Java and txt

Question

Hello!
I'm trying to write code, in order to be able to read/write from/to .txt files. I begun this morning and I've found some very useful things in the tutorials and the lists of this site. So I started writing code, but while everything's OK with Strings, I've encountered problems with mumbers.
To be more specefic:
public class TestForInt {
  public TestForInt() {
  public static void main(String args[]){
    try{
      DataOutputStream out = new DataOutputStream(new FileOutputStream("myTestText.txt", true));
      int test = 5;
      out.writeInt(test);
      out.writeChar('\r');
      out.close();
    }catch (IOException e){
      e.printStackTrace();
}I get nothing as a result. To be more accurate, I get an empty rectangle as If it didn't recognise the character. I tried both keys of the keyboard for inserting the number (I don't know if this could make any difference)! When I tried to write a double (5.5), I got as a result "@" and an empty rectange again!
I'm a bit puzzled, because I have no problems when writing strings and I can't think of anything else to do!
Thank you in advance for your time!
Athina
PS: Any suggestions on writing/reading .txt through java are welcome!

Answer 1

Hi,
Computers can only store numbers, so they don't know what a character is. In order to enable computers to store characters, every character has been assigned a numerical code called an 'ascii code'. You can look up the codes for all the characters in an ascii table. For instance, the characters a-z are represented by the codes 97-122. So, when you store a string in a file, what you are really storing are the ascii codes for each character in the string. For instance, if you store the string "ab" in a file, your file actually stores the numbers 97 and 98.
On the other hand, in Java there are two different ways to store a number, like 10:
1) you can store the number 10 as 10, or
2) you can store the number 10 as the characters '1' and '0'.
It may surprise you to learn that all characters have ascii codes--including digits. And, what may be more surprising is that the characters 0-9 don't have the ascii codes 0-9, instead the ascii codes for the characters 0-9 are 48-57. For instance, if you have a string like this:
"a10"
that will be stored in a file as 97, 49, and 48. However, if you have the number 10 and instruct Java to store it as a number, it will store 10 in the file. So, 10 can be stored two different ways in a file:
1) as the number 10
2) as the numbers 49 and 48
Java has different streams you can use for input and output. Strings are stored the same way no matter what stream you use, but some streams store numbers differently. So called "byte streams" (or binary streams) store numbers using method 1: the number 10 is stored as 10. On the other hand, "character streams" store numbers like 10 using method 2: the number 10 is stored as the ascii codes for the characters '1' and '0'.
Now, let's say you store 10 as a number in your file using method 1 where 10 is stored as 10. Then, suppose you try to display the file with a text editor. A text editor is programmed to interpret everything in a file as an ascii code. So, a text editor looks up the numbers it sees in a file in an ascii table and displays the corresponding character. So, when a text editor sees the number 10 that you stored in the file, it looks up 10 in an ascii table. The ascii code 10 happens to correspond to a line feed character, and the text editor displays a strange looking line feed character. The problem is: the text editor doesn't know that you stored 10 as a number. It assumes the 10 is an ascii code, and it looks it up in the table. A text editor doesn't realize that 10 was stored as a number using method 1.
The result is: you can't use a text editor to read a file that was created using a byte stream, which stores numbers as numbers. A text editor can only correctly interpret files which were created using a character stream where all the numbers in the file are ascii codes. Whenever a file has a mixture of numbers and ascii codes, which happens with byte streams, you will see a bunch of gibberish when you view the file in a text editor.