如何使用Java从最后一行读取文件到第一行

Question

    FileInputStream fstream = new FileInputStream("\\file path");
    DataInputStream in = new DataInputStream(fstream);
    BufferedReader br = new BufferedReader(new InputStreamReader(in));
     while (br.ready()) {

                                line = br.readLine();
    }

Please let me know how to read a file from the last line to first provided the row number is not fixed and is varying with time? I know the above is useful for reading it from first row...

Answer 1

read the file into a list, and process that list backwards... files and streams are usually designed to work forward; so doing this directly with streams might turn out a lite awkward. Only advised when the files are really huge...

Answer 2

You cannot read a Buffer backwards, you can however count the lines of your buffer as explained in the link below

http://www.java2s.com/Code/Java/File-Input-Output/Countthenumberoflinesinthebuffer.htm

And afterwards select your line using this code:

FileInputStream fs= new FileInputStream("someFile.txt");
BufferedReader br = new BufferedReader(new InputStreamReader(fs));
for(int i = 0; i < 30; ++i)
  br.readLine();
String lineIWant = br.readLine();

As you can see, you iterate, reading each line(and doing nothing) before you get to the one you want (here we got 31 lines passed and #32 is the one read). If your file is huge this will take a lot of time.

Other way to to this is to input everything in a List and then with a sizeof() and a for() you can select everything you want.

Answer 3

If you know the length of each line then you can work out how many lines there are by looking at the size of the file and dividing by the length of each line. (this of course ignores any possible metadata in the file)

You can then use some maths to get the start byte of the last line. Once you have then you can then open a RandomAccessFile on the file and then use seek to go to that point. Then using readline you can then read the last line

This does assume though that the lines are all the same length.

Answer 4

You can use FileUtils

and use this method

static List<String> readLines(File file) 
          Reads the contents of a file line by line to a 
          List of Strings using the default encoding for the VM.

This will return a List then use Collections.reverse()

Then simply iterate it to get the file lines in reverse order

Answer 5

这可能对您有帮助[1]： http : //mattfleming.com/node/11

Answer 6

只需向后保存信息，这就是我所做的全部。只需阅读Pryor即可保存并使用\\ n

Answer 7

You can save the lines in a list (in my code a arraylist) and "read" the lines backwards from the arraylist:

try
    {

        FileInputStream fstream = new FileInputStream("\\file path");
        DataInputStream in = new DataInputStream(fstream);
        BufferedReader br = new BufferedReader(new InputStreamReader(in));
        String line = "";
        ArrayList<String> lines = new ArrayList<String>();

        //Read lines and save in ArrayList 
        while (br.ready())
        {
            lines.add(br.readLine());
        }

        //Go backwards through the ArrayList
        for (int i = lines.size(); i >= 0; i--)
        {
            line = lines.get(i);
        }
    }
    catch (Exception e)
    {
        e.printStackTrace();
    }