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Duplicate characters in a string are characters that appear multiple times. These characters can be found using a nested loop or some other ways

Java program to find all duplicate characters in a string

public class DuplicateCharacter {
  public static void main(String arg[]) {
    String originalstring = "Java Proedu Questions";
    char[] originalCharArray = originalstring.toCharArray();
    System.out.println("The string is:" + originalstring);
    System.out.print("Duplicate Characters in string are: ");
    for (int i = 0; i < originalstring.length(); i++) {
      for (int j = i + 1; j < originalstring.length(); j++) {
        if (originalCharArray[i] == originalCharArray[j]) {
          System.out.print(originalCharArray[j] + " ");
          break;
        }
      }
    }
  }
}

Output

The string is:Java Proedu Questions
Duplicate Characters in string are: a o e u s

Java program to find all duplicate characters in a string using another way

public class DuplicateCharacter {
  public static void main(String[] args) {
    String originalString = "Java Proedu Questions";
    int count;

    char originalChar[] = originalString.toCharArray();

    System.out.println("Duplicate characters in a given string: ");

    for (int i = 0; i < originalChar.length; i++) {
      count = 1;
      for (int j = i + 1; j < originalChar.length; j++) {
        if (originalChar[i] == originalChar[j] && originalChar[i] != ' ') {
          count++;
          // Set string[j] to 0 to avoid printing visited character
          originalChar[j] = '0';
        }
      }
     
      if (count > 1 && originalChar[i] != '0')
        System.out.print(originalChar[i]+" ");
    }
  }
}

Output

Duplicate characters in a given string:
a o e u s