Java 8 Stream – Read a file line by line
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
Hi Mk, what would you recommend to process a large file, let say 50 gb, to for example, delete duplicate lines?
You have used Files.lines. Shouldn’t the stream be closed after Files.lines?
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!
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);
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 😀