4.6 Using Text Files

1. Files and the classes that open them

A file is storage for data that persists when the program is not running, and a program can retrieve that data while it runs 4.6.A.1. A variable disappears when the program ends; a file such as scores.txt stays on the disk.

A file is connected to a program with two classes: File and Scanner 4.6.A.2. You open a file by creating a File object, with the file name as the argument of the constructor File(String str), where str is the pathname of the file 4.6.A.3. Then you pass that File to the Scanner(File f) constructor, which creates a Scanner that reads from it 4.6.A.6.

Step	Code	What it does
1. Name the file	File f = new File("scores.txt");	Connects a File object to the file name
2. Make a reader	Scanner input = new Scanner(f);	A Scanner that reads from that file
3. Read	input.nextInt(), input.next(), …	Returns the next piece of data
4. Close	input.close();	Ends the connection

File and IOException are in the java.io package, so an import statement must make them available 4.6.A.5. Scanner is in java.util, so it needs an import too.

2. throws IOException

When you use the File class, Java requires you to say what to do if the file with that name cannot be opened. One way is to add throws IOException to the header of the method that uses the file 4.6.A.4. If the file name is invalid, the program then terminates 4.6.A.4.

The data file used in this section, scores.txt:

12 7 25
9 14
30

A complete program that reads the first two numbers:

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

public class FirstScores
{
    public static void main(String[] args) throws IOException
    {
        File f = new File("scores.txt");
        Scanner input = new Scanner(f);

        int a = input.nextInt();
        int b = input.nextInt();
        System.out.println(a + b);

        input.close();
    }
}

Output:

19

Real view: forget throws IOException and the code does not compile.

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

public class NoThrows
{
    public static void main(String[] args)
    {
        Scanner input = new Scanner(new File("scores.txt"));
        System.out.println(input.nextInt());
        input.close();
    }
}

Compiler error:

error: unreported exception FileNotFoundException; must be caught or declared to be thrown

A FileNotFoundException is a kind of IOException, which is why throws IOException covers it. Forget the import for File and you get a different compiler error:

import java.io.IOException;
import java.util.Scanner;

public class NoImport
{
    public static void main(String[] args) throws IOException
    {
        Scanner input = new Scanner(new File("scores.txt"));
        input.close();
    }
}

Compiler error:

error: cannot find symbol

With everything correct but a file name that does not exist (here score.txt, a typo), the program compiles and then terminates when it runs:

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

public class Typo
{
    public static void main(String[] args) throws IOException
    {
        Scanner input = new Scanner(new File("score.txt"));
        System.out.println("opened");
        input.close();
    }
}

Output:

Exception in thread "main" java.io.FileNotFoundException: score.txt (No such file or directory)

“opened” is never printed. (On some operating systems the message in brackets is worded differently.)

3. Scanner methods

These Scanner methods are in the Java Quick Reference 4.6.A.6. A token is a piece of text separated from the next one by whitespace (spaces, tabs or line breaks).

Method	Returns	If it goes wrong
int nextInt()	the next int	InputMismatchException if the next token is not an int or is out of range
double nextDouble()	the next double	InputMismatchException if the next token is not a double
boolean nextBoolean()	the next boolean (true or false)	InputMismatchException if it is not a boolean
String next()	the next token as a String
String nextLine()	the rest of the current line as a String (can be the empty string right after another Scanner method)
boolean hasNext()	true if there is another item to read, false otherwise
void close()	nothing; closes the scanner

The data file teams.txt, one team per line (name, wins, average goals, qualified):

Lions 3 4.5 true
Hawks 1 2.0 false

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

public class Teams
{
    public static void main(String[] args) throws IOException
    {
        Scanner input = new Scanner(new File("teams.txt"));

        String name = input.next();
        int wins = input.nextInt();
        double avg = input.nextDouble();
        boolean qualified = input.nextBoolean();

        System.out.println(name + " won " + wins + " games");
        System.out.println("Average goals: " + avg);
        System.out.println("Qualified: " + qualified);
        System.out.println("Next team: " + input.next());

        input.close();
    }
}

Output:

Lions won 3 games
Average goals: 4.5
Qualified: true
Next team: Hawks

Notice that the line break after true does not matter to next(): it is just whitespace between tokens.

When a token does not match the method, the program stops with an InputMismatchException 4.6.A.6. The file laps.txt contains 8 5 abc 4:

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

public class Laps
{
    public static void main(String[] args) throws IOException
    {
        Scanner input = new Scanner(new File("laps.txt"));
        int total = 0;
        while (input.hasNext())
        {
            total += input.nextInt();
            System.out.println("Total so far: " + total);
        }
        input.close();
    }
}

Output:

Total so far: 8
Total so far: 13
Exception in thread "main" java.util.InputMismatchException

hasNext() was true (the token abc exists), but nextInt() cannot turn it into an int.

Background: keyboard input is outside the scope of the AP® Computer Science A course and exam 4.6.A.6. Also, nextLine handles whitespace differently from the other methods, so mixing nextLine with nextInt, next and the others on the same file sometimes needs extra code to adjust 4.6.A.7. Writing or analyzing code that mixes them is outside the scope of the exam 4.6.A.7. A safe rule: either read tokens (next, nextInt, …) or read whole lines (nextLine), not both.

4. Reading until the end with hasNext

A while loop can keep reading as long as the file still contains items, using hasNext as the loop condition 4.6.A.9. This is the standard file traversal:

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

public class SumScores
{
    public static void main(String[] args) throws IOException
    {
        Scanner input = new Scanner(new File("scores.txt"));
        int sum = 0;
        int count = 0;

        while (input.hasNext())
        {
            int s = input.nextInt();
            sum += s;
            count++;
        }
        input.close();

        System.out.println(count + " scores, sum " + sum);
        System.out.println("Average: " + (double) sum / count);
    }
}

Output:

6 scores, sum 97
Average: 16.166666666666668

The numbers were on three lines in scores.txt, but the loop does not care: it reads token after token until hasNext() returns false. Calling nextInt() again after that would throw a NoSuchElementException.

When you are finished with the file, close it with the close method from Scanner 4.6.A.10. Closing releases the file for other programs and makes clear the reading is over.

5. Reading lines and splitting them

Many data files store one record per line, with the values separated by commas. Read each whole line with nextLine() and break it into parts with the String method split 4.6.A.8: String[] split(String del) returns a String array where each element is a substring of the original, split around matches of del 4.6.A.8.

The data file roster.txt:

Ana,12,true
Ben,9,false
Cleo,15,true

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

public class Roster
{
    public static void main(String[] args) throws IOException
    {
        Scanner input = new Scanner(new File("roster.txt"));

        while (input.hasNext())
        {
            String line = input.nextLine();
            String[] parts = line.split(",");
            System.out.println(parts[0] + " has " + parts.length + " fields; points = " + parts[1]);
        }
        input.close();
    }
}

Output:

Ana has 3 fields; points = 12
Ben has 3 fields; points = 9
Cleo has 3 fields; points = 15

For the line "Ana,12,true", split(",") gives this array:

index	0	1	2
parts	“Ana”	“12”	“true”

Every element is a String, even "12". To do arithmetic with it, convert it with Integer.parseInt (topic 4.7).

String date = "2026 11 12";
String[] pieces = date.split(" ");
System.out.println(pieces.length);
System.out.println(pieces[1] + pieces[2]);

Output:

3
1112

pieces[1] + pieces[2] joins two strings, so the result is 1112, not 23.

Background: the del argument is a regular expression, a pattern language. Some characters such as . and * have special meanings in it. Code that uses those special properties is outside the scope of the AP® exam 4.6.A.8, so on the exam expect simple separators like ",", " " or ":".