Select multiple record from list view using StandardSetController with Javascript validation

Create apex class 

public class conContactList{

   public Integer  numberofRecord{get;set;}

   public List<Contact> ContactList{get;set;}

    public conContactList(ApexPages.StandardSetController controller) 

    {

         ContactList=controller.getSelected();

        numberofRecord=ContactList.Size();

       

    }

    public List<Contact>getContactListSF()

    {

       List<Contact> NewContactList=[Select c.Id, FirstName,LastName,Phone from Contact c Where c.Id In : ContactList] ;

       return NewContactList;

    }

   

}

Visualforce page

<apex:page standardController="Contact" recordSetVar="Contact" extensions="conContactList" 

cache="true" >

<apex:outputText value="{!numberofRecord}"></apex:outputText>

<apex:pageblock Title="List of contacts" >

 <apex:pageBlockTable value="{!ContactListSF}" var="a" >

   <apex:column headervalue="First Name" value="{!a.FirstName}"  /> 

   <apex:column headervalue="Last Name" value="{!a.LastName}"  /> 

   <apex:column headervalue="Last Name" value="{!a.Phone}"  /> 

   

 </apex:pageBlockTable>

 </apex:pageblock>

</apex:page>

Javascript Button 

{!REQUIRESCRIPT("/soap/ajax/22.0/connection.js")}

{!REQUIRESCRIPT("/soap/ajax/16.0/apex.js")}

var records = {!GETRECORDIDS($ObjectType.Contact)};

var strIDs='';

if (records[0] == null)

{

    alert('Please select a record');

}

else

{

  var sApi='{!$Api.Partner_Server_URL_290}';

  var sindex=sApi.indexOf('//');

  var Eindex=sApi.indexOf('.');

 var server=sApi.substring(sindex+2,Eindex)  //*** getting server information *****

//this.form.action = 'https://c.'+server+'.visual.force.com/apex/ContactList?wrapMassAction=1&scontrolCaching=1'; 

this.form.action = 'https://gs78.'+server+'.visual.force.com/apex/ContactList?wrapMassAction=1&scontrolCaching=1'; //*** You must need to full url Other wise you loss selected record***

this.form.submit();

}