【思路】将文件中的内容按行读取存入一个字符串中，在输出时再为每一行加上行号。

1 import java.io.*; 2 public class Text { 3     private String strFinal = ""; 4     public void open(String fileName) { 5         try { 6             BufferedReader in = new BufferedReader(new FileReader(fileName)); 7             String s = null; 8             while ((s = in.readLine()) != null) { 9                 strFinal = strFinal + s + "\n";10             }11             in.close();12         } catch (IOException e) {13             System.out.println(e);14         }15     }16     public void save(String fileName){17         try{18             BufferedReader in = new BufferedReader(new StringReader(strFinal));19             PrintWriter out = new PrintWriter(new FileWriter(fileName));20             int lineCount = 1;21             String s = null;22             while((s = in.readLine())!=null){23                 out.println(lineCount+++": "+s);24             }25             in.close();26             out.close();27         }catch(IOException e){28             System.out.print(e);29         }30     }31     public static void main(String args[])throws IOException{32         Text obj = new Text();33         obj.open("D:/Java_workspace/Text/src/Text.java");34         obj.save("E:\\Example\\A.txt");35     }36 }