How to use inputField with Standard Controller and extensions

Page 
<apex:page standardController="Contact" extensions="datatrans">
<apex:form >
<apex:inputField value="{!contact.FirstName}" required="true" />
<apex:commandbutton value="save" action="{!datatrans}">

</apex:commandbutton>
</apex:form>


</apex:page>

Controller
public with sharing class datatrans {
public Contact record {get; set;}
    public String Name{
        get {
            return record.FirstName;
        }
        private set;
    }
    public datatrans(ApexPages.StandardController sc) {
record = (Contact)sc.getRecord();
    }
public pageReference datatrans()
{

string value=Name;
system.debug('*****value*****'+value);
return null;

}

}