Dynamic Field Example

testcontroller.java

public with sharing class TestController {
	
	public String mainContactField {get; set;}
	
	public TestController(ApexPages.StandardController std) {
		Contact c = (Contact)std.getRecord();
		mainContactField = 'Phone';
		
		List<Contact> cWithRecords = [SELECT Id, Phone, MobilePhone, AssistantPhone, OtherPhone from Contact where ID = :c.Id];
		if(cWithRecords.size() > 0) {
			if(ApexPages.currentPage().getParameters().get('area') != null) {
				if(cWithRecords[0].Phone.replaceAll('[^0-9]+','').startsWith(ApexPages.currentPage().getParameters().get('area'))) { 
                                    mainContactField = 'Phone'; 
                                }
				if(cWithRecords[0].MobilePhone != null && cWithRecords[0].MobilePhone.replaceAll('[^0-9]+','').startsWith(ApexPages.currentPage().getParameters().get('area'))) { 
                                    mainContactField = 'MobilePhone'; 
                                }
				if(cWithRecords[0].AssistantPhone != null && cWithRecords[0].AssistantPhone.replaceAll('[^0-9]+','').startsWith(ApexPages.currentPage().getParameters().get('area'))) { 
                                    mainContactField = 'AssistantPhone'; 
                                }
				if(cWithRecords[0].OtherPhone != null && cWithRecords[0].OtherPhone.replaceAll('[^0-9]+','').startsWith(ApexPages.currentPage().getParameters().get('area'))) { 
                                    mainContactField = 'OtherPhone'; 
                                }
			}
		}
	//this is just a comment
        }

}

testpage.jsp

<apex:page standardController="Contact" extensions="TestController">
        <apex:form >
            <apex:messages />
            Closest Phone Number: <apex:inputField value="{!Contact[mainContactField]}"/><apex:commandButton action="{!save}" value="Save" />
            <apex:outputPanel rendered="false">
                <!-- pull in values for std controller query -->
                <apex:outputText value="{!Contact.Phone}" />
                <apex:outputText value="{!Contact.MobilePhone}" />
                <apex:outputText value="{!Contact.AssistantPhone}" />
                <apex:outputText value="{!Contact.OtherPhone}" />
            </apex:outputPanel>
        </apex:form>
</apex:page>