Question about adding StringBuffer to a set

/*** I thought there was no problem with the simple code below in a large project, but it took me some time to figure out the BUG, this Code simply adds two strings. Let's take a look at the output content. **/@ Testpublic void test () {StringBuffer sb = new StringBuffer (); list list = new ArrayList (); sb. append ("First row data"); list. add (0, sb); sb. delete (0, sb. length (); sb. append ("second row of Data"); list. add (1, sb); System. out. println (list );}

After running the program, you will find that the output content of the program is: see the end. That is to say, the list set does not store the reference value, but stores the reference, however, in Java APIs, public void add (int index, E element) inserts the specified element into the specified position in this list. Move the current element (if any) at this position to the right and all subsequent elements (add its index to 1 ). According to the output result of the program, we can see that the list set gets the specific value pointed to by reference only when the set is output. ([Second row of data, second row of data])