question: public String replace ( CharSequence target , CharSequence replacement ) { if ( target == null ) { throw new NullPointerException ( "target should not be null" ) ; } if ( replacement == null ) { throw new NullPointerException ( "replacement should not be null" ) ; } String ts = target . toString ( ) ; int index = indexOf ( ts , 0 ) ; if ( index == - 1 ) return this ; String rs = replacement . toString ( ) ; StringBuilder buffer = new StringBuilder ( count ) ; int tl = target . length ( ) ; int tail = 0 ; do { buffer . append ( value , offset + tail , index - tail ) ; buffer . append ( rs ) ; tail = index + tl ; } while ( ( index = indexOf ( ts , tail ) ) != - 1 ) ; buffer . append ( value , offset + tail , count - tail ) ; return buffer . toString ( ) ; }
target:  Copies this String replacing occurrences of the specified  target sequence with another sequence.  The string is processed from the beginning to the end. 