Using Text Files — AP Computer Science A · ⁨AP Ciencias de la Computación A⁩

Vocabulary · ⁨Vocabulario⁩

English	Español
text file/tekst faɪl/	archivo de texto

A blank line is not an invalid token

A text file 文本文件 contains a blank line followed by 12. At the start, both hasNextLine() and · ⁨ y ⁩hasNextInt() are true: the integer-token check skips whitespace, including the blank line, to find 12.
Compare a file containing oops followed by 12: it has another line, but its next token is not an integer. A line-presence check cannot establish integer format; choose a guard that matches the read.

A line can exist without containing an integer

Scanner line and integer checks on blank and invalid input

Practice · ⁨Práctica⁩

With the default delimiter, a Scanner containing a blank line followed by 12 reports hasNextInt() as true at the start.

Attach the reader to its source

new Scanner(new File("scores.txt")) reads the named file, whereas new Scanner(System.in) reads keyboard input. Both expose the same Scanner methods; the source and its availability differ.
Opening a missing file can throw FileNotFoundException. The complete example below declares it on main; a real application could instead catch it and report a useful error. Neither choice creates missing data.

Practice · ⁨Práctica⁩

To read a text file, you attach a Scanner to a...

File object
System.in
String
an int

Practice · ⁨Práctica⁩

The same Scanner methods (nextInt, nextLine) work for both keyboard and file input.

Read complete lines

hasNextLine() checks whether another line exists without consuming it; nextLine() consumes the remainder of that line and its line separator. A blank line can produce the empty string "".
For line-based records, use while (in.hasNextLine()) { String line = in.nextLine(); ... }. Decide explicitly whether an empty record is meaningful, should be skipped, or should be reported as invalid.

Explore · ⁨Explorar⁩

Reading a text file · ⁨Lectura de un archivo de texto⁩

Follow the steps a program takes to read a file line by line.

Practice · ⁨Práctica⁩

Which method checks whether there is another line to read?

hasNextLine()
nextLine()
close()
length()

Practice · ⁨Práctica⁩

You should check hasNextLine() before each call to nextLine().

Read tokens with matching checks

Scanner's default delimiter is whitespace. hasNext() checks for a token; next() consumes it. hasNextInt() checks whether the next token can be read as an integer; nextInt() consumes that integer.
while (in.hasNextInt()) stops at oops, even if 12 follows. If the policy is to skip invalid tokens and retain later integers, loop with hasNext(), test hasNextInt(), and consume an invalid token with next().

Example · ⁨Ejemplo⁩

This complete program sums valid integer tokens and reports every skipped token. For a file containing oops followed by 12, it prints Skipped: oops and then Total: 12. The example assumes the sum fits in an int.

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

public class ReadScores {
    public static void main(String[] args) throws FileNotFoundException {
        int total = 0;
        try (Scanner in = new Scanner(new File("scores.txt"))) {
            while (in.hasNext()) {
                if (in.hasNextInt()) {
                    total += in.nextInt();
                } else {
                    System.out.println("Skipped: " + in.next());
                }
            }
        }
        System.out.println("Total: " + total);
    }
}

Practice · ⁨Práctica⁩

Match each read to the guard that checks that kind of input.

hasNextLine()

hasNextInt()

hasNext()

nextLine()
nextInt()
next()

Consume input and release the file

Every branch in the token loop consumes a token. Repeatedly checking an invalid token without consuming it leaves the Scanner at the same position and can make a loop run forever.
The try-with-resources block closes this file Scanner on exit, including exceptional exit. When managing it manually, call close() when finished; checks do not replace closing the resource.

Watch out · ⁨Precaución⁩

hasNextLine() followed by nextInt() is not a matching guard. A non-integer token can cause InputMismatchException, and that failed integer read leaves the token available. Choose whether to reject the file, report the record, or explicitly skip the token.

Practice · ⁨Práctica⁩

When you finish reading a file, you should...

close the Scanner
delete the file
print the file
do nothing ever

Check the data policy before the loop

For input 8 oops 12, a loop that reads only while hasNextInt() totals 8 and stops at oops. The shown skip-and-report policy totals 20. An unexpected token need not mean the file has ended.
These policies answer different questions. For a required score per student, silently skipping a bad token could misalign records; prefer reporting and rejecting that record rather than treating all later tokens as interchangeable scores.

Key idea · ⁨Idea clave⁩

Choose line or token reading from the file format. Checks inspect upcoming input without consuming it; matching reads consume it. Handle invalid data explicitly, advance every loop branch, and close the file reader.

Practice · ⁨Práctica⁩

For input 8 oops 12, what total does the shown skip-and-report program print?