First of all you can not have a string as you posted in question

String fname="C:\textfiles\db\query\query.txt";

this should be replaced by

String fname="C:\\textfiles\\db\\query\\query.txt";

as backslash("\") needs an escape as well.

Finally you need to do something like this to split them:

 String fname="C:\\textfiles\\db\\query\\query.txt";
 String[] items= fname.split("\\\\");
 System.out.println(Arrays.toString(items));

Hope this helps.

Just to add on

\ is a special character in regular expressions, as well as in Java string literals. If you want a literal backslash in a regex, you have to double it twice.

When you type "\\", this is actually a single backslash (due to escaping special characters in Java Strings).

Regular expressions also use backslash as special character, and you need to escape it with another backslash. So in the end, you need to pass "\\" as pattern to match a single backslash.

 public static void main(String[] args) {
        String taskOwner = "test\\00216243";
        String taskArray[] = taskOwner.split("\\\\");
        System.out.println(taskArray[0]);
    }

output

test
00216243

Oh no... please don't start messing with Windows filenames. One thing you don't want is to have platform-dependent code. Rather than this, use standard Java library:

System.out.println(new File("C:\\Users\\Owner\\Desktop\\foo.txt").getName());

Finally, if you really had to parse the path manually, I would use File.separatorChar to make the code portable.

// hardcoded here for the example, but you would actually get it from somewhere
String path = "C:\\Users\\Owner\\Desktop\\foo.txt"; 

int i = path.lastIndexOf(File.separatorChar);
String last = i < 0 || i == s.length() ? "" : path.substring(i + 1);
System.out.println(last);

This is also less expensive than splitting a string since you're only interested in the last element.

In regex-land, a \ is an escape character, so to obtain a literal \ we need to escape it: \\. However, in Java strings, \ is also an escape character, so we need to escape each \ a second time, resulting in \\\\. Therefore, this is what you want:

str.split("\\\\")

Don't split. Just parse:

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy\\MM\\dd");
LocalDate dPresent = LocalDate.parse(datum, formatter);

My guess is that you are missing that backslash ('\') characters are escape characters in Java String literals. So when you want to use a '\' escape in a regex written as a Java String you need to escape it; e.g.

Pattern.compile("\.");   // Java syntax error

// A regex that matches a (any) character
Pattern.compile(".");  

// A regex that matches a literal '.' character
Pattern.compile("\\.");  

// A regex that matches a literal '\' followed by one character
Pattern.compile("\\\\.");

The String.split(String separatorRegex) method splits a String into substrings separated by substrings matching the regex. So str.split("\\.") will split str into substrings separated by a single literal '.' character.