Learning Java - Input/Output & File Management

#Introduction

Almost every application interacts with files: reading configuration, writing logs, importing data, or exporting reports. Episode 8 covers input/output and file management in Java — from the classic java.io API to the modern java.nio. You will learn to read and write text, binary, and CSV files, and apply best practices for resource management.

#Java I/O Basics: java.io and java.nio

java.io is the classic stream-based API: FileInputStream, FileOutputStream, FileReader, and FileWriter. java.nio (introduced in Java 7) offers a more modern and flexible API: Path, Files, channels, and buffers. For everyday needs, java.nio.file.Files is a concise entry point.

#Reading a File with Files

Read an entire text file in a single line of code:

import java.nio.file.*;
 
public class BacaTeks {
    public static void main(String[] args) throws IOException {
        String isi = Files.readString(Path.of("catatan.txt"));
        System.out.println(isi);
    }
}

Files.readString(Path.of("catatan.txt")) reads the entire file into a single string. For large files, use readAllLines or a stream.

#Reading and Writing Text Files

#Writing Text Files

Use Files.writeString to write text, or Files.write for a list of lines:

import java.nio.file.*;
import java.util.List;
 
public class TulisTeks {
    public static void main(String[] args) throws IOException {
        Files.writeString(Path.of("output.txt"), "Halo Java");
        Files.write(Path.of("baris.txt"), List.of("Baris 1", "Baris 2"));
    }
}

#Reading Line by Line with BufferedReader

To process large files efficiently, read line by line:

import java.io.*;
import java.nio.file.*;
 
public class BacaBaris {
    public static void main(String[] args) throws IOException {
        try (BufferedReader reader = Files.newBufferedReader(Path.of("log.txt"))) {
            String baris;
            while ((baris = reader.readLine()) != null) {
                System.out.println(baris);
            }
        }
    }
}

#Reading and Writing Binary Files

Binary files such as images, audio, and archives cannot be read as text. Use byte streams with Files.copy to copy, or Files.readAllBytes to process:

import java.nio.file.*;
 
public class SalinBinary {
    public static void main(String[] args) throws IOException {
        Path sumber = Path.of("gambar.png");
        Path tujuan = Path.of("gambar-copy.png");
        Files.copy(sumber, tujuan, StandardCopyOption.REPLACE_EXISTING);
    }
}

Files.copy with the REPLACE_EXISTING option copies a binary file and overwrites the destination if it already exists.

#Reading and Writing CSV Files

CSV (Comma-Separated Values) is a text format with values separated by commas. Read line by line and split with split:

import java.io.*;
import java.nio.file.*;
 
public class BacaCsv {
    public static void main(String[] args) throws IOException {
        try (BufferedReader reader = Files.newBufferedReader(Path.of("produk.csv"))) {
            String baris;
            while ((baris = reader.readLine()) != null) {
                String[] kolom = baris.split(",");
                System.out.println(kolom[0] + " - " + kolom[1]);
            }
        }
    }
}

#Writing CSV

Writing CSV is as simple as composing a string per line:

List<String> baris = List.of("1,Java,50000", "2,Python,45000");
Files.write(Path.of("produk.csv"), baris);

For complex CSV, consider a library such as OpenCSV.

#Path API, Files Utility, and Working Directory

#Path and Files Operations

java.nio.file provides various file operations:

import java.nio.file.*;
 
public class OperasiFile {
    public static void main(String[] args) throws IOException {
        Path dir = Path.of("backup");
        Files.createDirectories(dir);
        Files.move(Path.of("a.txt"), Path.of("b.txt"));
        Files.deleteIfExists(Path.of("sementara.txt"));
    }
}

#Working Directory

The working directory is the directory where a process is run — the starting point for relative paths:

pwd

Relative paths such as Path.of("data.txt") are always interpreted relative to the working directory.

#Best Practices for Resource Management

The golden rule: every resource that is opened must be closed. Try-with-resources handles this automatically and is safe against exceptions:

try (BufferedReader reader = Files.newBufferedReader(Path.of("input.txt"));
     BufferedWriter writer = Files.newBufferedWriter(Path.of("output.txt"))) {
    String baris;
    while ((baris = reader.readLine()) != null) {
        writer.write(baris.toUpperCase());
        writer.newLine();
    }
}

#Closing

Episode 8 teaches I/O and file management: distinguishing java.io and java.nio, reading and writing text, binary, and CSV files, using the Path API and Files utility, understanding the working directory, and applying try-with-resources for resource management.

Key takeaways:

• java.nio.file.Files is a concise entry point for modern I/O.
• Text files are read with readString or readAllLines.
• Binary files use Files.copy or readAllBytes.
• CSV is read line by line and split with split(",").
• Relative paths are interpreted against the working directory.
• Always use try-with-resources to close resources automatically.

In the next episode, episode 9, we will discuss data binding and JSON processing — the process of JSON serialization and deserialization in Java, the popular libraries Jackson, Gson, and JSON-B, mapping Java objects to JSON and back, and validating input data plus error handling while parsing. Time to talk to the outside world!