File IO reader outside of main?

[nhmllr]

So I have a project with a few classes, including the one with the main() in it, but in my project I want to read from a .txt file from the user.

Here's a class (tested in a separate project) that I wrote to read a text file (it is the first I've ever made, so it might not be good)

import java.io.FileReader;

public class reader
{
	public static void main(String[] args) throws Exception
	{
		FileReader fr = new FileReader("textfile.txt");
		
		String string = "";
		
		int ch;
		do
		{
			ch = fr.read();
			if (ch != -1)
			{
				string += (char) ch;
			}
		} while (ch != -1);
		
		fr.close();
		
		System.out.println(string);
	}
}

Now, usually with multiclassed projects you can put what would be int he main() in the constructor, and have the main() in a different class that creates the new class. But that doesn't work here.

How can I get a text reader that reads the text in it's constructor and doesn't need the main?

[pbrockway2]

That program is more of a "text printer".

There are two ways to go. The first is just to call the main() method from wherever you like. main() is not special in any way.

public class MyOtherApp 
{
    public static void main(String[] args) throws exception 
    {
        reader.main();
    }
}

Note that reader should really be called Reader. And the .class file has to be accessible to the other program - since you are not using packages this means that Reader.class has to be in the same directory as MyOtherApp.class.

The other way is to create a class out of what you have. It could be given a constructor to make it a little more useful:

public class Cat 
{
    public Cat(String filename) throws Exception
    {
        String string = "";
         
        int ch;
        do
        {
            ch = fr.read();
            if (ch != -1)
            {
                string += (char) ch;
            }
        } while (ch != -1);
         
        fr.close();
         
        System.out.println(string);
    }
}

Then use this class as you would any other:

public class MyOtherApp 
{
    public static void main(String[] args) throws exception 
    {
        new Cat("anyfile.txt");
    }
}

-----

You're right, it's not such a good file printing program. (All those string concatenations!) But I guess that's no so much the point. Rather (1) main() is not special and (2) Build even small utilities as classes with well defined "functionality". What I mean is that it would have been better for the Cat constructor to just take the filename argument and store it as an instance variable. Then it could have a doCat() method or whatever to do the actual file printing. Or it could return the string. Or... anything else you wanted. Better to write the programs this way than shove everything into a main() method.

[JosAH]

Now, usually with multiclassed projects you can put what would be int he main() in the constructor, and have the main() in a different class that creates the new class. But that doesn't work here.

Why "doesn't it work"? Reading from a file isn't an exceptional action ...

kind regards,

Jos

[nhmllr]

Thanks for responding!
Your way does work really well.

I just have two questions:

Why do you need a "throws Exception" next to the main?
Is there a better way to read it than with concatenation? I knew it was a bad way when I did it...

[nhmllr]

To JoshAH:

I think it's just because I didn't have the "throws Exception" part next to the main

[pbrockway2]

Why do you need a "throws Exception" next to the main?

The question really is why did you have throws Exception in the original main(). I was only copying ;) I guess the reason is that reading can give rise to i/o exceptions (someone unplugs the drive etc), and you had declared the method to throw Exception rather than IOException. Either way once you declare a method or constructor to throw an exception like that it has to be caught (with try/catch) or the method using it has also to throw the exception.

It's explained in detail in the Exceptions chapter of Oracle's Tutorial.

Is there a better way to read it than with concatenation? I knew it was a bad way when I did it...

Again the best place to go would be the tutorial: in particular the Basic I/O material. Look for "buffered streams". Also find out about StringBuilder: the basic approach would be to read the file a line at a time and append the line to a string builder.