converting java.nio.file.Path to File got failed

Path#toString() won't return the path with forward slashes (/) like the string path you are comparing to. It returns it with back-slashes (\) so your comparison won't ever become true.

When reading paths from the local file System always use the path or file separator related to the System. To do this you can use the String#replaceAll() method to convert any path name slashes to what is appropriate for the file system your code is operating on, for example:

String pathToCompare = "BOOT-INF/classes/certs"
                       .replaceAll("[\\/]", "\\" + File.separator);

This should work fine:

private List<java.io.File> doSomething(java.nio.file.Path p) {
    java.util.List<java.io.File> FileList = new java.util.ArrayList<>();
    try {
        int level = 1;
        String pathToCompare = "BOOT-INF/classes/certs"
                               .replaceAll("[\\/]", "\\" + File.separator);
        if (p.toString().equals(pathToCompare)) {
            level = 2;
            java.util.stream.Stream<Path> walk = java.nio.file.Files.walk(p, level);
            for (java.util.Iterator<Path> it = walk.iterator(); it.hasNext();) {
                System.out.println(it.next());//getting all .cer files in the folder
                FileList.add(it.next().toFile());//getting an error UnsupportedOperationException Path not associated with
            }
        }
    }
    catch (Exception e) {
        System.err.println(e);
    }
    return FileList;
}

EDIT:

If you want to base your listing on specific file extensions and or the contents of a JAR file then you will need to do something a little different. The code below is very similar to the code you have been using with the exception that:

• the method name is changed to getFilesList();
• the listing is returned as List<String> instead of List<File>;
• the depth level is now an argument supplied to the method (always make sure the depth Level is adequate enough to carry out the task);
• An optional String args argument (named: onlyExtensions) has been added to the method so that one (or more) file name extensions can be applied to return a list that contains only paths where the file names contain the extension(s) applied. If an extension supplied happens to be ".jar" then the contents of that JAR file will also be applied to the returned List. If nothing is supplied then all files are returned in the list.

For JAR files a helper method is also provided:

Modify any of this code as you see fit:

public static List<String> getFilesList(String thePath, int depthLevel, String... onlyExtensions) {
    Path p = Paths.get(thePath);
    java.util.List<String> FileList = new java.util.ArrayList<>();
    try {
        java.util.stream.Stream<Path> walk = java.nio.file.Files.walk(p, depthLevel);
        for (java.util.Iterator<Path> it = walk.iterator(); it.hasNext();) {
            File theFile = it.next().toFile();
            if (onlyExtensions.length > 0) {
                for (String ext : onlyExtensions) {
                    ext = ext.trim();
                    if (!ext.startsWith(".")) {
                        ext = "." + ext;
                    }
                    if (!theFile.isDirectory() && theFile.getName().substring(theFile.getName().lastIndexOf(".")).equalsIgnoreCase(ext)) {
                        FileList.add(theFile.getName() + " --> " + theFile.getAbsolutePath());
                    }
                    else if (!theFile.isDirectory() && theFile.getName().substring(theFile.getName().lastIndexOf(".")).equalsIgnoreCase(".jar")) {
                        List<String> jarList = getFilesNamesFromJAR(theFile.getAbsolutePath());
                        for (String strg : jarList) {
                            FileList.add(theFile.getName() + " --> " + strg);
                        }
                    }
                }
            }
            else {
                FileList.add(theFile.getAbsolutePath());
            }
        }
    }
    catch (Exception e) {
        System.err.println(e);
    }
    return FileList;
}

The JAR file helper method:

public static java.util.List<String> getFilesNamesFromJAR(String jarFilePath) {
    java.util.List<String> fileNames = new java.util.ArrayList<>();
    java.util.zip.ZipInputStream zip = null;
    try {
        zip = new java.util.zip.ZipInputStream(new java.io.FileInputStream(jarFilePath));
        for (java.util.zip.ZipEntry entry = zip.getNextEntry(); entry != null; entry = zip.getNextEntry()) {
            fileNames.add(entry.getName());
        }
    }
    catch (java.io.FileNotFoundException ex) {
        System.err.println(ex);
    }
    catch (java.io.IOException ex) {
        System.err.println(ex);
    }
    finally {
        try {
            if (zip != null) {
                zip.close();
            }
        }
        catch (IOException ex) {
            System.err.println(ex);
        }
    }
    return fileNames;
}

To use the getFileList() method you might do something like this:

List<String> fileNames = getFilesList("C:\\MyDataFolder", 3, ".cer", ".jar");

// Display files in fileNames List
for (String str : fileNames) {
    System.out.println(str);
}