Unit 1 — Using Objects & Methods

File I/O with Scanner New in 2026

Reading data from external files is a brand-new topic in the 2025-2026 AP CSA curriculum. The Scanner class (combined with File) lets a Java program read text data from a file line by line or token by token. This skill appears in MCQ and may appear in FRQs.

Required Imports

import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;  // or java.io.IOException

These must appear at the top of any class that uses Scanner with files. On the AP exam, the question will specify which imports are provided.

Creating a Scanner for File Input

To read from a file, pass a File object to the Scanner constructor. This always requires handling a checked exception because the file might not exist.

Pattern 1 — try-catch block

try
{
    Scanner sc = new Scanner(new File("data.txt"));
    while (sc.hasNextLine())
    {
        String line = sc.nextLine();
        System.out.println(line);
    }
    sc.close();
}
catch (FileNotFoundException e)
{
    System.out.println("File not found: " + e.getMessage());
}

Pattern 2 — throws declaration (propagates exception)

public static void readFile() throws IOException
{
    Scanner sc = new Scanner(new File("scores.txt"));
    while (sc.hasNextLine())
    {
        String line = sc.nextLine();
        // process line
    }
    sc.close();
}

Scanner Reading Methods

Method	What it reads	Consumes newline?
nextLine()	Entire line up to (and including) \n	Yes
next()	Next whitespace-delimited token	No
nextInt()	Next token as int	No
nextDouble()	Next token as double	No
hasNextLine()	Returns true if another line exists	N/A
hasNext()	Returns true if another token exists	N/A
hasNextInt()	Returns true if next token is a valid int	N/A

Reading a File Line by Line

Scanner sc = new Scanner(new File("names.txt"));
while (sc.hasNextLine())
{
    String name = sc.nextLine();
    System.out.println("Hello, " + name + "!");
}
sc.close();

If names.txt contains:

Alice
Bob
Carol

Output:

Hello, Alice!
Hello, Bob!
Hello, Carol!

Reading Mixed Data Types

When a file contains numbers and text on the same line, read tokens individually with next() and nextInt() etc.

// File contents: "Alice 95\nBob 87\nCarol 91"
Scanner sc = new Scanner(new File("grades.txt"));
while (sc.hasNextLine())
{
    String name = sc.next();      // reads next token
    int score   = sc.nextInt();   // reads next integer token
    System.out.println(name + ": " + score);
    if (sc.hasNextLine()) {
        sc.nextLine();  // consume remaining newline
    }
}
sc.close();

The nextInt() + nextLine() Trap

This is the most common bug in file-reading code. When you call nextInt(), it reads the integer but leaves the newline character in the buffer. A subsequent nextLine() will read that empty newline instead of the next actual line.

// WRONG: nextLine() reads the leftover newline, not "Alice"
int count = sc.nextInt();
String name = sc.nextLine();   // reads "" (the leftover newline)

// CORRECT: consume the leftover newline first
int count = sc.nextInt();
sc.nextLine();                 // consumes the leftover newline
String name = sc.nextLine();   // now reads "Alice" correctly

Always Close the Scanner

Call sc.close() after you are done reading. This releases the file handle. On the exam, code that never closes the Scanner is technically a resource leak, but it will not cause a compile-time or runtime error in most questions.

Handling FileNotFoundException

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

FileNotFoundException is a checked exception — the compiler requires you to handle it (try-catch) or declare it (throws). Unlike runtime exceptions such as NullPointerException, it cannot be ignored.

Complete Example — Count Lines in a File

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

public class LineCounter
{
    public static int countLines(String filename) throws FileNotFoundException
    {
        Scanner sc = new Scanner(new File(filename));
        int count = 0;
        while (sc.hasNextLine())
        {
            sc.nextLine();
            count++;
        }
        sc.close();
        return count;
    }
}

Practice MCQs

Common Mistakes

• Passing a String instead of a File: new Scanner("data.txt") reads the string "data.txt", not the file. Always use new Scanner(new File("data.txt")).
• Ignoring the checked exception: Scanner(File) throws FileNotFoundException. Forgetting to handle it is a compile error.
• nextInt() + nextLine() mismatch: Call sc.nextLine() after any numeric read to consume the leftover newline before calling nextLine().
• Not closing the Scanner: Always call sc.close() when done. On the AP exam this is considered best practice.
• Using wrong check method: If the file might have non-integer lines, use hasNextInt() not hasNextLine() before calling nextInt().

Quick Reference

Task	Code
Open a file	Scanner sc = new Scanner(new File("file.txt"));
Read all lines	while (sc.hasNextLine()) { String s = sc.nextLine(); }
Read all integers	while (sc.hasNextInt()) { int n = sc.nextInt(); }
Handle exception	try { ... } catch (FileNotFoundException e) { ... }
Close scanner	sc.close();

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