How to swap position for two lines in a file Java accessing directly to the lines

Question

I need to swap the position of two lines in a file accessing directly to them.

All the lines in my file have the same size in bytes, I know where every line is since they are all the same size, but I need to point to them directly without going through all the file.

So I need to know how to position myself there and how to read and delete them, and I really couldn't find a solution I understand.

Thanks in advance.

Example: I want to swap the second and fourth line.

File content:

 1;first line                         ; 1
 2;second line                        ; 1
 3;third                              ; 2
 4;fourth                             ; 2
 5;fifth                              ; 2

How it should look:

 1;first line                         ; 1
 4;fourth                             ; 2   
 3;third                              ; 2
 2;second line                        ; 1
 5;fifth                              ; 2

Have you looked at [LineNumberReader](https://docs.oracle.com/javase/7/docs/api/java/io/LineNumberReader.html#mark(int))? — Joakim Danielson, Mar 06 '19 at 19:37
If you know the size of each line, there are many ways to do it. Can you show us what have you tried so far? — MS90, Mar 06 '19 at 19:42

score 0 · Accepted Answer · answered Mar 06 '19 at 19:49

Purely educational example. Don't use something like that in production. Use a library instead. Anyway, follow my comments.

File example

ciaocio=1
edoardo=2
lolloee=3

Target

ciaocio=1
lolloee=3
edoardo=2

final int lineSeparatorLength = System.getProperty("line.separator").getBytes().length;

// 9 = line length in bytes without separator
final int lineLength = 9 + lineSeparatorLength;

try (final RandomAccessFile raf = new RandomAccessFile(file, "rw")) {
    // Position the cursor at the beginning of the first line to swap
    raf.seek(lineLength);

    // Read the first line to swap
    final byte[] firstLine = new byte[lineLength];
    raf.read(firstLine);

    // Position the cursor at the beginning of the second line
    raf.seek(lineLength * 2);

    // Read second line
    final byte[] secondLine = new byte[lineLength];
    raf.read(secondLine);

    // Move the cursor back to the first line
    // and override with the second line
    raf.seek(lineLength);
    raf.write(secondLine);

    // Move the cursor to the second line
    // and override with the first
    raf.seek(lineLength * 2);
    raf.write(firstLine);
}

Thanks!! That's exactly what I was looking for and easy to understand — Carles Fitó Negre, Mar 06 '19 at 19:51

How to swap position for two lines in a file Java accessing directly to the lines

1 Answers1