String vs StringBuffer returned value

Question

Can you please explain why in the following code String and StringBuffer are treated differently and when value is appended in StringBuffer but not in String.

public class MyClass {
    
    public static void main(String args[]) {
        String str = new String("String Rohan ");
    StringBuffer strBfr = new StringBuffer("String Buffer Rohan "); 
        
        strUpdate(str);
        strBfrUpdate(strBfr);
        
        System.out.println(str);
        System.out.println(strBfr);
        
    }
    
    private static void strBfrUpdate(StringBuffer strBfr){
        strBfr.append("Kushwaha");
    }
    
    private static void  strUpdate(String str){
        str += "Kushwaha";
    }
}

Output is as follows:

String Rohan

String Buffer Rohan Kushwaha

Answer 1

In strUpdate method you keep a reference to a String called str:

private static void  strUpdate(String str){
    str += "Kushwaha";
}

When you write str += "..."; It means:

    str = new String(str + "...");

At this point str references the new String, but only in strUpdate method. It's not "taken back" to main.