Reading a File with Scanner

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Try This First

A program runs new File("scores.txt") for a file that does not exist, and no error appears. Decide why before reading on.

Before You Start

Check each box you can do from memory. A box you cannot check yet is not a problem; it points you to a quick refresher, not a grade.

• I can write a while loop and explain what condition makes it stop.
• I can describe what happens when a while loop condition never becomes false.
• I can trace a loop with a counter variable and predict the final value.

See Loops: for, while, and do-while for a quick review if any of the above felt uncertain.

What You Need To Walk In With

Walk into the next class able to state these:

• new File(filename) builds a path object and does not open anything. new Scanner(file) is the call that opens the file and can throw FileNotFoundException.
• A method that opens a file must either catch FileNotFoundException or declare throws FileNotFoundException.
• Picture a cursor on the file. A read advances it; hasNextLine() peeks without moving it and turns false at the end.
• Call close() when finished; a file Scanner holds an operating-system handle that should be released.

You should be able to: open a file with the two-step idiom and write a read loop that ends cleanly at the end of the file.

How It Works

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

public static int countLines(String filename) throws FileNotFoundException {
    File f = new File(filename);    // a path object; the disk is not touched yet
    Scanner sc = new Scanner(f);    // opens the file; can throw FileNotFoundException
    int n = 0;
    while (sc.hasNextLine()) {
        sc.nextLine();
        n = n + 1;
    }
    sc.close();                     // release the operating-system file handle
    return n;
}

Two parts are new compared to reading from the keyboard:

• new File(filename) is lexical: it stores the string and returns. You can ask f.exists() or f.canRead() before opening. The disk is consulted only when the Scanner constructor runs.
• The Scanner constructor on a File is declared throws FileNotFoundException, so the method that calls it must catch or declare that exception.

The cursor model

A l i c e \n B o b \n
^ cursor starts here

nextLine() reads from the cursor to the next newline, returns the text without the newline, and moves the cursor past it. hasNextLine() peeks ahead without moving the cursor and returns false at the end of the file, so the loop exits cleanly.

A file Scanner holds an operating-system file handle. Handles are limited and are not reclaimed reliably while the program runs, so call close() as soon as the reading is done.

Worked Example: Predict, Then Check

A file holds three lines: Alice, Bob, Carol. The loop above counts lines. Predict the returned value.

A Common Mistake

A frequent belief is that new File(name) opens the file, so a missing file should fail on that line. It does not: File is only a path. The open happens at new Scanner(file), which is where a missing file throws FileNotFoundException. Handle the exception on the method that builds the Scanner. (Source: BJP (Reges and Stepp), Ch 6.)

Go Deeper (optional)

For the curious: the explicit close() here is the foundation for try-with-resources, a later form that closes the Scanner automatically even if an error interrupts the reading. Learning the manual close() first makes clear what that automatic form is doing for you.

Check Yourself

Close the notes and answer each one from memory, then reveal it. Pulling an idea back from memory is one of the strongest ways to make it stick.