4.6 Using Text Files

What you need to know

• Open a file for reading: Scanner in = new Scanner(new File("data.txt"));. Both File (java.io) and Scanner (java.util) need imports.
• Creating a Scanner on a File can throw FileNotFoundException (a checked exception). The method must declare throws FileNotFoundException in its header.
• Read line by line: while (in.hasNextLine()) { String line = in.nextLine(); ... }. Read tokens: while (in.hasNext()) with next(), or hasNextInt() with nextInt().
• Close the file when done: in.close();
• Lines of text often need parsing: Integer.parseInt(str) and Double.parseDouble(str) convert String tokens to numbers.
• The same nextInt/nextLine buffer trap from 1.4 applies to files.
• A common pattern: read each line into an ArrayList so the data can be processed after the file is closed.

Worked example

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

public static ArrayList<Integer> readScores(String filename)
        throws FileNotFoundException
{
    Scanner in = new Scanner(new File(filename));
    ArrayList<Integer> scores = new ArrayList<Integer>();
    while (in.hasNextInt())
    {
        scores.add(in.nextInt());
    }
    in.close();
    return scores;
}

Going deeper

The nuance, edge cases, and connections that turn a 3 into a 5.

• The pattern: Scanner in = new Scanner(new File("name.txt"));. File represents the path; Scanner reads from it using the exact same methods as keyboard input. Both need imports: java.io.File and java.util.Scanner.
• FileNotFoundException is a checked exception — the compiler requires you to acknowledge it. Since you won't write try/catch on the exam, the acknowledgment is throws FileNotFoundException in the method header (import java.io.FileNotFoundException). Forget it → compile error.
• Reading loops: pair the check with the read. hasNextLine() ↔ nextLine(); hasNext() ↔ next(); hasNextInt() ↔ nextInt(). Mismatching (hasNext with nextLine) can miss data or throw on trailing whitespace.
• Parsing: file data is text. Integer.parseInt(token) and Double.parseDouble(token) convert. If a line has several values separated by spaces, next() reads them one at a time.
• The nextInt/nextLine trap applies to files exactly as to keyboard input (1.4).
• Close the Scanner when done: in.close(). Releases the file. The exam expects it.
• Typical structure: open → loop reading into an ArrayList → close → process the list. Separating reading from processing keeps the file open briefly and makes the processing code testable.
• This topic is new in the 2025–26 CED. Expect it to be tested — it's a signal the College Board wanted students to handle real data.

Mistakes that cost points

• No throws clause. Compile error. Every method that opens a file needs it (or the method that calls it does).
• Mismatched has/next. while (hasNext()) with nextLine() can behave unexpectedly on blank lines.
• Reading a number with nextLine and not parsing. You get a String. Parse it.
• Forgetting the imports. Three of them: File, FileNotFoundException, Scanner.

Practice questions

Written in the style of the real exam. Try each one before revealing the answer.