File Handling: File, FileReader, FileWriter, BufferedReader

Java's I/O classes let a program interact with the file system: checking whether files exist, creating or deleting them, and reading or writing their content. The classes are layered — some represent the file itself as metadata, while others actually stream bytes or characters into and out of that file.

Why it matters

Saving data to a file and reading it back is among the first practical things a program needs: exporting a report, keeping a log, importing a list. This lesson is the complete basic cycle of create, write, read and delete, and the try-with-resources habit it teaches applies to every resource you will ever open.

The File Class — Metadata, Not Content

java.io.File represents a path on the file system and lets you query or manipulate it without reading its contents: exists() checks if the path is present, mkdir()/mkdirs() create a directory (or the full directory chain), createNewFile() creates an empty file, and delete() removes a file or empty directory. A File object can represent a path that does not exist yet — creating the object does not touch the disk.

FileReader and FileWriter — Character Streams

FileReader and FileWriter are character streams: they read and write text one character (or char buffer) at a time, handling the platform's default character encoding. They are low-level and inefficient for line-based work on their own — every call to read() can involve a system call — which is why they are almost always wrapped by a buffering class.

BufferedReader — Efficient Line Reading

Wrapping a FileReader in a BufferedReader adds an internal buffer, dramatically reducing the number of actual disk reads, and adds the convenient readLine() method, which returns one line of text at a time and returns null when the end of the file is reached — the standard pattern for reading a text file line by line.

try-with-resources for Automatic Closing

Every stream class in java.io implements Closeable, and forgetting to close a stream leaks file handles. The try-with-resources statement, try (Resource r = ...) { ... }, automatically calls close() on each declared resource when the block exits, whether normally or via an exception, which is far safer than a manual finally block.

Example

import java.io.File;
import java.io.FileWriter;
import java.io.FileReader;
import java.io.BufferedReader;
import java.io.IOException;

public class FileHandlingDemo {
    public static void main(String[] args) throws IOException {
        File file = new File("notes.txt");
        System.out.println("Exists before write: " + file.exists());

        try (FileWriter writer = new FileWriter(file)) {
            writer.write("Line one\n");
            writer.write("Line two\n");
        }

        System.out.println("Exists after write: " + file.exists());

        try (BufferedReader reader = new BufferedReader(new FileReader(file))) {
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println("Read: " + line);
            }
        }

        file.delete();
        System.out.println("Exists after delete: " + file.exists());
    }
}

Exists before write: false
Exists after write: true
Read: Line one
Read: Line two
Exists after delete: false

Exercise

Write two lines to a temporary file, read the file back line by line printing each line, and then print how many lines were read. Delete the file at the end.

Expected output: first, second, 2 (one per line)

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

public class WriteThenRead {
    public static void main(String[] args) throws IOException {
        File file = File.createTempFile("notes", ".txt");

        // TODO: write "first" and "second" on separate lines
        // TODO: read the file line by line, printing each line and counting them
        // TODO: print the count

        file.delete();
    }
}

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

public class WriteThenRead {
    public static void main(String[] args) throws IOException {
        File file = File.createTempFile("notes", ".txt");

        try (BufferedWriter writer = new BufferedWriter(new FileWriter(file))) {
            writer.write("first");
            writer.newLine();
            writer.write("second");
            writer.newLine();
        }

        int count = 0;
        try (BufferedReader reader = new BufferedReader(new FileReader(file))) {
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line); // first, second
                count++;
            }
        }
        System.out.println(count); // 2

        file.delete();
    }
}

Quick quiz

Interview questions