Exceptions and Files

Files fail, networks drop and users type the wrong thing. Java separates recoverable errors from programming bugs with exceptions.

try and catch

Catch the most specific exception first. Print a useful message, not just the stack trace, for expected problems.

try {
    int value = Integer.parseInt("not a number");
    System.out.println(value);
} catch (NumberFormatException error) {
    System.out.println("Please enter a whole number.");
}

try-with-resources

Files and other resources must be closed. try-with-resources closes them automatically, even when an exception occurs.

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

try (BufferedReader reader = new BufferedReader(new FileReader("scores.txt"))) {
    String line;
    while ((line = reader.readLine()) != null) {
        System.out.println(line);
    }
} catch (IOException error) {
    System.out.println("Could not read the file: " + error.getMessage());
}

Writing a File

Use Files.writeString for simple text output. It creates or replaces the file.

import java.nio.file.Files;
import java.nio.file.Path;
import java.io.IOException;

try {
    Files.writeString(Path.of("output.txt"), "Amina,88\nBola,74\n");
} catch (IOException error) {
    System.out.println("Could not write the file.");
}

Practice

1. Handle invalid number input with try/catch.
2. Read a file line by line and count the lines.
3. Write two lines to a file and read them back.

Hands-on exercise

Write a program that reads scores.txt.
1. Print each line.
2. If the file does not exist, print 'No scores file found.'
3. Use try-with-resources so the reader always closes.

Recap

Catch expected failures, close resources automatically and give users a message they can act on.