public without sharing class RelatedController{


 Public Contact cont{get;set;}
 Public Account acc{get;set;}
    
 ApexPages.StandardController m_sc = null;
  
 public RelatedController(ApexPages.StandardController sc)
 {
        m_sc  = sc;
 }
  
 public void AccountPopulated()
 { 

    this.cont = (Contact)m_sc.getRecord();  
 
     List<Account> acct =[select Account.BillingStreet, Account.BillingCity, Account.BillingPostalCode, Account.BillingState, Account.BillingCountry, Account.Phone from Account where id=:cont.AccountId limit 1];
  
     if (acct.size() > 0) {
         cont.Account=[select Account.BillingStreet, Account.BillingCity, Account.BillingPostalCode, Account.BillingState, Account.BillingCountry, Account.Phone from Account where id=:cont.AccountId limit 1];}
     
     else
     {Account acc = new Account();

      Contact cont = new Contact(AccountId = acc.id,
                              MailingStreet = acc.BillingStreet,
        MailingCity = acc.BillingCity,
        MailingPostalCode = acc.BillingPostalCode,
        MailingState = acc.BillingState,
        MailingCountry = acc.BillingCountry,
        Phone = acc.Phone);
         }
 }
                 


    public PageReference save() {
          this.cont = (Contact)m_sc.getRecord();
        cont.MailingStreet = cont.Account.BillingStreet;
        cont.MailingCity = cont.Account.BillingCity;
        cont.MailingPostalCode = cont.Account.BillingPostalCode;
        cont.MailingState = cont.Account.BillingState;
        cont.MailingCountry = cont.Account.BillingCountry;
        cont.Phone = cont.Account.Phone; 

        upsert cont;
        return(new ApexPages.StandardController(cont)).view();
     
  }}

<apex:page standardController="Contact" extensions="RelatedController" action="{!AccountPopulated}" title="Contact Edit" showHeader="true" sidebar="true">

    <apex:sectionHeader title="Contact Edit" subtitle="New Contact"/>
    <apex:form >
              
        <apex:pageMessages escape="false"/>     
              
        <apex:pageBlock title="Main Detail" mode="edit">
            <apex:pageBlockButtons >
                <apex:commandButton value="Save" action="{!save}"/>
                <apex:commandButton value="Cancel" action="{!cancel}"/>
            </apex:pageBlockButtons>

                <apex:pageBlockSection title="Contact Information" columns="2">

                       // your fields on contact information come here

                    </apex:pageBlockSection>
                
                  <apex:pageBlockSection title="Address Information" columns="2" id="addressinfo">
                     <apex:pageBlockSectionItem id="AccountName">
                     <apex:outputLabel value="Account Name" for="AccountName"/> <apex:actionRegion >
                     <apex:inputField value="{!Contact.AccountId}" id="AccountName" required="true">
                     <apex:actionSupport event="onchange" action="{!AccountPopulated}" reRender="addressinfo"/>
                     </apex:inputField></apex:actionRegion>
                     </apex:pageBlockSectionItem>
                    <apex:pageBlockSectionItem > </apex:pageBlockSectionItem> 
                    <apex:inputField value="{!Contact.Account.BillingStreet}" label="Mailing Street" id="MailingStreet"/>
                    <apex:inputField value="{!Contact.OtherStreet}" id="OtherStreet"/>
                    <apex:inputField value="{!Contact.Account.BillingCity}" label="Mailing City" id="MailingCity" required="true"/>
                    <apex:inputField value="{!Contact.OtherCity}" id="OtherCity"/>
                    <apex:inputField value="{!Contact.Account.BillingPostalCode}" label="Mailing Zip/Postal Code" id="MailingPostalCode"/>
                    <apex:inputField value="{!Contact.OtherPostalCode}" id="OtherPostalCode"/>
                    <apex:inputField value="{!Contact.Account.BillingState}" label="Mailing State/Province" id="MailingState"/>
                    <apex:inputField value="{!Contact.OtherState}" id="OtherState"/>
                    <apex:inputField value="{!contact.Account.BillingCountry}" label="Mailing Country" id="MailingCountry" required="true"/> 
                    <apex:inputField value="{!contact.OtherCountry}" id="OtherCountry"/>
                </apex:pageBlockSection>
                
// your additional sections/fields come here

        </apex:pageBlock>
    </apex:form>
</apex:page>

@Istest
public class TestNewContact {
    static testMethod void test_NewContact() {

//Test converage for the NewContact visualforce page
PageReference pageRef = Page.NewContact;
Test.setCurrentPageReference(pageRef);
Account newAccount = new Account (
name='XYZ',
Phone = '+49-12345',
BillingStreet = 'Am Tierpark 16',
BillingCity = 'Cologne',
BillingPostalCode = '54321',
BillingState = 'Nordrhein-Westfalen',
BillingCountry = 'Germany'
);
insert newAccount;

Account acc=[select id,name from account where id=:newAccount.id];
System.assertEquals(acc.name, 'XYZ');

//create first contact
Contact myContact = new Contact (
FirstName='Joe',
LastName='Schmoe',
Email='Joe.Schmoe@test.de',
AccountId=newAccount.id,
MailingStreet=newAccount.BillingStreet,
MailingState = newAccount.BillingState,
MailingCity=newAccount.BillingCity,
MailingPostalCode=newAccount.BillingPostalCode,
MailingCountry = newAccount.BillingCountry,
Phone=newAccount.Phone);
insert myContact;


Contact con=[select id,MailingCity from contact where id=:myContact.id];
System.assertEquals(con.MailingCity, 'Cologne');

ApexPages.StandardController sc = new ApexPages.standardController(myContact);

// create an instance of the controller
RelatedController rc = new RelatedController(sc);

//try calling methods/properties of the controller
    rc.AccountPopulated();      
    rc.Save();
    

}
}