Lesson 3: Reading Files with Scanner

1 Scanner: Your Old Friend

The teacher held up the Scanner and said, "You've used Scanner before to read input from the keyboard. Now, you can hand the File object to Scanner, and it will read text from that file instead!"

Key Insight

We've used Scanner before with System.in to read from the keyboard. Now, we'll use the same Scanner but give it a File object instead!

Reading from Keyboard

Scanner input = new Scanner(
    System.in
);

String name = input.nextLine();
                        

Reading from File

File myFile = new File(
    "data.txt"
);

Scanner input = new Scanner(
    myFile
);

String line = input.nextLine();
                        

2 Creating a Scanner for Files

Here's the basic syntax to read from a file:

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

public class ReadFile {
    public static void main(String[] args) {
        try {
            // Step 1: Create File object
            File myFile = new File("data.txt");

            // Step 2: Create Scanner with the File
            Scanner fileReader = new Scanner(myFile);

            // Step 3: Read from the file
            String line = fileReader.nextLine();
            System.out.println(line);

            // Step 4: Close the Scanner
            fileReader.close();

        } catch (FileNotFoundException e) {
            System.out.println("Error: File not found!");
        }
    }
}
                

FileNotFoundException

When creating a Scanner with a File, Java requires us to handle the possibility that the file doesn't exist. We use try-catch to handle this.

3 Scanner Reading Methods

The same methods you used for keyboard input work for file input:

Method	Returns	Description
`next()`	String	Reads the next word (token, stops at whitespace)
`nextLine()`	String	Reads the entire line (until newline)
`nextInt()`	int	Reads the next integer
`nextDouble()`	double	Reads the next decimal number
`nextBoolean()`	boolean	Reads true or false
`hasNext()`	boolean	Returns true if there's more data
`hasNextLine()`	boolean	Returns true if there's another line
`hasNextInt()`	boolean	Returns true if next token is an integer

4 Reading an Entire File

To read all lines in a file, use a while loop with hasNextLine():

                        📄
                        students.txt
                    
Ahmed 85

Sara 92

Mohammed 78

Fatima 95

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

public class ReadAllLines {
    public static void main(String[] args) {
        try {
            File myFile = new File("students.txt");
            Scanner fileReader = new Scanner(myFile);

            // Read line by line until end of file
            while (fileReader.hasNextLine()) {
                String line = fileReader.nextLine();
                System.out.println(line);
            }

            fileReader.close();

        } catch (FileNotFoundException e) {
            System.out.println("Error: File not found!");
        }
    }
}
                

Output

Ahmed 85
Sara 92
Mohammed 78
Fatima 95
                    

5 Reading Specific Data Types

You can read individual tokens of different types:

                        📄
                        scores.txt
                    
Ahmed 85

Sara 92

Mohammed 78

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

public class ReadStudentScores {
    public static void main(String[] args) {
        try {
            File myFile = new File("scores.txt");
            Scanner fileReader = new Scanner(myFile);

            while (fileReader.hasNext()) {
                // Read name (String)
                String name = fileReader.next();

                // Read score (int)
                int score = fileReader.nextInt();

                System.out.println(
                    name + " scored " + score + " points"
                );
            }

            fileReader.close();

        } catch (FileNotFoundException e) {
            System.out.println("Error: File not found!");
        }
    }
}
                

Output

Ahmed scored 85 points
Sara scored 92 points
Mohammed scored 78 points
                    

6 Always Close the Scanner!

Why Close?

When you open a file for reading, the operating system allocates resources. If you don't close the Scanner, these resources remain locked and can cause problems - especially if you try to modify the file later!

📂

Open File

→

📖

Read Data

→

✅

Close Scanner

// Always remember to close!
fileReader.close();
                

Good Practice

Always close your Scanner in a finally block or use try-with-resources (which automatically closes):

// Try-with-resources: Automatically closes!
try (Scanner reader = new Scanner(new File("data.txt"))) {
    while (reader.hasNextLine()) {
        System.out.println(reader.nextLine());
    }
} // Scanner automatically closed here!
                    

7 Complete Example

Let's put it all together with a practical example:

                        📄
                        grades.txt
                    
3

Ahmed 85 90 78

Sara 92 88 95

Mohammed 78 82 80

The first line tells us there are 3 students. Each following line has a name and 3 test scores.

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

public class GradeCalculator {
    public static void main(String[] args) {
        try {
            File file = new File("grades.txt");
            Scanner reader = new Scanner(file);

            // Read number of students
            int numStudents = reader.nextInt();

            System.out.println(
                "Processing " + numStudents + " students...\n"
            );

            // Process each student
            for (int i = 0; i < numStudents; i++) {
                String name = reader.next();
                int test1 = reader.nextInt();
                int test2 = reader.nextInt();
                int test3 = reader.nextInt();

                double average = (test1 + test2 + test3) / 3.0;

                System.out.println(
                    name + ": Average = " + average
                );
            }

            reader.close();

        } catch (FileNotFoundException e) {
            System.out.println("Error: grades.txt not found!");
        }
    }
}
                

Output

Processing 3 students...

Ahmed: Average = 84.33333333333333
Sara: Average = 91.66666666666667
Mohammed: Average = 80.0
                    

8 Summary

Key Takeaways

✓ Scanner can read from files the same way it reads from keyboard.
✓ Pass a File object to Scanner instead of System.in.
✓ Use hasNextLine() or hasNext() to check for more data.
✓ Use next(), nextLine(), nextInt(), nextDouble() to read data.
✓ Always close() the Scanner when done reading!
✓ Handle FileNotFoundException with try-catch.

The student exclaimed, "Reading is easy! But how do I save data to a file?"

The teacher replied, "That's where PrintWriter comes in - let's learn about writing files next!"