java - Recursively find all text files in directory

Question

I am trying to get the names of all of the text files in a directory. If the directory has subdirectories then I also want to get any text files in those as well. I am not sure how to make the process continue for any number of subdirectories.

Right now the code below just gets all the text files in the current directory and and subdirectories in the directory. For each subdirectory found, it also finds any text files and deeper subdirectories. The problem is that if those deeper subdirectories have yet deeper subdirectories then I am not finding all the text files. This seems to be a problem that requires recursion because I don't know how deep this will go.

Here is my code so far:

File rootDirectory = new File(rootDir);
if (rootDirectory.isDirectory()) {
    System.out.println("Valid directory");

    File[] listOfFiles = rootDirectory.listFiles(); 
    for (int i = 0; i < listOfFiles.length; i++) {
        String iName = listOfFiles[i].getName();
        if (listOfFiles[i].isFile()) {
            if (iName.endsWith(".txt") || iName.endsWith(".TXT")) {
                System.out.println("File: "+iName);
            }
        }
        if (listOfFiles[i].isDirectory()) {
            System.out.println("Directory: "+iName);

            File[] subList = listOfFiles[i].listFiles();
            for (int j = 0; j < subList.length; j++) {
                String jName = subList[j].getName();
                if (subList[j].isFile()) {
                    if (jName.endsWith(".txt") || jName.endsWith(".TXT")) {
                        System.out.println("File: "+jName);
                    }
                }
                if (subList[j].isDirectory()) {
                    System.out.println("Directory: "+jName);
                }
            }
        }
    }
}
else System.out.println("Invalid directory");

Edit: Got it working, thank you Olaf Dietsche:

public void findFiles(File root, int depth) {
    File[] listOfFiles = root.listFiles();
    for (int i = 0; i < listOfFiles.length; i++) {
        String iName = listOfFiles[i].getName();
        if (listOfFiles[i].isFile()) {
            if (iName.endsWith(".txt") || iName.endsWith(".TXT")) {
                for (int j = 0; j < depth; j++) System.out.print("");
                System.out.println("File: "+iName);
            }
        }
        else if (listOfFiles[i].isDirectory()) {
            for (int j = 0; j < depth; j++) System.out.print("");
            System.out.println("Directory: "+iName);
            findFiles(listOfFiles[i], depth+1);
        }
    }
}

Answer 1

This is a recursive problem

public void find_files(File root)
{
    File[] files = root.listFiles(); 
    for (File file : files) {
        if (file.isFile()) {
            ...
        } else if (file.isDirectory()) {
            find_files(file);
        }
    }
}