How can I validate and parse Keyref Unique and minOccurs?

Hi, Everyone:
I made a .xsd and .xml file and parser with xerces-2_2_1. I set this 
following element:

<xs:element name="friend-of" type="nameType" minOccurs="0" maxOccurs="0"/>
It still prints the element "friend-of" even though minOccurs=0.
Do I need to change something in my parser?

Thanks,
Wenpeng
The XML document is as follows:
?xml version="1.0" encoding="UTF-8"?>
<book xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="/tmp/example/xml_1/book_2.xsd"
      isbn="012345678900">
            <title>
            Being a Dog Is a Full-Time Job
      </title>
      <author>Charles M. Schulz</author>
      <character>
      <name>Snoopy</name>
      <friend-of>Peppermint Patty</friend-of>
      <since>1950-10-04</since>
      <qualification>
      extroverted beagle
      </qualification>
      </character>
      <character>
      <name>Peppermint Patty</name>
       <friend-of>I test</friend-of>
      <since>1966-08-22</since>
      <qualification>bold, brash and tomboyish</qualification>
      </character>
      </book>

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