String str1 = "ABCDABCD";
String result1 = "";

for (int a = 0; a <= str1.length()-1; a++) {
if (result1.contains("" + str1.charAt(a))) { 
// charAt methodda you provide index number ve sana character olarak donuyor,
// If the string result does not contains str.CharAt(i), 
// then we concate it to the result. if it does we will not
   continue;
}
result1 += str1.charAt(a);
}
System.out.println(result1);

import java.util.*;

public class RemoveDuplicatesFromArrayList {

    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1,2,2,2,3,5);

        System.out.println(numbers);

        Set<Integer> hashSet = new LinkedHashSet(numbers);
        ArrayList<Integer> removedDuplicates = new ArrayList(hashSet);

        System.out.println(removedDuplicates);
    }
}

public static void main(String[] args) {

        String result = removeDup("AAABBBCCC");
        System.out.println(result); // ABC

public static  String  removeDup( String  str) {
        String result = "";
        for (int i = 0; i < str.length(); i++)
            if (!result.contains("" + str.charAt(i)))
                result += "" + str.charAt(i);
        return result;
    }
}

String str2 = "ABABABCDEF";// ABCDEF
        String[] arr2 = str2.split("");
str2 = new LinkedHashSet<>(Arrays.asList(arr2)).toString().replace(", ", "");
        System.out.println(str2); // ABCDEF