Java inserts an object and JS at the specified position in the list collection method __js

Java inserts a piece of data into the collection at the specified location:

List<string> list =new arraylist<> ();
List.add ("a");
List.add ("B");
List.add ("C");
//...
At this point, list ={a,b,c}

//insert "Please select"
list.add (0, "Please select") in the 0 of the list;
At this point, list={"Please select", A, B, C}, and a will not be replaced, just use backward to move a

JS: Inserts a piece of data into the array at the specified location: Use JS's Splice () method to add/remove items to/from the array, please read the brochure

var ar =[' b ', ' C ', ' d ', ' e ']
//to AR 0 position insert ' a '
ar.splice (0,0, ' a ')//At this time ar =[a,b ...]
Insert to AR 1 position ' Please select '
ar.splice (1, 0, ' Please select ')//At this time ar=[' a ', ' Please choose ', b ...]