How to detect duplicate words from a String in Java?

Question

What are the ways by which duplicate word in a String can be detected?

e.g. \"this is a test message for duplicate test\" contains one duplicate word test.

Answer 1

The following Java code resolves the problem of detecting duplicates from a String. There should not be any problem if the duplicate word is separated by newline or punctuation symbols.

    String duplicatePattern = "(?i)\\b(\\w+)\\b[\\w\\W]*\\b\\1\\b";
    Pattern p = Pattern.compile(duplicatePattern);
    String phrase = "this is#$;%@;<>?|\\` p is a is Test\n of duplicate test";
    Matcher m = p.matcher(phrase);
    String val = null;
    while (m.find()) {
        val = m.group();
        System.out.println("Matching segment is \"" + val + "\"");
        System.out.println("Duplicate word: " + m.group(1)+ "\n");
    }

The output of the code will be:

Matching segment is "is#$;%@;<>?|\` p is a is"
Duplicate word: is

Matching segment is "Test
 of duplicate test"
Duplicate word: Test

Here, m.group(1) statement represents the String matched against 1st group of Pattern [here, it's (\\w+)].

Answer 2

The best you can do with regexes is O(N^2) search complexity. You can easily achieve O(N) time and space search complexity by splitting the input into words and using a HashSet to detect duplicates.