ClassCastException with StringBuilder and CharSequence

Question

Why the following code will yield a nice ClassCastException:

public static void main(String []args){
        StringBuilder s1 = new StringBuilder("quaqua");
        StringBuilder s12= (StringBuilder)s1.subSequence(0,3);
        System.out.println(s12);
     }

whereas the following code ( substituting StringBuilder with String):

public static void main(String []args){
        String s1 = new String("quaqua");
        String s12= (String)s1.subSequence(0,3);
        System.out.println(s12);
     }

works fine?;

I understand that there is the method substring(int begin, int end); however I am just curious why the cast with StringBuilder (which implements CharSequence) does not work whereas with String works.

Thanks in advance.

Answer 1

This is because the actual class that StringBuilder.subSequence returns is String:

...
public CharSequence subSequence(int start, int end) {
    return substring(start, end);
}

public String substring(int start, int end) {
    ...
    return new String(value, start, end - start);
}

You cannot cast String to StringBuilder.