Lesson 4: Writing Files with PrintWriter

1 PrintWriter: Your Magic Pen

The teacher took out a magic pen called PrintWriter.

"This," they said, "lets us write text into a file, just like System.out.print writes text to the screen."

Key Insight

Just as we use System.out.println() to print to the console, we use PrintWriter to "print" to a file. The methods are almost identical!

Writing to Console

System.out.println(
    "Hello!"
);

System.out.print(
    "World"
);
                        

Writing to File

writer.println(
    "Hello!"
);

writer.print(
    "World"
);
                        

2 Creating a PrintWriter

There are several ways to create a PrintWriter:

import java.io.File;
import java.io.PrintWriter;
import java.io.FileNotFoundException;

public class WriteFile {
    public static void main(String[] args) {
        try {
            // Method 1: Using File object
            File myFile = new File("output.txt");
            PrintWriter writer = new PrintWriter(myFile);

            // Method 2: Directly with filename
            PrintWriter writer2 = new PrintWriter("output.txt");

            // Write to the file
            writer.println("Hello, file!");

            // IMPORTANT: Close the writer!
            writer.close();

        } catch (FileNotFoundException e) {
            System.out.println("Error creating file!");
        }
    }
}
                

File Creation

If the file doesn't exist, PrintWriter will create it. If it does exist, PrintWriter will overwrite it (erasing old content)!

3 PrintWriter Methods

Method	Description
`print(data)`	Writes data without a newline at the end
`println(data)`	Writes data followed by a newline
`printf(format, args)`	Writes formatted output (like C's printf)
`flush()`	Forces buffered data to be written to file
`close()`	Flushes and closes the writer

Example: Using Different Write Methods

PrintWriter writer = new PrintWriter("output.txt");

// println - adds newline
writer.println("Hello, file!");

// print - no newline
writer.print("Writing some more text...");
writer.println(" And a new line!");

// printf - formatted output
String name = "Ahmed";
int score = 95;
writer.printf(
    "%s scored %d points%n",
    name,
    score
);

writer.close();
                

                        📄
                        output.txt (Result)
                    
Hello, file!

Writing some more text... And a new line!

Ahmed scored 95 points

4 Why Must We Close or Flush?

The teacher shared this secret: "When you write data to a file, Java might keep some of that data in a temporary area called a buffer. This makes writing more efficient."

"But if you never say 'write everything to the file now' (by closing or flushing), the data could stay in that buffer and never appear in the actual file!"

Understanding the Buffer

Your Code Writes:

writer.println("Hello")

→

Buffer (Memory):

"Hello\n"

→

File (Disk):

After close()/flush()

Common Mistake!

If you forget to call close(), your file might be empty or incomplete! The data is sitting in the buffer, waiting to be written.

💻

Java Program

writer.println()

→

📦

Buffer

Temporary storage

→

⚙

OS Kernel

System calls

→

💾

Disk

Permanent storage

Behind the Scenes

When a Java program wants to write data to the hard disk, it eventually calls system calls provided by the operating system. Java's I/O classes talk to the JVM, which invokes the OS's file I/O system calls (like write() on Linux or WriteFile() on Windows). These system calls pass data to the OS kernel, which interacts with the file system and disk driver.

Note: The deep details will be covered in Operating Systems course.

5 Complete Example: Student Grade Report

Let's write a program that creates a grade report file:

import java.io.PrintWriter;
import java.io.FileNotFoundException;

public class GradeReport {
    public static void main(String[] args) {
        // Student data
        String[] names = {"Ahmed", "Sara", "Mohammed"};
        int[] scores = {85, 92, 78};

        try {
            PrintWriter writer = new PrintWriter("report.txt");

            // Write header
            writer.println("=== Grade Report ===");
            writer.println();

            // Write each student
            for (int i = 0; i < names.length; i++) {
                writer.printf(
                    "Student: %-10s Score: %d%n",
                    names[i],
                    scores[i]
                );
            }

            // Calculate and write average
            double sum = 0;
            for (int score : scores) {
                sum += score;
            }
            double average = sum / scores.length;

            writer.println();
            writer.printf(
                "Class Average: %.2f%n",
                average
            );
            writer.println("====================");

            // IMPORTANT: Close the writer!
            writer.close();

            System.out.println("Report saved to report.txt");

        } catch (FileNotFoundException e) {
            System.out.println("Error creating report file!");
        }
    }
}
                

                        📄
                        report.txt (Output File)
                    
=== Grade Report ===

Student: Ahmed      Score: 85

Student: Sara       Score: 92

Student: Mohammed   Score: 78

Class Average: 85.00

====================

6 Reading and Writing Together

Often, you'll need to read from one file and write to another:

import java.io.File;
import java.io.PrintWriter;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class CopyAndModify {
    public static void main(String[] args) {
        try {
            // Open input file for reading
            Scanner reader = new Scanner(
                new File("input.txt")
            );

            // Open output file for writing
            PrintWriter writer = new PrintWriter(
                "output.txt"
            );

            int lineNum = 1;

            while (reader.hasNextLine()) {
                String line = reader.nextLine();

                // Add line numbers to each line
                writer.printf(
                    "%3d: %s%n",
                    lineNum,
                    line
                );

                lineNum++;
            }

            // Close both!
            reader.close();
            writer.close();

            System.out.println("File processed successfully!");

        } catch (FileNotFoundException e) {
            System.out.println(
                "Error: " + e.getMessage()
            );
        }
    }
}
                

Best Practice

Always close your readers and writers when done. A good pattern is to close them in the reverse order of opening (close writer first, then reader).

7 Summary

Key Takeaways

✓ PrintWriter writes text to files like System.out writes to console.
✓ Use print(), println(), and printf() to write data.
✓ Data is buffered - stored temporarily before writing to disk.
✓ Always close() the writer to ensure data is saved!
✓ PrintWriter creates new files or overwrites existing ones.

And so, the student learned:

Files store data even when programs stop running
The File class lets us explore file properties
Scanner reads from files like it reads from keyboard
PrintWriter writes to files like System.out writes to screen
Always close your writers to save data correctly!

End of the Tale!