9.5. Reading from a File

Use Scanner, FileReader, or BufferedReader to read from a file.

To read data from a file in Java, you have several options: Scanner, FileReader, and BufferedReader. Each of these classes provides a way to access and retrieve information stored in files. Choosing which one to use often depends on how you want to process the data. Using a Scanner is particularly easy for beginners since it includes methods that allow you to read data line by line or by specific types like integers or strings.

Think of reading a file like reading a book. Just as you turn the pages to see one line after another, in programming, you need a tool (like Scanner) that helps you access the text from the book one line at a time.

✅ Code Example Using Scanner:

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
public class ReadFile {
public static void main(String[] args) {
try {
File file = new File("example.txt");
Scanner reader = new Scanner(file);
while (reader.hasNextLine()) {
String line = reader.nextLine();
System.out.println(line);
}
reader.close();
} catch (FileNotFoundException e) {
System.out.println("File not found.");
e.printStackTrace();
}
}
}

In this code example, we first import the necessary classes. A File object is created to represent the target file. We then create a Scanner to read from that file. The while loop checks if there is another line to read, and if so, it reads that line using nextLine() and outputs it to the console. It's important to close the Scanner after we're done reading the file to prevent any resource leaks.

Imagine using a highlighter while reading a book. You highlight what you've read to remember it better. In this code, the Scanner acts like your highlighter, helping you pick out and read one line at a time from the file.

catch (FileNotFoundException e) {
System.out.println("File not found.");
e.printStackTrace();
}```

When working with files, it is crucial to be prepared for the possibility of errors. This specific example uses a try-catch block, which is a way to handle situations where something might go wrong. If the file specified does not exist, a FileNotFoundException will be thrown, and the catch block will execute. This will print a message to the console informing the user that the file was not found, and it also prints the stack trace for debugging purposes.

Consider planning a trip to a restaurant without checking if it’s open. If it’s closed, you might be disappointed. In this code, the try-catch block serves as your check—it warns you if your 'restaurant' (or the file) is not available, allowing you to handle the disappointment gracefully.

Don't forget to close the reader.

One of the best practices when working with file operations is to always close your readers or writers. Failing to do so can lead to memory leaks, where your program consumes more resources than necessary. Closing the reader helps to free up these resources, ensuring that your program runs efficiently and does not crash.

Think of it like finishing a glass of water. Once you’re done drinking, you should put the glass in the sink rather than leaving it on the table. Closing the reader is like putting the glass away; it keeps your environment (or code) tidy and functional.

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