Java Assignment Challange Question

[alanp]

Hi everyone, I have this "Challange Question" with my Java assignment, can anyone please tell me how to do it?

Note: This is should be a simple Command Line program, made up of only one class and compiled using javac.exe, run by java.exe from J2SDK

  Assignment Question said:

Write a program that prints out its own source code to the standard output. The printed version must be an exact duplicate of the original source code (in other words, if you saved the output into a file with the correct name, you could compile and run it exactly the same as original - and it would in turn produce the exact same output as the original).

It seems pretty hard to me :pinch:

Can anyone with some Java programming experience tell me how please!

Thanks in advanced!!

[Germano]

Do you have to do it with access to the original .java source file, or without it?

If you have the .java file, just open it, and output the content to the screen...

[Edit]Forget the text bellow, it doesn't give you the most important, the methods code...[/Edit]

  Quote

Start with the java.lang.Class class (http://java.sun.com/j2se/1.4/docs/api/java...lang/Class.html).

Get the needed one by doing this.getClass() (http://java.sun.com/j2se/1.4/docs/api/java...html#getClass()).

Then use the java.lang.reflect API (http://java.sun.com/j2se/1.4/docs/api/java...ge-summary.html).

Edited March 12, 2003 by Germano

[Germano]

To read a file, use a BufferedReader (http://java.sun.com/j2se/1.4/docs/api/java...eredReader.html)

BufferedReader buffer = new BufferedReader(new InputStreamReader(new FileInputStream(the_file)));

the_file should be the class name (maybe get it with the above this.getClass()) plus ".java".

This is an important link: http://java.sun.com/j2se/1.4/docs/api/index.html

[alanp]

Hi, thanks for replying so fast, but this should be done with some *simple* statments, so... reading the source file might not be simple enough... any other ideas?

Edited March 12, 2003 by alanp

[kjordan2001]

I don't see how you can get more simple than reading a file and outputting it to the screen. I'm not sure how complicated it is to reverse and get the source code from a .class file, but seems like reading and outputting to the screen is the simplest way.

[alanp]

Oh, it have the following hints:

  hint said:

The Unicode value for the double quotes is 34 (decimal). You can print out the

double quote character using (char)34 or '\u0022'. Your program will need to use

variables, and you are likely to find the information about double quotes very useful.

You will also have to use the String class, and in particular the substring method. This

program can be written using only a few statements.

By the way... this is Computer Science 101, and it's Assignmnet 1...

[kjordan2001]

Wow, sure seems like a lot for the first assignment in a low level course....but I suppose you've been talking over the concepts for weeks?

Yeah, and it really does only need a few statements. A read statement for the source file. An output statement (either screen or output file). And then closing those files. And any other output you want.

Example of Output:
PrintWriter fw = new PrintWriter(new FileWriter("C:\\testing.txt"));
fw.println("Hello world");
fw.close()

Example of Input:
BufferedReader input = new BufferedReader(new FileReader("C:\\testing.txt"));
String line = input.readLine();
while (input.ready()) {
    System.out.println(line);
 ? ?line = input.readLine();
}
input.close()

Edited March 12, 2003 by kjordan2001

[alanp]

Hi, thanks for the help,

I tried the following

import java.io.*;

public class printsource{

	public static void main(String[] args){

  BufferedReader input = new BufferedReader(new FileReader("printsource.java"));

  String line = input.readLine();

  while (input.ready()) {

     System.out.println(line);
     line = input.readLine();

  }

  input.close();


	}

}

but I get errors like "unreported exception java.io.IOException; must be caught or declared to be thrown" when compiling, canyou tell me what's wrong please?

[kjordan2001]

Well, there's a couple ways to handle that....

Put:

public void myMethod() throws IOException {

or

try {

line = input.readLine();

}

catch (IOException e) {

System.out.println("Invalid");

}

For something like this where you have multiple lines where you're getting input, I like throws IOException, but sometimes when an error comes up, you're expected to do something about it. That's where the try catch stuff comes in.

But if you're calling a method that throws an IOException, where you call it needs a try catch.

Ex from Sun's Java Tutorial:

import java.io.*;

public class Copy {
 ? ?public static void main(String[] args) throws IOException {
	File inputFile = new File("farrago.txt");
	File outputFile = new File("outagain.txt");

 ? ? ? ?FileReader in = new FileReader(inputFile);
 ? ? ? ?FileWriter out = new FileWriter(outputFile);
 ? ? ? ?int c;

 ? ? ? ?while ((c = in.read()) != -1)
 ? ? ? ? ? out.write(c);

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

That incorporates both input and output. You need IOException handling of some sort with both input and output. The throws IOException handles any errors you might get. And since this is main, Java will handle the errors by printing out a long message since Java calls main.

More on IO http://java.sun.com/docs/books/tutorial/es...l/io/index.html

Edited March 12, 2003 by kjordan2001

[Germano]

Well, don't help him too much, he must learn...

Exception handling is something he must understand by himself what it is, and for what it is for.

Just my opinion...

[mackol]

import java.io.*;

public class ReadSource {
    public static void main(String[] arguments) {
        try {
            FileReader file = new
                FileReader("ReadSource.java");
            BufferedReader buff = new
                BufferedReader(file);
            boolean eof = false;
            while (!eof) {
                String line = buff.readLine();
                if (line == null)
                   eof = true;
                else
                    System.out.println(line);
            }
            buff.close();
        } catch (IOException e) {
            System.out.println("Error + e.toString());
        }
    }
}

here you go. it works for sure . at least it did for me. this is the code but whether u wanna learn and udnerstand it is up to u :)

[Germano]

import java.io.*;

public class ReadSource {
 ? public static void main(String[] arguments) {
 ? ? ? try {
 ? ? ? ? ? BufferedReader buff = new BufferedReader(new FileReader("ReadSource.java"));
 ? ? ? ? ? for(String line = buff.readLine(); line != null; line = buff.readLine())
 ? ? ? ? ? ? ? System.out.println(line);
 ? ? ? ? ? buff.close();
 ? ? ? } catch (IOException e) {
 ? ? ? ? ? System.out.println("Error + e.toString());
 ? ? :p }
 ? }
}

:p Less lines.

[kjordan2001]

  Germano said:

Well, don't help him too much, he must learn...

Exception handling is something he must understand by himself what it is, and for what it is for.

Just my opinion...

Just showing him stuff he could easily find on the java tutorial.

[kjordan2001]

Yep, line != null is another way, but I like the isready() method :yes:

[Germano]

  kjordan2001 said:

  Germano said:

Well, don't help him too much, he must learn...

Exception handling is something he must understand by himself what it is, and for what it is for.

Just my opinion...

Just showing him stuff he could easily find on the java tutorial.

Yes, you're right. I didn't notice the java tutorial link.

My experience indicates that absentee of documentation reading is the most common newbie problem. So I tend to always give links.

Also, programmers must learn to search by themselves solutions and examples in the internet. Google is one of the most powerful tools of a developer.

[kjordan2001]

  Germano said:

  kjordan2001 said:

  Germano said:

Well, don't help him too much, he must learn...

Exception handling is something he must understand by himself what it is, and for what it is for.

Just my opinion...

Just showing him stuff he could easily find on the java tutorial.

Yes, you're right. I didn't notice the java tutorial link.

My experience indicates that absentee of documentation reading is the most common newbie problem. So I tend to always give links.

Also, programmers must learn to search by themselves solutions and examples in the internet. Google is one of the most powerful tools of a developer.

Yeah, and Sun's tutorial and the API are a Java programmer's best friends, get to know them well if you're using java.

[alanp]

You guys are a great help! Thanks alot!!! :)

[Germano]

  Quote

Write a program that prints out its own source code to the standard output. The printed version must be an exact duplicate of the original source code (in other words, if you saved the output into a file with the correct name, you could compile and run it exactly the same as original - and it would in turn produce the exact same output as the original).

The Unicode value for the double quotes is 34 (decimal). You can print out the double quote character using (char)34 or '\u0022'. Your program will need to use variables, and you are likely to find the information about double quotes very useful. You will also have to use the String class, and in particular the substring method. This program can be written using only a few statements.

I think the source .java file isn't available. Notice "if you saved the output into a file with the correct name"... Doing that wouldn't be possible, with the .java on the path.

Being this the first assignment, I suppose the output is to be "hard coded"...

A String variable would contain the class code... Some double quotes would be necessary (why not use \" ?).

The problem would be that we would never stop from rewriting the source code, one inside the other, inside the other, inside the other?

This is called a Self Generating Program (or Self Reproducing Program).

The solution will have to use substrings? Well, looks like this way we accomplish all the assignment requiremen:). :)

Now, should we give him the source code?

Edited March 13, 2003 by Germano

[alanp]

  Germano said:

  Quote

Write a program that prints out its own source code to the standard output. The printed version must be an exact duplicate of the original source code (in other words, if you saved the output into a file with the correct name, you could compile and run it exactly the same as original - and it would in turn produce the exact same output as the original).

The Unicode value for the double quotes is 34 (decimal). You can print out the double quote character using (char)34 or '\u0022'. Your program will need to use variables, and you are likely to find the information about double quotes very useful. You will also have to use the String class, and in particular the substring method. This program can be written using only a few statements.

I think the source .java file isn't available. Notice "if you saved the output into a file with the correct name"... Doing that wouldn't be possible, with the .java on the path.

Being this the first assignment, I suppose the output is to be "hard coded"...

A String variable would contain the class code... Some double quotes would be necessary (why not use \" ?).

The problem would be that we would never stop from rewriting the source code, one inside the other, inside the other, inside the other?

This is called a Self Generating Program (or Self Reproducing Program).

The solution will have to use substrings?Well, looks like this way we accomplish all the assignment requirements. :)

Now, should we give him the source code?

yeah, I think printing out the source file is not what they really wanted... been trying to figure out how to do it other ways...

[alanp]

Hey, I got it!

public class Ass01Optional{

	public static void main(String args[]){

	String a="public class Ass01Optional{public static void main(String args[]){String a=;System.out.println(a.substring(0,56)+((char)34)+a+((char)34)+a.substring(56));}}";

	System.out.println(a.substring(0,75)+((char)34)+a+((char)34)+a.substring(75));

	}
}

well, just gotta get the numbers right :cool:

Edited March 13, 2003 by alanp

[kjordan2001]

Hmm, now I'm just confused on what your assignment was....I figured it was read in your .java file and do something with it, but seems like you're just hard coding the stuff into a string. Well, at least that's easier if that's what the program is supposed to do.