public static void main(String[] args) {
    String[] a = {"java","c++","c++","java","c#","bootstrap","java"};
    Set<String> nonDuplicateSet = new HashSet<String>();
    Set<String> duplicateSet = new HashSet<String>();
    for(String abc:a){
        if(!nonDuplicateSet.contains(abc)){
            nonDuplicateSet.add(abc);
        }else{
            duplicateSet.add(abc);
        }
    }
    System.out.println(duplicateSet);