My goal is to read a file (line by line), however, starting at the end of the file (last line) to the beginning (first line).
Could anyone give you a possible idea of doing this?
I was using RandomAcessFile with the intention of using seek to the end of the file and going back, but it did not work very well.
Files#readAllLines returns you a list of all the lines in the file. Then you can use Collections.reverse to reverse the order of this list:
List<String> linhas = Files.readAllLines(Paths.get("foo/bar.txt"));
Collections.reverse(linhas);
You can use List#get to get each line of the file later: linhas.get(0); (which would return the first line) will return the last, and so on ...
Here's the implementation:
/**
* Input stream utilizado para ler um arquivo em ordem reversa
* http://stackoverflow.com/questions/8664705/how-to-read-file-from-end-to-start
* -in-reverse-order-in-java
*
*
*/
class ReverseLineInputStream extends InputStream {
RandomAccessFile in;
long currentLineStart = -1;
long currentLineEnd = -1;
long currentPos = -1;
long lastPosInFile = -1;
public ReverseLineInputStream(File file) throws FileNotFoundException {
in = new RandomAccessFile(file, "r");
currentLineStart = file.length();
currentLineEnd = file.length();
lastPosInFile = file.length() - 1;
currentPos = currentLineEnd;
}
public void findPrevLine() throws IOException {
currentLineEnd = currentLineStart;
// There are no more lines, since we are at the beginning of the file
// and no lines.
if (currentLineEnd == 0) {
currentLineEnd = -1;
currentLineStart = -1;
currentPos = -1;
return;
}
long filePointer = currentLineStart - 1;
while (true) {
filePointer--;
// we are at start of file so this is the first line in the file.
if (filePointer < 0) {
break;
}
in.seek(filePointer);
int readByte = in.readByte();
// We ignore last LF in file. search back to find the previous LF.
if (readByte == 0xA && filePointer != lastPosInFile) {
break;
}
}
// we want to start at pointer +1 so we are after the LF we found or at
// 0 the start of the file.
currentLineStart = filePointer + 1;
currentPos = currentLineStart;
}
public int read() throws IOException {
if (currentPos < currentLineEnd) {
in.seek(currentPos++);
int readByte = in.readByte();
return readByte;
} else if (currentPos < 0) {
return -1;
} else {
findPrevLine();
return read();
}
}
@Override
public void close() throws IOException {
super.close();
in.close();
}
}
Using it:
ReverseLineInputStream reverseLineInputStream = new RverseLineInputStream(file);
InputStreamReader inputStreamReader = new InputStreamReader(reverseLineInputStream);
BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
String readLine = bufferedReader.readLine();