Using Text Files
in Java
Master reading from and writing to text files using Java's I/O classes: from basic Scanner input to robust exception handling.
AP Computer Science A
What Are Text Files?
A text file stores data as a sequence of human-readable characters, organized line by line. Unlike binary files, you can open a text file in any editor and read its contents directly.
.txt
Plain text with no formatting: the simplest file type
.csv
Comma-separated values: tabular data in text form
.dat
Generic data files often read line by line in programs
Why Use Text Files?
1
Data persistence: Save results so they survive after the program ends.
2
Large input: Process hundreds of values without typing them every time.
3
Interoperability: Share data between programs, languages, and platforms.
4
Testing and debugging: Feed consistent test data from files instead of manual input.
The Java I/O Pipeline
Reading with Scanner
import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
public class ReadDemo {
public static void main(String[] args) {
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!");
}
}
}
Writing with PrintWriter
import java.io.PrintWriter;
import java.io.FileNotFoundException;
public class WriteDemo {
public static void main(String[] args) {
try {
PrintWriter out = new PrintWriter("output.txt");
out.println("Hello, file!");
out.println("Score: " + 95);
out.close(); // MUST close to flush data
} catch (FileNotFoundException e) {
System.out.println("Cannot create file!");
}
}
}
Scanner vs BufferedReader
| Scanner | BufferedReader | |
|---|---|---|
| Package | java.util | java.io |
| Parses tokens? | Yes: nextInt(), nextDouble() | No: returns Strings only |
| Speed | Slower (regex parsing) | Faster (buffered reads) |
| Exception | FileNotFoundException | IOException |
| AP CSA Focus | ★ Primary | Good to know |
Handling Exceptions
Step-by-Step: Reading a File
1
Import java.io.File, java.util.Scanner, and java.io.FileNotFoundException.
2
Create a File object with the path: new File("data.txt")
3
Wrap it in a Scanner inside a try-catch block.
4
Loop with hasNextLine() or hasNext() to process each line or token.
5
Close the Scanner when done to release the file resource.
Example: Counting Lines
// Count the number of lines in a file
Scanner sc = new Scanner(new File("poem.txt"));
int count = 0;
while (sc.hasNextLine()) {
sc.nextLine(); // read and discard
count++;
}
sc.close();
System.out.println("Lines: " + count);
Example: Summing Numbers
Given a file scores.txt with one integer per line, compute the total:
Scanner sc = new Scanner(new File("scores.txt"));
int total = 0;
while (sc.hasNextInt()) {
total += sc.nextInt();
}
sc.close();
System.out.println("Total: " + total);
Example: Parsing CSV Data
Given students.csv with lines like "Alice,95,88,72", extract names and compute averages:
while (sc.hasNextLine()) {
String line = sc.nextLine();
String[] parts = line.split(",");
String name = parts[0];
double sum = 0;
for (int i = 1; i < parts.length; i++) {
sum += Integer.parseInt(parts[i]);
}
double avg = sum / (parts.length - 1);
System.out.println(name + ": " + avg);
}
Read, Process, Write
// Read numbers, double them, write to new file
Scanner in = new Scanner(new File("input.txt"));
PrintWriter out = new PrintWriter("output.txt");
while (in.hasNextInt()) {
int num = in.nextInt();
out.println(num * 2);
}
in.close();
out.close(); // Essential!
Key Scanner Methods
| Method | Returns | Use Case |
|---|---|---|
| nextLine() | String | Read an entire line |
| next() | String | Read next whitespace-delimited token |
| nextInt() | int | Read and parse an integer |
| nextDouble() | double | Read and parse a decimal |
| hasNextLine() | boolean | Check if more lines exist |
| hasNextInt() | boolean | Check if next token is an int |
Common Mistakes
✕ Forgetting close()
PrintWriter buffers output. Without close(), the file may be empty or incomplete.
✕ No try-catch
FileNotFoundException is checked: the code won't compile without handling it.
✕ Wrong file path
The file must be in the project root directory, not inside src/. Use an absolute path if unsure.
✕ Mixing nextLine() with nextInt()
After nextInt(), the newline stays in the buffer. A subsequent nextLine() reads an empty string.
✕ Reading past end of file
Calling nextLine() without checking hasNextLine() throws a NoSuchElementException.
Summary Table
| Task | Class | Key Method | Exception |
|---|---|---|---|
| Read text | Scanner | nextLine() | FileNotFoundException |
| Read numbers | Scanner | nextInt() / nextDouble() | FileNotFoundException |
| Write text | PrintWriter | println() / print() | FileNotFoundException |
| Parse tokens | String | split(",") | None (runtime errors) |
| Convert types | Integer / Double | parseInt() / parseDouble() | NumberFormatException |
Question 1 of 5
Which class is used to read from a text file in Java?
Scanner wraps a File object to read data line by line or token by token. PrintWriter is for writing, FileWriter is a lower-level writer, and System.in reads from the console, not a file.
Question 2 of 5
What happens if you forget to call close() on a PrintWriter?
PrintWriter buffers output in memory. Calling close() flushes the buffer to disk. Without it, data may never actually be written to the file, leaving it empty or incomplete.
Question 3 of 5
Consider the following code:
Scanner sc = new Scanner(new File("data.txt"));
while (sc.hasNextLine()) {
String line = sc.nextLine();
System.out.println(line.toUpperCase());
}
sc.close();
▼ Tap to read passage
What exception must be handled (or declared) for this code to compile?
Creating a Scanner with a File object can throw FileNotFoundException, which is a checked exception. The compiler requires you to handle it with try-catch or declare it with throws.
Question 4 of 5
A file numbers.txt contains: 10 20 30 on one line. What does the following code print?
hasNextInt() returns true for each space-separated integer. The loop reads 10, 20, and 30, adding each to sum: \( 10 + 20 + 30 = 60 \).
Question 5 of 5
What does String.split(",") return when called on the String "a,b,c"?
split(",") divides the string at each comma and returns a String[] (String array). Each segment between commas becomes one element: {"a", "b", "c"} with length 3.