There are several ways to find duplicate characters in a string in Java. Here are a few examples:

1. 1. Using a nested loop:

2. public class DuplicateCharacters {
       public static void main(String[] args) {
           String str = "hello world";
           int count;
   
           // Convert the string to lowercase to make the comparison case-insensitive
           str = str.toLowerCase();
   
           // Convert the string to a char array
           char[] charArray = str.toCharArray();
   
           System.out.println("Duplicate characters in the string: ");
   
           // Iterate over each character in the string
           for (int i = 0; i < charArray.length; i++) {
               count = 1;
               // Compare each character with the remaining characters in the string
               for (int j = i + 1; j < charArray.length; j++) {
                   if (charArray[i] == charArray[j] && charArray[i] != ' ') {
                       count++;
                       // Set the character to '\0' to mark it as visited and avoid counting it again
                       charArray[j] = '\0';
                   }
               }
               // If a character is found to be a duplicate, print it
               if (count > 1 && charArray[i] != '\0')
                   System.out.println(charArray[i]);
           }
       }
   }

2. 2. Using a HashMap:
3. 
   

4. import java.util.HashMap;
   import java.util.Map;
   
   public class DuplicateCharacters {
       public static void main(String[] args) {
           String str = "hello world";
           str = str.toLowerCase();
           char[] charArray = str.toCharArray();
   
           Map<Character, Integer> charCountMap = new HashMap<>();
   
           for (char c : charArray) {
               if (charCountMap.containsKey(c)) {
                   charCountMap.put(c, charCountMap.get(c) + 1);
               } else {
                   charCountMap.put(c, 1);
               }
           }
   
           System.out.println("Duplicate characters in the string: ");
   
           for (Map.Entry<Character, Integer> entry : charCountMap.entrySet()) {
               if (entry.getValue() > 1 && entry.getKey() != ' ') {
                   System.out.println(entry.getKey());
               }
           }
       }
   }

3. 3. Using Java 8 Streams:
4. 
   

5. public class DuplicateCharacters {
       public static void main(String[] args) {
           String str = "hello world";
           str = str.toLowerCase();
           str.chars()
                   .mapToObj(c -> (char) c)
                   .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
                   .entrySet()
                   .stream()
                   .filter(e -> e.getValue() > 1 && e.getKey() != ' ')
                   .forEach(e -> System.out.println(e.getKey()));
       }
   }

All three of these programs produce the same output, which is a list of the duplicate characters in the input string:

Duplicate characters in the string:
l
o