3 Ways to Read File line by line in Java 8? Examples
Java 8 has added a new method called lines() in the Files class which can be used to read a file line by line in Java. The beauty of this method is that it reads all lines from a file as Stream of String, which is populated lazily as the stream is consumed. So, if you have a huge file and you only read the first 100 lines then the rest of the lines will not be loaded into memory, which results in better performance. This is slightly different from the Files.readAllLines() method (which reads all lines into a List) because this method reads the file lazily, only when a terminal operation is called on Stream like forEach() , count(), etc. By using the count() method you can actually count a number of lines in files or the number of empty lines by filtering empty lines.
In fact, you can do a lot more than just reading content from a file, you can filter them on some criterion e.g. filter lines that do not start with a specific word, filter lines whose length is greater than 100, trim all lines to remove leading and trailing space, convert each line into uppercase or lowercase, etc.
In short, you can use different methods from java.util.stream.Streams class to process lines read from a file before printing them or returning them to the caller. It’s not just lambda expression, which is introduced in Java 8, there are many more goodies like this that are hidden behind the aura of big features like lambdas and streams.
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How to Read File line by line in Java 8? Example Tutorial
In this short example, I have discussed three ways to read a text file line by line in Java 1.8. My first example is about the classical approach of reading files line by line using BufferedReader . You can also use a Scanner in place of BufferedReader if you are using Java 1.5 but you can see that it’s not smooth.
You need to first create a FileReader to read a file, which uses the platform’s default character encoding for converting bytes to characters. Then, you need to wrap that inside BufferedReader to take advantage of the in-memory buffering and readLine() method of BufferedReader class. This method can be used to read files line by line, it returns null if there are no more lines to read.
If you also want to count a total number of lines or want to display line numbers along with each line, you can use a count variable as shown in our first example. You can see, almost 9 to 10 lines of code are required to read a file line by line prior to Java 8.
There are a couple of ways to read a file line by line in Java 8 like by using Files.readAllLines() method, which returns a List of String, which is nothing but lines from File. There are two overloaded versions of this method, one which accepts a character encoding and the other which uses UTF-8 charset.
The only problem with this method is that it’s not lazy like the next method, I am going to show you guys, but it also has an inherent advantage, it ensures that file is closed when all bytes have been read or an I/O error or another runtime exception occurs. You can see this Java 8 tutorial to see this method in action.
A better way to read a text file line by line in Java 8 is by using Files.lines() method which takes advantage of the Stream API introduced in Java 8. This method is lazy and only reads lines when some terminal operation is performed on Stream e.g. when you call forEach() method to display lines from the file or call count() method to calculate a total number of lines from a file.
This method also comes in two overloaded versions, one which accepts a given character encoding and the other which by default uses UTF-8 encoding to convert bytes to character from file. The only disadvantage of this method is that it doesn’t ensure that file is closed once all lines are read.
The returned stream by this method encapsulates a Reader and if you don’t want to rely on the operating system for disposing of file handlers, you make sure to call this method inside try-catch , try-finally, or try-with-resource block to ensure that the close() method is called once stream operation is completed. I have not wrapped the code inside the try-with-resource statement to improve readability but that is a must if you are doing it for production code.
Java 8 Example of Reading File line by line
Here is our sample Java program for reading a text file, actually manifest. mf file from Eclipse’s project directory in Java 8. The first method is a classic way to read a file using BufferedReader, but the rest of the code demonstrates how Java 8 can help you not only read file line by line but also to do filtering, transformation, and many more things with powerful Java 8 Stream API.
If you are wondering about the 3rd empty line from a manifest.mf file then it’s worth remembering that it does contain an empty last line, you can check that by opening manifest.mf file in a text editor like Notepad++ or Wordpad.
import java.io.BufferedReader; import java.io.File; import java.io.FileReader; import java.io.IOException; import java.nio.file.Files; import java.nio.file.Paths; /** * Java Program to demonstrate how to read a file line by line in Java 8 * @author Javin Paul */ public class Java8FileReader < public static void main(String args[]) throws IOException < // reading a file line by line before Java 8 FileReader fr = new FileReader("manifest.mf"); BufferedReader bufr = new BufferedReader(fr); int count = 1; String line = bufr.readLine(); System.out.println("Old way of reading file line by line in Java : "); while(line != null)< System.out.println(count + " : " + line); line = bufr.readLine(); count++; > bufr.close(); // reading file line by line in Java 8 System.out.println("Reading file line by line using Files.lines() in Java 8"); Files.lines(Paths.get("manifest.mf")).forEach(System.out::println); // You can do even better, you can read all lines // trim them and filter out all empty lines // before printing as shown in following example System.out.println("Doing more things than just reading file using Java 8 Streams"); Files.lines(new File("manifest.mf").toPath()) .map(s -> s.trim()) .filter(s -> !s.isEmpty()) .forEach(System.out::println); // You can also filter line using String methods // e.g. print only lines which starts with " System.out.println("Printing lines which startswith ); Files.lines(Paths.get("build.xml")) .map(s -> s.trim()) .filter(s -> s.startsWith(")) .forEach(System.out::println); > > Output Old way of reading file line by line in Java : 1 : Manifest-Version: 1.0 2 : X-COMMENT: Main-Class will be added automatically by build 3 : Reading file line by line using Files.lines() in Java 8 Manifest-Version: 1.0 X-COMMENT: Main-Class will be added automatically by build Doing more thing then just reading file using Java 8 Streams Manifest-Version: 1.0 X-COMMENT: Main-Class will be added automatically by build Printing lines which startswith ? from file ?xml version="1.0" encoding="UTF-8"?>
You can see that in the first example all three lines are printed with line numbers. In the second example also I have printed all lines without any filtering or transformation, but in a subsequent example, I have trimmed each line and filtered out empty lines, that’s why you are seeing only two lines there. In the last example, I have only printed the line which is starting with an opening HTML tag ‘
That’s all about 3 ways to read files line by line in Java 8. There is no need to use BufferedReader or Scanner anymore, you can either use Files.readAllLines() if the file is small and you are not concerned about loading all lines in memory, or better use Files.lines() to read a text file line by line lazily.
This method will only read lines when a terminal operation will be called on Stream like the forEach() method to print lines from a file.
It’s also worth remembering that, Files.readAllLines() uses UTF-8 character encoding and ensures that the file is closed when all bytes are read or an I/O error or runtime exception occurred, but Files.lines() doesn’t provide such guarantee. If you want to timely release resources make sure to call Files.lines() method inside try-with-resource statement.
If you want to learn more about new features in Java 1.8, I suggest you read Java SE 8 for Really Impatient By Cay S. Horstmann, one of the best books for learning Java 8. It also covers some significant enhancements from Java 1.7 release e.g. new File API, try with resource statements, and improved exception handling
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How to read text file line by line in Java
In Java File I/O programming, the classes BufferedReader and LineNumberReader allows reading a sequence of lines from a text file, in a line-by-line fashion, by using their readLine() method. The LineNumberReader is a subclass of the BufferedReader class. The only difference is that, the LineNumberReader class keeps track of the current line number, whereas the BufferedReader class does not.
Reading all lines from a text file using the BufferedReader class is as easy as follows:
String filePath = «Path/To/Your/Text/File.txt»; try < BufferedReader lineReader = new BufferedReader(new FileReader(filePath)); String lineText = null; while ((lineText = lineReader.readLine()) != null) < System.out.println(lineText); >lineReader.close(); > catch (IOException ex)
The above code snippet takes a given text file, reads every line and prints content of each line to the console output. It’s also common to read and put all the lines into a collection (e.g. an array list) for processing later, like this:
BufferedReader lineReader = new BufferedReader(new FileReader(filePath)); String lineText = null; List listLines = new ArrayList(); while ((lineText = lineReader.readLine()) != null) < listLines.add(lineText); >lineReader.close();
Using the LineNumberReader class is similar to the BufferedReader class, for example:
String filePath = «Path/To/Your/Text/File.txt»; try < LineNumberReader lineReader = new LineNumberReader(new FileReader(filePath)); String lineText = null; while ((lineText = lineReader.readLine()) != null) < System.out.println(lineReader.getLineNumber() + ": " + lineText); >lineReader.close(); > catch (IOException ex)
The LineNumberReader class provides the getLineNumber() method which returns the current line number, so we can use it for checking conditions on line numbers, for example:
- Printing only the even lines:
while ((lineText = lineReader.readLine()) != null) < int lineNumber = lineReader.getLineNumber(); if (lineNumber % 2 == 0) < System.out.println(lineNumber + ": " + lineText); >>
while ((lineText = lineReader.readLine()) != null) < int lineNumber = lineReader.getLineNumber(); if (lineNumber >= 50 && lineNumber >
NOTES: The LineNumberReader class also provides the setLineNumber() method but it doesn’t change to the current position in the file stream. It only changes the value that will be returned by the getLineNumber() method.
API References:
Related File IO Tutorials:
Other Java File IO Tutorials:
About the Author:
Nam Ha Minh is certified Java programmer (SCJP and SCWCD). He started programming with Java in the time of Java 1.4 and has been falling in love with Java since then. Make friend with him on Facebook and watch his Java videos you YouTube.