How can I read a large text file line by line using Java?

Question

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

How can I do this quickly?

Answer 1

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. } 

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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.

Answer 2

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();