如何添加从文件读取的字符串到ArrayList？

Question

I have a super beginner's question. I have a computer science test today and one of the practice problems is this:

• Write a program that carries out the following tasks:
• Open a file with the name hello.txt.
• Store the message “Hello, World!” in the file.
• Close the file.
• Open the same file again.
• Read the message into a string variable and print it.

This is the code I have for it so far:

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

public class ReadFile
{
    public static void main(String[] args) throws FileNotFoundException
    {
        PrintWriter out = new PrintWriter("hello.txt");
        out.println("Hello, World");
        File readFile = new File("hello.txt");
        Scanner in = new Scanner(readFile);
        ArrayList<String> x = new ArrayList<String>();
        int y = 0;

        while (in.hasNext())
        {
            x.add(in.next());
            y++;
        }

        if (x.size() == 0)
        {
            System.out.println("Empty.");
        }
        else
        {
            System.out.println(x.get(y));
        }

        in.close();
        out.close();     
    }
}

What's wrong with this code?

Answer 1

1) You need to close the stream

2) You need to refer to the x Arraylist with (y-1) otherwise you will get a java.lang.IndexOutOfBoundsException . The indexes starts from 0 and not from 1.

http://www.tutorialspoint.com/java/util/arraylist_get.htm

   public static void main(String[] args) throws FileNotFoundException
        {
            PrintWriter out = new PrintWriter("hello.txt");
            out.println("Hello, World");
            out.close();
            File readFile = new File("hello.txt");
            Scanner in = new Scanner(readFile);
            ArrayList<String> x = new ArrayList<String>();
            int y = 0;

            while (in.hasNext())
            {
                x.add(in.next());
                y++;
            }

            in.close();  

            if (x.size() == 0)
            {
                System.out.println("Empty.");
            }
            else
            {
                System.out.println(x.get(y-1));
            }

        }
    }

Answer 2

I guess what's wrong with the code ist that you cant read anything from the file.

this is because PrintWriter is buffered

fileName - The name of the file to use as the destination of this writer. If the file exists then it will be truncated to zero size; otherwise, a new file will be created. The output will be written to the file and is buffered .

You need to close the file you have just writen to before openning it for reading so that the changes are fluched to the physical storage. Thus moving out.close(); right after out.println("Hello, World");

Answer 3

class FileWritingDemo {
public static void main(String [] args) {
char[] in = new char[13]; // to store input
int size = 0;
try {
File file = new File("MyFile.txt"); // just an object

FileWriter fw = new FileWriter(file); // create an actual file & a FileWriter obj
fw.write("Hello, World!"); // write characters to the file
fw.flush(); // flush before closing
fw.close(); // close file when done

FileReader fr = new FileReader(file); // create a FileReader object
size = fr.read(in); // read the whole file!
for(char c : in) // print the array
System.out.print(c);
fr.close(); // again, always close
} catch(IOException e) { }
}
}