[SalesForce] Using apex value retrived from Visualforce page in SOQL Query

Please help me figure out a way on how to solve this. The below query gives me error :

Error: Compile Error: Didn't understand relationship 'contactData' in field path. If you are attempting to use a custom relationship, be sure to append the '__r' after the custom relationship name. Please reference your WSDL or the describe call for the appropriate names. at line 18 column 27

    displayContacts = [select id,
                       Name,
                       Domain__c,
                       City__c,
                       FirstName,
                       LastName,
                       Email,
                       Country__c from Contact where 
                       (Domain__c=:contactData.Domain__c OR contactData.Domain__c==NULL)
                       AND (City__c=:contactData.City__c OR contactData.City__c == NULL)
                       AND (Country__c=:contactData.Country__c OR contactData.Country__c == NULL];   

I am trying to use the value generated from Visualforce page to add condition in the query.

myClass

class getpicklstval1{

public String searchString {get;set;}
public Boolean allContacts{get;set;}   
public Contact ContactData{get;set;}
public List<Contact> displayContacts{get;set;}
public getpicklstval1(ApexPages.StandardController controller){
    ContactData = new Contact();
    displayContacts = new List<Contact>();       
}

public void called(){
    if(contactData.Domain__c!=NULL && contactData.City__c!=NULL && contactData.Country__c!=NULL){
    displayContacts = [select id,
                       Name,
                       Domain__c,
                       City__c,
                       FirstName,
                       LastName,
                       Email,
                       Country__c from Contact where 
                       (Domain__c=:contactData.Domain__c OR contactData.Domain__c==NULL)
                       AND (City__c=:contactData.City__c OR contactData.City__c == NULL)
                       AND (Country__c=:contactData.Country__c OR contactData.Country__c == NULL];                           
   }                
}

public List<Contact> getreturnContact(){
    return displayContacts;
}
public void SearchKeyword(){
}    }  

Visualforce Page

    <apex:page StandardController="Contact" extensions="getpicklstval1">
    <apex:form >
    <apex:inputField id="Domain" value="{!ContactData.Domain__c}" style="width:159px;">
        <apex:actionSupport event="onchange" action="{!called}" rerender="displayContacts"/>
    </apex:inputfield>  
    <apex:inputField id="City" value="{!ContactData.City__c}" style="width:159px;">
        <apex:actionSupport event="onchange" action="{!called}" rerender="displayContacts"/>
    </apex:inputfield>    
    <apex:inputField id="Country" value="{!ContactData.Country__c}" style="width:159px;">
        <apex:actionSupport event="onchange" action="{!called}" rerender="displayContacts"/>
    </apex:inputfield>    

    <apex:pageBlock id="displayContacts">
    <apex:pageblockTable id="searchContactList" var="contact" value="{!returnContact}">     

         <apex:column style="width:3px;">
                 <apex:facet name="header"/>                  
                         <apex:inputCheckbox value="{!contact.Name}" id="checkedone" style="width:15px;">                 
                 </apex:inputCheckbox>                 
         </apex:column>
         <apex:column style="width:200px; height:60px;">
            <apex:facet name="header">
                <apex:outputPanel layout="block" style="text-align:center;">
                     First Name
                </apex:outputPanel>                    
            </apex:facet>                   
            <apex:outputLink value="/{!contact.id}">{!contact.FirstName}</apex:outputLink>               
         </apex:column>                      
         </apex:pageblockTable>
        </apex:pageBlock> 
    </apex:form>
    </apex:page>