程序员的小窝

I need to read a large text file of around 5-6 GB line by line using Java.

How can I do this quickly?

A common pattern is to use

try (BufferedReader br = new BufferedReader(new FileReader(file))) {
    String line;
    while ((line = br.readLine()) != null) {
       // process the line.
    }
}

You can read the data faster if you assume there is no character encoding. e.g. ASCII-7 but it won't make much difference. It is highly likely that what you do with the data will take much longer.

EDIT: A less common pattern to use which avoids the scope of line leaking.

try(BufferedReader br = new BufferedReader(new FileReader(file))) {
    for(String line; (line = br.readLine()) != null; ) {
        // process the line.
    }
    // line is not visible here.
}

UPDATE: In Java 8 you can do

try (Stream<String> stream = Files.lines(Paths.get(fileName))) {
        stream.forEach(System.out::println);
}

NOTE: You have to place the Stream in a try-with-resource block to ensure the #close method is called on it, otherwise the underlying file handle is never closed until GC does it much later.

Look at this blog:

• Java Read File Line by Line - Java Tutorial

The buffer size may be specified, or the default size may be used. The default is large enough for most purposes.

// Open the file
FileInputStream fstream = new FileInputStream("textfile.txt");
BufferedReader br = new BufferedReader(new InputStreamReader(fstream));

String strLine;

//Read File Line By Line
while ((strLine = br.readLine()) != null)   {
  // Print the content on the console
  System.out.println (strLine);
}

//Close the input stream
fstream.close();