question: public StrBuilder insert ( final int index , String str ) { validateIndex ( index ) ; if ( str == null ) { str = nullText ; } if ( str != null ) { final int strLen = str . length ( ) ; if ( strLen > 0 ) { final int newSize = size + strLen ; ensureCapacity ( newSize ) ; System . arraycopy ( buffer , index , buffer , index + strLen , size - index ) ; size = newSize ; str . getChars ( 0 , strLen , buffer , index ) ; } } return this ; }
target:  Inserts the string into this builder. Inserting null will use the stored null text value.