Gets all the unique characters from a string in Java

UniqueCharacters.java

// Find all unique characters in the string
String uniqueLettersInTestString = "";
for (int i = 0; i < testString.length(); i++) {
  if (!uniqueLettersInTestString.contains(String.valueOf(testString.charAt(i)))) {
    uniqueLettersInTestString = uniqueLettersInTestString + String.valueOf(testString.charAt(i));
  }
}
System.out.println("Unique Letters are: " + uniqueLettersInTestString);