file
JFileChooser chooser = new JFileChooser();
chooser.showOpenDialog(null);
File file = chooser.getSelectedFile();
String filename = file.getName();
//System.out.println("You have selected: " + filename); testing to see if file seleected was right
String path = file.getAbsolutePath();
//Reads contents of file into terminal
//FileReader fr = new FileReader("filename");
// FileReader fr = new FileReader("D:/Documents/" + "filename"));
BufferedReader br = null;
try {
br = new BufferedReader(new FileReader(path));
List<String> list = new ArrayList<String>();
String line;
while ((line = br.readLine()) != null) {
System.out.println(line);
list.add(line);
}
printer();
} catch (IOException exp) {
exp.printStackTrace();
} finally {
try {
br.close();
} catch (Exception e) {
}
}
//fr.close();
}
});
}
//private static final String mText =
// "This is a test to see if this text will be printed "; //This works perfectly fine
//AttributedString mStyledText = new AttributedString(mText);
/**
* Print a single page containing some sample text.
*/
public void printer() {
/* Get the representation of the current printer and
* the current print job.
*/
PrinterJob printerJob = PrinterJob.getPrinterJob();
/* Build a book containing pairs of page painters (Printables)
* and PageFormats. This example has a single page containing
* text.
*/
Book book = new Book();
book.append(this, new PageFormat());
/* Set the object to be printed (the Book) into the PrinterJob.
* Doing this before bringing up the print dialog allows the
* print dialog to correctly display the page range to be printed
* and to dissallow any print settings not appropriate for the
* pages to be printed.
*/
printerJob.setPageable(book);
/* Show the print dialog to the user. This is an optional step
* and need not be done if the application wants to perform
* 'quiet' printing. If the user cancels the print dialog then false
* is returned. If true is returned we go ahead and print.
*/
boolean doPrint = printerJob.printDialog();
if (doPrint) {
try {
printerJob.print();
} catch (PrinterException exception) {
System.err.println("Printing error: " + exception);
}
}
}
/**
* Print a page of text.
*/
public int print(Graphics g, PageFormat format, int pageIndex) {
System.out.println("About to obtain an attributed string, list=" + list);
AttributedString mStyledText = new AttributedString(list);
// etc
/* We'll assume that Jav2D is available.
*/
Graphics2D g2d = (Graphics2D) g;
/* Move the origin from the corner of the Paper to the corner
* of the imageable area.
*/
g2d.translate(format.getImageableX(), format.getImageableY());
/* Set the text color.
*/
g2d.setPaint(Color.black);
/* Use a LineBreakMeasurer instance to break our text into
* lines that fit the imageable area of the page.
*/
Point2D.Float pen = new Point2D.Float();
AttributedCharacterIterator charIterator = mStyledText.getIterator();
LineBreakMeasurer measurer = new LineBreakMeasurer(charIterator, g2d.getFontRenderContext());
float wrappingWidth = (float) format.getImageableWidth();
while (measurer.getPosition() < charIterator.getEndIndex()) {
TextLayout layout = measurer.nextLayout(wrappingWidth);
pen.y += layout.getAscent();
float dx = layout.isLeftToRight() ? 0 : (wrappingWidth - layout.getAdvance());
layout.draw(g2d, pen.x + dx, pen.y);
pen.y += layout.getDescent() + layout.getLeading();
}
return Printable.PAGE_EXISTS;
}
}
完整的堆栈跟踪：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at java.text.AttributedString.<init>(AttributedString.java:127)
at PrintText.print(PrintText.java:109)
at sun.print.RasterPrinterJob.printPage(RasterPrinterJob.java:1973)
at sun.print.RasterPrinterJob.print(RasterPrinterJob.java:1461)
at sun.print.RasterPrinterJob.print(RasterPrinterJob.java:1277)
at PrintText.printer(PrintText.java:97)
at PrintText$1.run(PrintText.java:51)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:721)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:682)
at java.awt.EventQueue$3.run(EventQueue.java:680)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:691)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)