我正在嘗試編寫(xiě)一個(gè) Java 程序來(lái)計(jì)算 PDF 文件的頁(yè)數(shù)。但是當(dāng)我運(yùn)行這個(gè)程序時(shí)，我得到一個(gè)錯(cuò)誤，我不知道為什么。這是錯(cuò)誤：Exception in thread "main" java.lang.NullPointerException at pdfpagecount.Pdfpagecount.main(Pdfpagecount.java:12)這是產(chǎn)生錯(cuò)誤的代碼：package pdfpagecount;import java.io.File;import java.io.FileInputStream;import com.lowagie.text.pdf.PdfReader;public class Pdfpagecount {public static void main(String[] args) {    File gopi = new File("C:\\Users\\Gopinath Muruti\\Desktop\\test.pdf");    File listOfFile[] = gopi.listFiles();    for(int i = 0; i < listOfFile.length; i++) {        File tempFile = listOfFile[i];        String fileName = tempFile.getName();        System.out.println("File Name = " + fileName);        try {            if(fileName.toLowerCase().indexOf(".pdf") != -1) {                PdfReader document = new PdfReader(new FileInputStream(new File("filename")));                int noPages = document.getNumberOfPages();                System.out.println("Number of Pages in the PDF document is = " + noPages);            }        }        catch(Exception e) {            System.out.println("Exception : " + e.getMessage());            e.printStackTrace();        }    }}}