File Handling with java.nio.file

Programs need to keep data after they stop: reports, logs, exports, configuration. Modern Java handles files with the java.nio.file package – Path for locations and Files for operations. It is shorter and safer than the old FileReader/FileWriter code you will still see in older tutorials.

Write and read a 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, "Day 1: installed the JDK\nDay 2: variables\n");     // create / overwrite
        Files.writeString(file, "Day 3: loops\n", 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, first: " + lines.get(0));
        System.out.println(Files.exists(file) + " " + Files.size(file) + " bytes");
    }
}

Day 1: installed the JDK
Day 2: variables
Day 3: loops
3 lines, first: Day 1: installed the JDK
true 55 bytes

• File operations throw IOException (a checked exception), so methods must catch or declare it. Here main declares throws IOException to keep the example short.
• writeString and readString use UTF-8 by default – ₹, é and Indian scripts work correctly.
• A relative path like "notes.txt" is resolved against the folder you ran the program from.

Large files: read line by line

readAllLines loads the entire file into memory. For big logs, stream the lines so only a little is in memory at once:

import java.io.BufferedReader;
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 log = Path.of("server.log");
        Files.writeString(log, "INFO start\nERROR db timeout\nINFO login asha\nERROR payment failed\nWARN disk 85%\n");

        try (Stream<String> lines = Files.lines(log)) {               // closed automatically
            System.out.println(lines.filter(l -> l.startsWith("ERROR")).count() + " errors");
        }

        try (BufferedReader reader = Files.newBufferedReader(log)) {   // classic loop style
            String line;
            int n = 0;
            while ((line = reader.readLine()) != null) {
                n++;
                if (line.startsWith("WARN")) System.out.println("line " + n + ": " + line);
            }
        }
    }
}

2 errors
line 5: WARN disk 85%

Writing many lines

import java.io.BufferedWriter;
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 {
        List<String> tasks = List.of("buy milk", "call bank", "finish chapter 14");
        Path todo = Path.of("todo.txt");
        Files.write(todo, tasks);                                  // one element per line

        try (BufferedWriter w = Files.newBufferedWriter(Path.of("report.txt"))) {
            for (int i = 0; i < tasks.size(); i++) {
                w.write((i + 1) + ". " + tasks.get(i));
                w.newLine();
            }
        }
        System.out.print(Files.readString(Path.of("report.txt")));
    }
}

1. buy milk
2. call bank
3. finish chapter 14

Directories and file operations

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

public class Main {
    public static void main(String[] args) throws IOException {
        Path dir = Path.of("exports", "2026");
        Files.createDirectories(dir);                         // creates parents too
        Path a = dir.resolve("sales.csv");                    // exports/2026/sales.csv
        Files.writeString(a, "id,amount\n1,500\n");
        Path b = dir.resolve("sales-backup.csv");
        Files.copy(a, b, StandardCopyOption.REPLACE_EXISTING);

        System.out.println(a.getFileName() + " | parent: " + a.getParent());
        try (Stream<Path> files = Files.list(dir)) {
            files.map(p -> p.getFileName().toString()).sorted().forEach(System.out::println);
        }
        Files.delete(b);
        System.out.println(Files.exists(b));
    }
}

sales.csv | parent: exports/2026
sales-backup.csv
sales.csv
false

On Windows the parent prints as exports\2026; Path uses the right separator for each system, so never hard-code "/" or "\\" when building paths – use Path.of(a, b) or resolve.

A real task: summarise a CSV

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

public class Main {
    public static void main(String[] args) throws IOException {
        Path csv = Path.of("sales.csv");
        Files.writeString(csv, """
            region,rep,amount
            North,Asha,1200
            South,Ben,800
            North,Chen,450
            South,Dia,1500
            East,Eli,not-a-number
            """);

        Map<String, Integer> totals = new TreeMap<>();
        List<String> lines = Files.readAllLines(csv);
        for (String line : lines.subList(1, lines.size())) {      // skip the header
            String[] f = line.split(",");
            try {
                totals.merge(f[0], Integer.parseInt(f[2]), Integer::sum);
            } catch (NumberFormatException e) {
                System.out.println("Skipped bad row: " + line);
            }
        }
        System.out.println(totals);
        System.out.println("Grand total: " + totals.values().stream().mapToInt(Integer::intValue).sum());
    }
}

Skipped bad row: East,Eli,not-a-number
{North=1650, South=2300}
Grand total: 3950

For real CSV files – with quoted commas, for example – use a library such as OpenCSV or Apache Commons CSV. For JSON, Spring Boot uses Jackson.

Practice

Summary