JAXB generated classes for fixed attribute in subtype

Question

My java classes are being generated from xsd file.
The goal I want to accomplish is to have some "known" properties based on the type of elements. For instance, I have a list of animals. After the xml is being parsed, I want to know in the code how many legs have my animals. But the number of legs is a characteristic of the animal type, so, in case I have a cat it will have 4 legs and the kangaroo will have 2 legs.
In case I define the xsd like this:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">

    <xs:element name="Animals" type="animals" />

    <xs:complexType name="animals">
        <xs:sequence>
            <xs:element name="Animal" maxOccurs="unbounded"  type="animal"/>
        </xs:sequence>
    </xs:complexType>

    <xs:complexType name="animal" abstract="true">
        <xs:simpleContent>
            <xs:extension base="xs:string">
             <xs:attribute name="name" />
            </xs:extension>
        </xs:simpleContent>
    </xs:complexType>

    <xs:complexType name="catType">
        <xs:simpleContent>
            <xs:extension base="animal">
                <xs:attribute name="nbOfLegs" type="xs:integer" fixed="4" />
            </xs:extension>
        </xs:simpleContent>
    </xs:complexType>

    <xs:complexType name="kangarooType">
        <xs:simpleContent>
            <xs:extension base="animal">
                <xs:attribute name="nbOfLegs" type="xs:integer" fixed="2" />
            </xs:extension>
        </xs:simpleContent>
    </xs:complexType>

</xs:schema>

The generated classes are as expected (I removed the annotations):

public abstract class Animal {
    protected String name;
    public String getName() {return name;}
    public void setName(String value) {this.name = value;}
}

public class CatType extends Animal {
    protected BigInteger nbOfLegs;
    public BigInteger getNbOfLegs() {
        if (nbOfLegs == null) {
            return new BigInteger("4");
        } else {
            return nbOfLegs;
        }
    }
    public void setNbOfLegs(BigInteger value) {this.nbOfLegs = value;}
}

This way, in case the user sets the number of legs for cats in the xml, it can only be 4, and if he doesn't, in code I will receive 4 anyway. Similar for kangaroo I always receive 2 legs.
But the problem with this approach is that I cannot use polymoprhism like this:

for(Animal animal : animals) {
    System.out.println(animal.getNbOfLegs());
}

So I tried a different approach:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">

    <xs:element name="Animals" type="animals" />

    <xs:complexType name="animals">
        <xs:sequence>
            <xs:element name="Animal" maxOccurs="unbounded"  type="animal"/>
        </xs:sequence>
    </xs:complexType>

    <xs:complexType name="animal" abstract="true">
        <xs:simpleContent>
            <xs:extension base="xs:string">
                <xs:attribute name="name" />
                <xs:attribute name="nbOfLegs" type="xs:integer"/>
            </xs:extension>
        </xs:simpleContent>
    </xs:complexType>

    <xs:complexType name="catType">
        <xs:simpleContent>
            <xs:restriction base="animal">
                <xs:attribute name="nbOfLegs" type="xs:integer" fixed="4" />
            </xs:restriction>
        </xs:simpleContent>
    </xs:complexType>

    <xs:complexType name="kangarooType">
        <xs:simpleContent>
            <xs:restriction base="animal">
                <xs:attribute name="nbOfLegs" type="xs:integer" fixed="2" />
            </xs:restriction>
        </xs:simpleContent>
    </xs:complexType>

</xs:schema>

The Animal class is generated as expected:

public abstract class Animal {  
    protected String name;
    protected BigInteger nbOfLegs;
    public String getName() {return name;}
    public void setName(String value) {this.name = value;}
    public BigInteger getNbOfLegs() {return nbOfLegs;}
    public void setNbOfLegs(BigInteger value) {this.nbOfLegs = value;}
}

But the generated CatType is empty.
I expected it to be like this:

public class CatType extends Animal {
    @Override
    public BigInteger getNbOfLegs() {
        if (nbOfLegs == null) {
            return new BigInteger("4");
        } else {
            return nbOfLegs;
        }
    }
}

Is it possible to customize the bindings file in order to achieve the desired generated CatType class?
Thank you.