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.
f9f9
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();
}

}

}

References

Java 8 File.lines()
Java 8 Stream
Java BufferedReader

link: https://www.mkyong.com/java8/java-8-stream-read-a-file-line-by-line/