How to Read a File in Reverse Order (From End to Start) in Java?

How to Read File from End to Start (in Reverse Order) in Java?

Reading a file in reverse order, from the end to the start, can be a useful operation in various situations. Here's a breakdown of how it can be achieved using a BufferReader:

The Problem:
You have a file and want to read it in a reverse line order.

The Solution:
To accomplish this task, we can utilize the ReverseLineInputStream class, which allows us to read lines from a file in the reverse order without loading the entire file into memory. The advantage of this method is that it can handle large files efficiently.

The ReverseLineInputStream class maintains pointers to the file and uses the standard InputStream methods. Its core functionality involves:

1. FindPrevLine Method:

   • Determines the starting point of the previous line by identifying the last newline character (LF).
   • Adjusts the currentLineStart and currentLineEnd pointers accordingly.

2. Read Method:

   • Checks if there are more characters to read in the current line.
   • Advances the file pointer and returns the character.
   • If there are no more characters in the current line, it calls findPrevLine to move to the previous line and continues reading from there.
   • Returns -1 when there are no more lines to read.

Here's an example of how to use the ReverseLineInputStream:

<code class="java">File file = new File("/var/nagios.log");
ReverseLineInputStream rlis = new ReverseLineInputStream(file);
BufferedReader br = new BufferedReader(new InputStreamReader(rlis));

while (true) {
    String line = br.readLine();
    if (line == null) {
        break;
    }
    System.out.println(line);
}</code>

This code will read the contents of the /var/nagios.log file in reverse order, line by line.

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