File Handling
Read and write files with java.nio.file.Files, BufferedReader and try-with-resources.
Java has two file APIs. The modern java.nio.file package (Path and Files) is the easiest for most tasks. The older java.io classes (FileReader, BufferedReader, FileWriter) are still common, especially for reading large files line by line.
File operations can fail (missing file, no permission, full disk), so they throw IOException, a checked exception. You must catch it or declare it with throws.
Writing and reading a whole 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, "first line\nsecond line\n");
Files.write(file, List.of("third line"), java.nio.file.StandardOpenOption.APPEND);
String all = Files.readString(file);
System.out.print(all);
List<String> lines = Files.readAllLines(file);
System.out.println(lines.size() + " lines, last: " + lines.get(lines.size() - 1));
System.out.println(Files.exists(file) + " " + Files.size(file) + " bytes");
}
}Output
first line second line third line 3 lines, last: third line true 34 bytes
try-with-resources
Readers and writers hold an open file handle that must be closed. A try-with-resources statement closes them automatically at the end of the block, even if an exception is thrown.
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
public class Main {
public static void main(String[] args) {
try (BufferedWriter out = new BufferedWriter(new FileWriter("scores.csv"))) {
out.write("Asha,91\n");
out.write("Ravi,72\n");
out.write("Meera,85\n");
} catch (IOException e) {
System.out.println("Could not write: " + e.getMessage());
}
int total = 0, count = 0;
try (BufferedReader in = new BufferedReader(new FileReader("scores.csv"))) {
String line;
while ((line = in.readLine()) != null) {
String[] parts = line.split(",");
System.out.println(parts[0] + " scored " + parts[1]);
total += Integer.parseInt(parts[1]);
count++;
}
} catch (IOException e) {
System.out.println("Could not read: " + e.getMessage());
}
System.out.println("Average: " + (double) total / count);
}
}Output
Asha scored 91 Ravi scored 72 Meera scored 85 Average: 82.66666666666667
Handling a missing file
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.NoSuchFileException;
import java.nio.file.Path;
public class Main {
public static void main(String[] args) {
try {
String text = Files.readString(Path.of("missing.txt"));
System.out.println(text);
} catch (NoSuchFileException e) {
System.out.println("File not found: " + e.getMessage());
} catch (IOException e) {
System.out.println("Other I/O problem: " + e.getMessage());
}
}
}Output
File not found: missing.txt
Working with folders
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.stream.Stream;
public class Main {
public static void main(String[] args) throws IOException {
Path dir = Path.of("reports");
Files.createDirectories(dir);
Files.writeString(dir.resolve("jan.txt"), "January");
Files.writeString(dir.resolve("feb.txt"), "February");
try (Stream<Path> files = Files.list(dir)) {
files.map(p -> p.getFileName().toString()).sorted().forEach(System.out::println);
}
System.out.println(dir.resolve("jan.txt"));
}
}Output
feb.txt jan.txt reports/jan.txt