One class calling another class

Hi I am new to this and I hope to be using this forum allot from now on.

My first question is that I created a little java class called ListFiles.class that when I run it gives me a output of a list of files using:

System.out.println(list);

The output looks like this:

C:\Users\Admin\Testing\bkdb.txt
C:\Users\Admin\Testing\FolderOne\FolderTwo\MainTex tFile.txt
C:\Users\Admin\Testing\FolderOne\MainImage.bmp
C:\Users\Admin\Testing\Main_Document.doc

What I want is for a second class (separate file) to use this output and write it to a txt_file.txt, preferably if read the txt_file.txt and if any line already exist then ignore it, only adding new lines.

Is that possible?

Please see api docs for FileWriter and BufferedWriter.

And you will have to apply logic to check the condition you mentioned.

For updating the txt_file.txt it's easier to create a new file and then replace the old one.
So that new class will contain one method that takes a List<String> as a parameter.
Then it will have a loop that reads all the lines in the old file into a List<String>.
After that, addAll the values in both Lists into a Set<String> and write out the results (Set's values) to the new file. Then rename the new file to the old file's name.

Quote:

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Originally Posted by r035198x

For updating the txt_file.txt it's easier to create a new file and then replace the old one.
So that new class will contain one method that takes a List<String> as a parameter.
Then it will have a loop that reads all the lines in the old file into a List<String>.
After that, addAll the values in both Lists into a Set<String> and write out the results (Set's values) to the new file. Then rename the new file to the old file's name.

---

Hi r035198x,

Thank you for your advise I do see the benefit on creating a new file and then replace the old one. Second part I will try to figure out as I only a shell scripter and trying to learn java while converting a few of my scripts :)

Thanks for all the help.

I think I am doing something wrong as the output is not what I expect, I am sorry for my stupidity but can some one please advise.

THIS IS THE RESULT WHEN I RUN THE FIRST CLASS

C:\>java -cp Details.jar packages.ListFiles
C:\temp\bkdb.txt
C:\temp\nbu6.5.6\BitmapImage.bmp
C:\temp\nbu6.5.6\Something.txt
C:\temp\New Bitmap Image.bmp
C:\temp\New Text Document.txt

NEW CLASS

Code:

---

package packages;

import java.util.List;
import packages.ListFiles;

public class Test {
 public static void main(String[] args) {

    List<String> ListFiles = null;

    for (String aList : ListFiles) {
    System.out.println(aList);
    }
  }
}

---

THIS IS THE RESULT WHEN I RUN THE SECOND CLASS

C:\>java -cp Details.jar packages.Test
Exception in thread "main" java.lang.NullPointerException
at packages.Test.main(Test.java:16)


I know I am duplicating work but this is just step one for me to understand how I can call one class with another and I would expect to be able to see the same result when I run both classes.

Any help would be greatly appreciated.

:)

You have set

Code:

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List<String> ListFiles = null;

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which is basically setting it to null you get Nullpointer problems when you try to use it like that.
How you call your other method depends on how it is written.
First change it so that instead of pronting the files to the console, it builds a List of the file and returns it.
Then if the method is static you can do

Code:

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List<String> listFiles = YourFirstClassName.yourMethodName();

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If it's not static you would need something like

Code:

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List<String> listFiles = new YourFirstClassName().yourMethodName();

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r035198x you being great and really helping and cheating me, unfortunately this is only a new hobbie for me so I am not going spend money on courses and just learn by playing/ trying with it.

Now, you mentioned that I had to change the first class which has taken me weeks to get it to work by looking at examples and adding / removing bit of code until it did what I wanted.

So I have another question then, can I take this code I have with a for loop on specific files and convert that to a method so that the second class can access it?

Code:

---


public static void main(String[] args) throws FileNotFoundException {
    // USING java.io.File
    File folder = new File("C:/temp");
    // USING java.util.List
      List<File> contents = ListFiles1.getFileListing(folder);


      // PRINT OUT THE LIST OF FILES


        for (File list : contents) {
            if (list.isFile() && list.getName().toLowerCase().endsWith(".txt")
                || list.getName().toLowerCase().endsWith(".bmp")
                || list.getName().toLowerCase().endsWith(".doc")
        )
        System.out.println(list);
        }
}

---

This is the end bit from the First Class ( ListFiles.class )

Sorry about the comments, I have put allot of comments in to help me to remind what is what :)