Text files: write and read with Scanner

Reading and writing text files

• A text file stores text that stays on disk after the program ends.
• To read a file in Java we use a Scanner connected to a File.
• To write a file we use a PrintWriter. In this lesson each program writes a file, then reads it back.
• Bonus lesson: the AP CSA exam does not test file input/output. Learn this for real programs — you will not be examined on it.

The imports you need

• File tools live in two places, so we import both at the top.
• import java.io.*; — for File, PrintWriter, and IOException.
• import java.util.Scanner; — for the Scanner reader.

import java.io.*;
import java.util.Scanner;

Files can fail, so we say throws IOException

• Reading or writing a file can go wrong (the file may be missing).
• Java makes you handle this. The simplest way is to add throws IOException to main.
• This says "if a file error happens, let the program stop". That is fine for these tasks.

Writing a file with PrintWriter

• new PrintWriter("data.txt") makes (or empties) a file called data.txt.
• Use writer.println(...) to write one line, just like printing to the screen.
• Always call writer.close() when you finish, so the text is saved.

import java.io.*;

public class WriteDemo {
    public static void main(String[] args) throws IOException {
        PrintWriter writer = new PrintWriter("data.txt");
        writer.println(10);
        writer.println(20);
        writer.close();           // save the file
        System.out.println("wrote data.txt");
    }
}

Reading numbers with Scanner

• new Scanner(new File("data.txt")) opens the file for reading.
• in.hasNextInt() is true while there is another whole number to read.
• in.nextInt() reads the next whole number. Call in.close() when done.

import java.io.*;
import java.util.Scanner;

public class ReadNumbers {
    public static void main(String[] args) throws IOException {
        Scanner in = new Scanner(new File("data.txt"));
        int total = 0;
        while (in.hasNextInt()) {     // more numbers?
            total = total + in.nextInt();
        }
        in.close();
        System.out.println(total);
    }
}

Reading whole lines

• Sometimes a line is text, not a number. Then read lines instead.
• in.hasNextLine() is true while there is another line.
• in.nextLine() reads the whole next line as a String.

import java.io.*;
import java.util.Scanner;

public class ReadLines {
    public static void main(String[] args) throws IOException {
        Scanner in = new Scanner(new File("data.txt"));
        while (in.hasNextLine()) {
            String line = in.nextLine();
            System.out.println("read: " + line);
        }
        in.close();
    }
}

• hasNext() (no Int or Line) checks for the next word, and next() reads one word.
• The pattern is always: check with a hasNext... method, then read with the matching method.

Write then read in one program

• In each task your program first writes a file, then reads the same file back.
• This means you can run it again and again and get the same result.

import java.io.*;
import java.util.Scanner;

public class RoundTrip {
    public static void main(String[] args) throws IOException {
        PrintWriter writer = new PrintWriter("data.txt");
        writer.println(3);
        writer.println(4);
        writer.close();

        Scanner in = new Scanner(new File("data.txt"));
        int total = 0;
        while (in.hasNextInt()) {
            total = total + in.nextInt();
        }
        in.close();
        System.out.println(total);   // 7
    }
}

Common mistakes

• nextInt() leaves the newline, so a following nextLine() reads an empty line.
• Check hasNext() before reading to avoid running off the end.

Now you try

• Each task pre-fills a class. Write the file first, then read it back, as the prompt asks.
• Remember the throws IOException on main, and close() both the writer and the Scanner.
• Press Run to compile and run, then Check answer.