How to append String to StringBuffer variable in JAVA

My code is as below. i need to add single quotes for each word in string with single quotes after appending DD to it.

public class Main
{
    public static void main(String[] args) {
        String ChkboxIds = "'a1b2c3','321cba','123abc'";
        String checkBoxId = null;
        String checkId = null;
        StringBuilder checkIDS = new StringBuilder("'");
        for(int i=0;i<=ChkboxIds.split(ChkboxIds, ',').length;i++){
            checkBoxId = "DD"+ChkboxIds.split(",")[i].replace("'","")+","+checkBoxId;
            checkId = checkBoxId.substring(0, checkBoxId.length() - 5);
            System.out.println("---PRINT---"+checkId);
            for(int j=0;j<i;j++){
                checkIDS..append('\'').append(checkId.split(",")).append('\'').append(',');
                System.out.println("---PRINT123----"+checkIDS);
            }
        }
    }
}

I have tried using StringBuffer too. please point your answers here. The output i get is some junk data while i need the words with dd attached at the start.

Expected output:'DDa1b2c3','DD321cba','DD123abc'

Problem

• issue at .append(checkId.split(",")) where you append an String[] so it's representation is it's hashcode

• don't need a second loop, each word need one loop round, no inner loop needed

• your split is wrong, you need ChkboxIds.split(","), you don't need with the same string as delimiter

Fix

You can do much more simpler than that

• split on comma
• remove quotes, append DD, add quotes
• save at same place in array
• join array with comma

String chkboxIds = "'a1b2c3','321cba','123abc'";

String[] splitted = chkboxIds.split(",");
String checkBoxId;

for (int i = 0; i < splitted.length; i++) {
    checkBoxId = "DD" + splitted[i].replace("'", "");
    splitted[i] = "'" + checkBoxId + "'";
}

String result = String.join(",", splitted);
System.out.println(result);
// 'DDa1b2c3','DD321cba','DD123abc'

Regex power

String chkboxIds = "'a1b2c3','321cba','123abc'";
String result = chkboxIds.replaceAll("'(\\w+)'", "'DD$1'");