Java 8 Stream – Read a file line by line

野性酷女 2022-06-04 07:19 148阅读 0赞

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(); } } }

## References ##

1.  [Java 8 File.lines()][Java 8 File.lines]
2.  [Java 8 Stream][]
3.  [Java BufferedReader][]

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

[Java 8 File.lines]: http://docs.oracle.com/javase/8/docs/api/java/nio/file/Files.html#lines-java.nio.file.Path-
[Java 8 Stream]: https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html
[Java BufferedReader]: https://docs.oracle.com/javase/8/docs/api/java/io/BufferedReader.html