Writing Files

IB Syllabus: B2.5.1 – Construct code to perform file-processing operations: open files in write/append mode, write to files, close files.

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Key Concepts

Writing with FileWriter

FileWriter writes text to a file. By default, it overwrites any existing content.

import java.io.FileWriter;
import java.io.IOException;

try {
    FileWriter writer = new FileWriter("output.txt");

    writer.write("Alice,85\n");
    writer.write("Bob,72\n");
    writer.write("Charlie,91\n");

    writer.close();
    System.out.println("File written successfully.");

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

try:
    f = open("output.txt", "w")    # "w" = write mode - overwrites existing content

    f.write("Alice,85\n")
    f.write("Bob,72\n")
    f.write("Charlie,91\n")

    f.close()
    print("File written successfully.")

except OSError:    # OSError is Python's IOException - raised when writing fails
    print("Error writing file.")

Always call writer.close(). Without closing, data may remain in a buffer and never reach the file on disk.

Overwrite vs Append

Mode	Constructor	Behaviour
Overwrite	new FileWriter("file.txt")	Erases existing content, starts fresh
Append	new FileWriter("file.txt", true)	Adds to end of existing content

(Python: overwrite is open("file.txt", "w"), append is open("file.txt", "a") - the mode string does the job of Java’s extra argument.)

// Append mode -- adds to end of file
FileWriter writer = new FileWriter("log.txt", true);
writer.write("New entry added.\n");
writer.close();

# Append mode - "a" adds to the end of the file
f = open("log.txt", "a")
f.write("New entry added.\n")
f.close()

Use overwrite when saving the complete current state (e.g., saving all records). Use append when adding to a log or history.

Writing with BufferedWriter

BufferedWriter wraps FileWriter for better efficiency and adds the newLine() method.

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;

try {
    BufferedWriter writer = new BufferedWriter(new FileWriter("results.txt"));

    writer.write("Name,Score");
    writer.newLine();
    writer.write("Alice,85");
    writer.newLine();
    writer.write("Bob,72");
    writer.newLine();

    writer.close();

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

# Python has no separate BufferedWriter - open() already buffers writes.
# There is no newLine() either: write "\n" yourself (write() adds nothing automatically).
try:
    f = open("results.txt", "w")

    f.write("Name,Score\n")
    f.write("Alice,85\n")
    f.write("Bob,72\n")

    f.close()

except OSError:
    print("Error writing file.")

newLine() inserts the correct line separator for the operating system (Windows: \r\n, macOS/Linux: \n). This is more portable than hardcoding "\n".

try-with-resources

The cleanest approach – automatically closes the writer:

try (BufferedWriter writer = new BufferedWriter(new FileWriter("data.csv"))) {
    writer.write("Alice,85");
    writer.newLine();
    writer.write("Bob,72");
    writer.newLine();
}  // writer is automatically closed here

(Python has a similar shortcut, the with statement, but IB Python uses explicit open() and close() - so we do too.)

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Worked Examples

Example 1: Save an ArrayList to a File

public static void saveNames(ArrayList<String> names, String filename)
        throws IOException {
    BufferedWriter writer = new BufferedWriter(new FileWriter(filename));
    for (int i = 0; i < names.size(); i++) {
        writer.write(names.get(i));
        writer.newLine();
    }
    writer.close();
}

def save_names(names, filename):
    f = open(filename, "w")
    for i in range(len(names)):
        f.write(names[i] + "\n")
    f.close()

Example 2: Write CSV Data

Save structured data with comma-separated fields.

public static void saveStudents(ArrayList<Student> students, String filename)
        throws IOException {
    BufferedWriter writer = new BufferedWriter(new FileWriter(filename));
    for (int i = 0; i < students.size(); i++) {
        Student s = students.get(i);
        writer.write(s.getName() + "," + s.getGrade());
        writer.newLine();
    }
    writer.close();
}

def save_students(students, filename):
    f = open(filename, "w")
    for i in range(len(students)):
        s = students[i]
        f.write(s.get_name() + "," + str(s.get_grade()) + "\n")    # str() needed - Python will not join a number onto a string automatically
    f.close()

Example 3: Append a Log Entry

public static void logAction(String action, String filename)
        throws IOException {
    BufferedWriter writer = new BufferedWriter(new FileWriter(filename, true));
    writer.write(action);
    writer.newLine();
    writer.close();
}

def log_action(action, filename):
    f = open(filename, "a")    # "a" = append mode
    f.write(action + "\n")
    f.close()

Example 4: Complete Read-Process-Write Pipeline

Read scores, calculate the average, and write a summary file.

import java.util.*;
import java.io.*;

public class ScoreSummary {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>();
        ArrayList<Integer> scores = new ArrayList<>();

        // READ
        try (Scanner reader = new Scanner(new File("students.csv"))) {
            while (reader.hasNextLine()) {
                String[] parts = reader.nextLine().split(",");
                names.add(parts[0].trim());
                scores.add(Integer.parseInt(parts[1].trim()));
            }
        } catch (FileNotFoundException e) {
            System.out.println("Input file not found.");
            return;
        }

        // PROCESS
        int total = 0;
        for (int i = 0; i < scores.size(); i++) {
            total += scores.get(i);
        }
        double average = (scores.size() > 0) ? (double) total / scores.size() : 0;

        // WRITE
        try (BufferedWriter writer = new BufferedWriter(new FileWriter("summary.txt"))) {
            writer.write("Student Score Summary");
            writer.newLine();
            writer.write("=====================");
            writer.newLine();
            for (int i = 0; i < names.size(); i++) {
                writer.write(names.get(i) + ": " + scores.get(i));
                writer.newLine();
            }
            writer.write("---------------------");
            writer.newLine();
            writer.write("Average: " + average);
            writer.newLine();
        } catch (IOException e) {
            System.out.println("Error writing summary: " + e.getMessage());
        }

        System.out.println("Summary written to summary.txt");
    }
}

# main() is a function here so that "return" can stop the program early,
# exactly like Java's return inside main.
def main():
    names = []
    scores = []

    # READ
    try:
        f = open("students.csv", "r")
        line = f.readline()
        while line != "":
            parts = line.split(",")
            names.append(parts[0].strip())
            scores.append(int(parts[1].strip()))
            line = f.readline()
        f.close()
    except FileNotFoundError:
        print("Input file not found.")
        return    # stop - there is nothing to process

    # PROCESS
    total = 0
    for i in range(len(scores)):
        total = total + scores[i]
    if len(scores) > 0:
        average = total / len(scores)
    else:
        average = 0

    # WRITE
    try:
        f = open("summary.txt", "w")
        f.write("Student Score Summary\n")
        f.write("=====================\n")
        for i in range(len(names)):
            f.write(names[i] + ": " + str(scores[i]) + "\n")
        f.write("---------------------\n")
        f.write("Average: " + str(average) + "\n")
        f.close()
    except OSError:
        print("Error writing summary.")

    print("Summary written to summary.txt")

main()

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Quick Check

Q1. What happens to existing content when you use new FileWriter("data.txt")?

Q2. How do you open a file in append mode?

Q3. What does writer.newLine() do?

Q4. What can happen if you forget to call writer.close()?

Q5. Which exception should you catch when writing to a file?

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Code Completion

Complete the method that saves student names and grades to a CSV file.

Fill in the blanks: Save each student as a CSV line.

public static void save(ArrayList<Student> students, String filename)
        throws IOException {
    BufferedWriter writer = new BufferedWriter(new (filename));
    for (int i = 0; i < students.size(); i++) {
        Student s = students.get(i);
        writer.(s.getName() + "," + s.getGrade());
        writer.;
    }
    writer.;
}

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Spot the Error

This code should append a new score to a file, but it erases everything each time.

Bug Hunt: Every time this method runs, previous scores disappear and only the latest score remains.

1public static void addScore(int score) throws IOException { 2 FileWriter writer = new FileWriter("scores.txt"); 3 writer.write(score + "\n"); 4 writer.close(); 5}

Pick the correct fix for line 2:

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Predict the Output

Predict: What does the file test.txt contain after this code runs?

FileWriter writer = new FileWriter("test.txt");
writer.write("Hello");
writer.write("World");
writer.close();

Scanner reader = new Scanner(new File("test.txt"));
System.out.println(reader.nextLine());
reader.close();

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Practice Exercises

Core

1. Write five names – Ask the user for five names (via Scanner on keyboard) and write them to names.txt, one per line.

2. Append mode test – Modify exercise 1 to use append mode. Run it twice and verify the file contains 10 names.

3. Copy uppercase – Read all lines from input.txt and write them to output.txt with each line converted to uppercase using .toUpperCase().

Extension

1. CSV report – Read students.csv (name,score), calculate the average, and write a formatted report to report.txt listing each student, their score, and whether they are above or below average.

2. Log with timestamps – Write a method log(String message) that appends a line to log.txt in the format [YYYY-MM-DD HH:MM:SS] message. Use java.time.LocalDateTime.now().

Challenge

1. File-based address book – Build a complete program that stores contacts (name, phone, email) in a CSV file. Support four operations: add a contact (append), list all contacts (read + display), search by name (read + filter), and delete by name (read all, write back without the match). The file is the only storage – no data is kept in memory between operations.

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Connections

• Prerequisites: Reading Files – reading is the counterpart to writing
• Prerequisites: Exception Handling – try/catch for IOExceptions
• Related: ArrayList – saving ArrayList contents to files
• Forward: File I/O in Apps – file I/O integrated into MVC applications