Java File Handling

Java can create, read, write and delete files. The modern way is the java.nio.file package (Files and Path), which is simple and short.

Write and read a text file

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.List;

public class Main {
    public static void main(String[] args) throws IOException {
        Path file = Path.of("notes.txt");

        Files.writeString(file, "Java is fun.\nFiles are easy.\n");
        System.out.println(Files.exists(file));
        System.out.println(Files.size(file) + " bytes");

        String text = Files.readString(file);
        System.out.println(text);

        List<String> lines = Files.readAllLines(file);
        System.out.println(lines.size() + " lines, first: " + lines.get(0));
    }
}

Append and write many lines

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.StandardOpenOption;
import java.util.List;

public class Append {
    public static void main(String[] args) throws IOException {
        Path file = Path.of("log.txt");
        Files.write(file, List.of("line 1", "line 2"));                         // overwrite
        Files.writeString(file, "line 3\n", StandardOpenOption.APPEND);          // add at the end
        System.out.println(Files.readString(file));
    }
}

Read a big file line by line

For big files do not load everything in the memory. Read with a stream or a BufferedReader. Both need try with resources, so the file is closed.

import java.io.BufferedReader;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.List;

public class Big {
    public static void main(String[] args) throws IOException {
        Path file = Path.of("data.txt");
        Files.write(file, List.of("apple", "banana", "cherry", "apricot"));

        try (BufferedReader reader = Files.newBufferedReader(file)) {
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line.toUpperCase());
            }
        }

        try (var lines = Files.lines(file)) {
            System.out.println(lines.filter(l -> l.startsWith("a")).count() + " lines start with a");
        }
    }
}

Copy, move and delete

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.StandardCopyOption;

public class Ops {
    public static void main(String[] args) throws IOException {
        Path a = Path.of("a.txt");
        Files.writeString(a, "hello");

        Path b = Path.of("b.txt");
        Files.copy(a, b, StandardCopyOption.REPLACE_EXISTING);
        Files.move(b, Path.of("c.txt"), StandardCopyOption.REPLACE_EXISTING);
        System.out.println(Files.exists(a) + " " + Files.exists(b) + " " + Files.exists(Path.of("c.txt")));

        Files.delete(a);
        System.out.println(Files.deleteIfExists(a));      // false: already gone
    }
}

Folders

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.stream.Stream;

public class Dirs {
    public static void main(String[] args) throws IOException {
        Path dir = Files.createDirectories(Path.of("project/docs"));
        Files.writeString(dir.resolve("readme.txt"), "Hello");
        Files.writeString(Path.of("project", "main.txt"), "Main");

        try (Stream<Path> paths = Files.walk(Path.of("project"))) {
            paths.sorted().forEach(System.out::println);
        }
        System.out.println(Files.isDirectory(dir));
    }
}

File not found

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.NoSuchFileException;
import java.nio.file.Path;

public class Missing {
    public static void main(String[] args) {
        try {
            Files.readString(Path.of("does-not-exist.txt"));
        } catch (NoSuchFileException e) {
            System.out.println("File not found: " + e.getMessage());
        } catch (IOException e) {
            System.out.println("Other problem: " + e);
        }
    }
}

Old classes you will still see

In older code you will find File, FileReader, FileWriter and Scanner(new File(...)). They still work, but Files and Path are simpler and safer for new code.

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