question: public static String maskRight ( String s , int len , char mask_ch ) { if ( len <= 0 ) { return s ; } len = Math . min ( len , s . length ( ) ) ; StringBuilder sb = new StringBuilder ( ) ; sb . append ( s . substring ( 0 , s . length ( ) - len ) ) ; for ( int i = 0 ; i < len ; i ++ ) { sb . append ( mask_ch ) ; } return sb . toString ( ) ; }
target:  Returns a string consisting of "s", with each of the last "len" characters replaces by "mask_ch" character.