Thread: Copying from one file(source) to another file(destination)

Hello, please, I have a file(source.txt) that contains some texts. Now, the java code is to run through each line in the source file and check if that line begins with a number and if true, copy that line to the destination file(destination.txt). I have the code but I am not sure why I can't loop through each line in the source.txt file. Below is my java code:

 
public static void copyFile(File source, File destination) {
 
		BufferedReader br;
		try {
			FileWriter fw = new FileWriter(destination.getAbsoluteFile());
			PrintWriter bw = new PrintWriter(fw);
			br = new BufferedReader(new FileReader(source));
			String line = new String();
			while ((line = br.readLine()) != null) {
				String[] lineParts = line.split("\\s+");
				   if(lineParts[0].matches("\\d+")){
 
						bw.printf("%s" + "%n", line);
				   }
			}
			br.close();
			bw.close();
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
 
	}

As shown in the source.txt file, there are 4 lines in the file but when the execution gets to
while ((line = br.readLine()) != null) , it executes to false.. Anyone knows why this is so?

below is the source.txt file

12 is a number
Green is bad
5 is not so cool
you are right

Maybe this method here might help you:

Files.readAllLines(path)

It will read a file and return a List<String> of all lines.

Maybe it will make your life a little easier.

But why is it false? the source.txt has 4 lines. So, the loop should run atleast 4 times.

--- Update ---

I will try this but why is the readline not true?

Your method works fine just as it is in my environment. I suspect your input file is not being found.

As Norm suggested, add some print statements to the loop to figure out what's going on.

Hello, the problem is a bit strange as there are no errors. The only logical thing is in the method, this line is a suspect:

br = new BufferedReader(new FileReader(source));

because when I added println() round the br.readLine() it returned null which shouldn't be. How can I be sure it is really reading this source.txt file?

Below is my main function

 
public static void main(String[] args) {	
		//the source file
		File source = new File("source.txt");
		System.out.println(source.getAbsoluteFile());
 
		if (!source.exists()) {
			try {
				source.createNewFile();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
 
 
		//the destination file
		File destination = new File("destination.txt");
		if (!destination.exists()) {
			try {
				destination.createNewFile();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
 
 
 
		copyFile(source, destination);
	}

This is a good example of a try/catch that isn't doing you much good. I suspect the source file, "source.txt", isn't being found in the main() method so null is being passed to the method as the source file. Your if() statement above may not be helping at all or at best is creating an empty file, but I haven't completely worked through what's happening there.

How can I be sure it is really reading this source.txt file?

Ensure the File "source" is being created and pointing to the right resource. This may take some experimentation in your environment to determine how to specify the path to the desired file. For example, in my environment, using Eclipse on Linux, I have a folder named "files" at the same level as my /src directory. When I test a simple program that requires a text file, I put the text file in the folder "files" and specify the path to the file in the program as "./files/filename.txt". I don't know what will work in your environment, but you can figure it out.

Hi, ok I will try that . Please, this is just a side note. Imagine I have an array of integers with the following numbers {1,2,3,4} but I want to replace the 4 with ? so that in the end, the array looks something like this : {1,2,3,?}. Is this achievable? I just want to know if this can be done?

Thanks

All elements of an array must be of the same type.

So, is there no possible way to I could replace an integer value in an integer array to some character like "?" ? I have an array with the values {1,3,2,4} but want it to be {1,3,2,?} . Is this not possible by any means in Java?

You could chose one of the possible int values to represent a "?" and use that.

But how could I do this such that I could represent the 4 in the array with "?" and still use it in the integer array? I don't see this being logically right but I am asking to be sure that I am right this can't be done. The goal I am trying to achieve is to pass an array repeatedly into some method(acting like a machine). So, say you feed it with 5 integers{1,2,3,4,5} , it should replace the last number with a "?" so, after passing it through the machine for the first time, one gets this {1,2,3,4,?} now on a second pass, one gets this {1,2,3,?,?} and on a third pass {1,2,?,?,?}..and so on..until one has the whole array filled with "?" {?,?,?,?,?}. This is what I am trying to achieve but I noticed replacing an integer with "?" inside an array is not possible. So, could I be wrong?

There's always a way, and the way won't always (usually isn't) the first one you think of.

If the machine you describe is mostly for show, then it could return a String representation of the desired int array: "{1,2,3,4,?}", "{1,2,3,?,?}", and "{1,2,?,?,?}". If there's some function required from those values, then you may have to parse the String representations into the necessary values. You could also use an Object[] array that could contain both Integer and String objects, but that could get messy, depending on what you do next with it.

How the data is stored in your program doesn't have to be what you show the user.