question: public java . lang . StringBuilder insert ( int index , java . lang . String string ) { if ( string == null ) { string = "null" ; } int min = string . length ( ) ; if ( min != 0 ) { move ( min , index ) ; string . getChars ( 0 , min , value , index ) ; count += min ; } return this ; }
target:  Inserts the string into this string buffer. The characters of the String argument are inserted, in order, into this string buffer at the indicated offset, moving up any characters originally above that position and increasing the length of this string buffer by the length of the argument. If str is null, then the four characters "null" are inserted into this string buffer. The character at index k in the new character sequence is equal to: the character at index k in the old character sequence, if k is less than offset the character at index k-offset in the argument str, if k is not less than offset but is less than offset+str.length() the character at index k-str.length() in the old character sequence, if k is not less than offset+str.length() The offset argument must be greater than or equal to 0, and less than or equal to the length of this string buffer.