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