In Java 8, you can use Files.lines to read file as Stream.
c://lines.txt – A simple text file for testing
line1
line2
line3
line4
line5
1. Java 8 Read File + Stream
TestReadFile.java
package com.mkyong.java8;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.stream.Stream;
public class TestReadFile {
public static void main(String args[]) {
String fileName = "c://lines.txt";
//read file into stream, try-with-resources
try (Stream<String> stream = Files.lines(Paths.get(fileName))) {
stream.forEach(System.out::println);
} catch (IOException e) {
e.printStackTrace();
}
}
}
Output
line1
line2
line3
line4
line5
2. Java 8 Read File + Stream + Extra
This example shows you how to use Stream to filter content, convert the entire content to upper case and return it as a List.
TestReadFile2.java
package com.mkyong.java8;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class TestReadFile2 {
public static void main(String args[]) {
String fileName = "c://lines.txt";
List<String> list = new ArrayList<>();
try (Stream<String> stream = Files.lines(Paths.get(fileName))) {
//1. filter line 3
//2. convert all content to upper case
//3. convert it into a List
list = stream
.filter(line -> !line.startsWith("line3"))
.map(String::toUpperCase)
.collect(Collectors.toList());
} catch (IOException e) {
e.printStackTrace();
}
list.forEach(System.out::println);
}
}
Output
LINE1
LINE2
LINE4
LINE5
3. BufferedReader + Stream
A new method lines() has been added since 1.8, it lets BufferedReader returns content as Stream.
TestReadFile3.java
package com.mkyong.java8;
import java.io.BufferedReader;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
public class TestReadFile3{
public static void main(String args[]) {
String fileName = "c://lines.txt";
List<String> list = new ArrayList<>();
try (BufferedReader br = Files.newBufferedReader(Paths.get(fileName))) {
//br returns as stream and convert it into a List
list = br.lines().collect(Collectors.toList());
} catch (IOException e) {
e.printStackTrace();
}
list.forEach(System.out::println);
}
}
Output
line1
line2
line3
line4
line5
4. Classic BufferedReader And Scanner
Enough of Java 8 and Stream, let revisit the classic BufferedReader (JDK1.1) and Scanner (JDK1.5) examples to read a file line by line, it is working still, just developers are moving toward Stream.
4.1 BufferedReader + try-with-resources example.
TestReadFile4.java
package com.mkyong.core;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
public class TestReadFile4{
public static void main(String args[]) {
String fileName = "c://lines.txt";
try (BufferedReader br = new BufferedReader(new FileReader(fileName))) {
String line;
while ((line = br.readLine()) != null) {
System.out.println(line);
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
4.2 Scanner + try-with-resources example.
TestReadFile5.java
package com.mkyong.core;
import java.io.File;
import java.io.IOException;
import java.util.Scanner;
public class TestReadFile5 {
public static void main(String args[]) {
String fileName = "c://lines.txt";
try (Scanner scanner = new Scanner(new File(fileName))) {
while (scanner.hasNext()){
System.out.println(scanner.nextLine());
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
Which one is the fastest way to read and print file among these all ways?
I had the same query
Hello @mykong,
Thank you for your examples. I would like to parse a YAML file to object without using jakson or known librairies. Only Java8.
Could you help?
Thank you.
Nawel
Hi MK,
¿how can we read a txt file in a deployed webapp? ¿Do we have to create a folder in the src>resources>static folder ?
Thanks
Thanks!
You have used Files.lines. Shouldn’t the stream be closed after Files.lines?
Hi MK,
Is there any way through which we can read row record on the basis of value. For example my csv file is :-
ProductID,ProductName,price,availability,type
12345,Anaox,300,yes,medicine
23456,Chekmeter,400,yes,testing
i want to get the row whose ProductID is ‘23456’. i was checking the new CsvReader(“D:\roche-poc.csv”).getRawRecord() method. but it doesn’t have any method parameters.
Thanks
Supplier<Stream> supl = () -> {
try {
return Files.lines(Paths.get(“Products.csv”));
} catch (IOException e) {
e.printStackTrace();
}
return Stream.empty();
};
List header = Arrays.asList(supl.get().findFirst().get().split(“,”));
String r = supl.get().filter(l -> l.split(“,”)[header.indexOf(“ProductID”)].equals(“12345”)).findAny().orElse(“”);
System.out.println(r);
Hi Mk, what would you recommend to process a large file, let say 50 gb, to for example, delete duplicate lines?
Are streams faster than scanner, at this issue? I ve benchmarked a simple text file to time scanner vs lines.collect and the scanner is always faster by far
hi MK,
i have a file with data in dual lines
eg:
line1:hello, how are
you, ?
line2: hi, how are you?
how do i read line1, as one line, rather than 2 lines?
much appreciate your response for this
-Cal
string tokenizer may do the trick for ya
hi MK , you have a spike with lambda ?
thx
??? Sorry, don’t get you.
he might be saying Do lambda expressions have Spikes on their heasd or not 😀