Java Fastest way to read through text file with 2 million lines

Question

Currently I am using scanner/filereader and using while hasnextline. I think this method is not highly efficient. Is there any other method to read file with the similar functionality of this?

public void Read(String file) {         Scanner sc = null;           try {             sc = new Scanner(new FileReader(file));              while (sc.hasNextLine()) {                 String text = sc.nextLine();                 String[] file_Array = text.split(" ", 3);                  if (file_Array[0].equalsIgnoreCase("case")) {                     //do something                 } else if (file_Array[0].equalsIgnoreCase("object")) {                     //do something                 } else if (file_Array[0].equalsIgnoreCase("classes")) {                     //do something                 } else if (file_Array[0].equalsIgnoreCase("function")) {                     //do something                 }                  else if (file_Array[0].equalsIgnoreCase("ignore")) {                     //do something                 }                 else if (file_Array[0].equalsIgnoreCase("display")) {                     //do something                 }             }          } catch (FileNotFoundException e) {             System.out.println("Input file " + file + " not found");             System.exit(1);         } finally {             sc.close();         }     } 

Answer 1

You will find that BufferedReader.readLine() is as fast as you need: you can read millions of lines a second with it. It is more probable that your string splitting and handling is causing whatever performance problems you are encountering.

Answer 2

I made a gist comparing different methods:

import java.io.*; import java.nio.file.Files; import java.nio.file.Paths; import java.util.ArrayList; import java.util.LinkedList; import java.util.List; import java.util.Scanner; import java.util.function.Function;  public class Main {      public static void main(String[] args) {          String path = "resources/testfile.txt";         measureTime("BufferedReader.readLine() into LinkedList", Main::bufferReaderToLinkedList, path);         measureTime("BufferedReader.readLine() into ArrayList", Main::bufferReaderToArrayList, path);         measureTime("Files.readAllLines()", Main::readAllLines, path);         measureTime("Scanner.nextLine() into ArrayList", Main::scannerArrayList, path);         measureTime("Scanner.nextLine() into LinkedList", Main::scannerLinkedList, path);         measureTime("RandomAccessFile.readLine() into ArrayList", Main::randomAccessFileArrayList, path);         measureTime("RandomAccessFile.readLine() into LinkedList", Main::randomAccessFileLinkedList, path);         System.out.println("-----------------------------------------------------------");     }      private static void measureTime(String name, Function<String, List<String>> fn, String path) {         System.out.println("-----------------------------------------------------------");         System.out.println("run: " + name);         long startTime = System.nanoTime();         List<String> l = fn.apply(path);         long estimatedTime = System.nanoTime() - startTime;         System.out.println("lines: " + l.size());         System.out.println("estimatedTime: " + estimatedTime / 1_000_000_000.);     }      private static List<String> bufferReaderToLinkedList(String path) {         return bufferReaderToList(path, new LinkedList<>());     }      private static List<String> bufferReaderToArrayList(String path) {         return bufferReaderToList(path, new ArrayList<>());     }      private static List<String> bufferReaderToList(String path, List<String> list) {         try {             final BufferedReader in = new BufferedReader(                 new InputStreamReader(new FileInputStream(path), StandardCharsets.UTF_8));             String line;             while ((line = in.readLine()) != null) {                 list.add(line);             }             in.close();         } catch (final IOException e) {             e.printStackTrace();         }         return list;     }      private static List<String> readAllLines(String path) {         try {             return Files.readAllLines(Paths.get(path));         } catch (IOException e) {             e.printStackTrace();         }         return null;     }      private static List<String> randomAccessFileLinkedList(String path) {         return randomAccessFile(path, new LinkedList<>());     }      private static List<String> randomAccessFileArrayList(String path) {         return randomAccessFile(path, new ArrayList<>());     }      private static List<String> randomAccessFile(String path, List<String> list) {         try {             RandomAccessFile file = new RandomAccessFile(path, "r");             String str;             while ((str = file.readLine()) != null) {                 list.add(str);             }             file.close();         } catch (IOException e) {             e.printStackTrace();         }         return list;     }      private static List<String> scannerLinkedList(String path) {         return scanner(path, new LinkedList<>());     }      private static List<String> scannerArrayList(String path) {         return scanner(path, new ArrayList<>());     }      private static List<String> scanner(String path, List<String> list) {         try {             Scanner scanner = new Scanner(new File(path));             while (scanner.hasNextLine()) {                 list.add(scanner.nextLine());             }             scanner.close();         } catch (FileNotFoundException e) {             e.printStackTrace();         }         return list;     }   } 

run: BufferedReader.readLine() into LinkedList, lines: 1000000, estimatedTime: 0.105118655

run: BufferedReader.readLine() into ArrayList, lines: 1000000, estimatedTime: 0.072696934

run: Files.readAllLines(), lines: 1000000, estimatedTime: 0.087753316

run: Scanner.nextLine() into ArrayList, lines: 1000000, estimatedTime: 0.743121734

run: Scanner.nextLine() into LinkedList, lines: 1000000, estimatedTime: 0.867049885

run: RandomAccessFile.readLine() into ArrayList, lines: 1000000, estimatedTime: 11.413323046

run: RandomAccessFile.readLine() into LinkedList, lines: 1000000, estimatedTime: 11.423862897

BufferedReader is the fastest, Files.readAllLines() is also acceptable, Scanner is slow due to regex, RandomAccessFile is inacceptable