XSD中的子元素和命名空间

<fun xmlns="http://ttdi.us/I/am/having/fun"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://ttdi.us/I/am/having/fun
                          test.xsd">
    <activity>rowing</activity>
    <activity>eating</activity>
    <activity>coding</activity>
</fun>

使用以下XSD(诚然,我只是一个凡人):

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema targetNamespace="http://ttdi.us/I/am/having/fun" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://ttdi.us/I/am/having/fun">
    <xsd:element name="fun" type="activityList"></xsd:element>

    <xsd:complexType name="activityList">
        <xsd:sequence>
            <xsd:element name="activity" type="xsd:string" maxOccurs="unbounded" minOccurs="0"></xsd:element>
        </xsd:sequence>
    </xsd:complexType>
</xsd:schema>

cvc-complex-type.2.4.a: Invalid content was found starting with element 'activity'. One of '{activity}' is expected.

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema targetNamespace="http://ttdi.us/I/am/having/fun"
    xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://ttdi.us/I/am/having/fun">
    <xsd:element name="activity" type="xsd:string"></xsd:element>

    <xsd:element name="fun">
        <xsd:complexType>
            <xsd:sequence>
                <xsd:element ref="activity" minOccurs="0" maxOccurs="unbounded"/>
            </xsd:sequence>
        </xsd:complexType>
    </xsd:element>
</xsd:schema>

<actvity> 在我的文档的根?如果 ref 应该按原样替换,那么为什么我不能替换 ref="actvity" name="activity" type="xsd:string"

DocumentBuilderFactory dbf;
// initialize dbf
dbf.setNamespaceAware(true);