public void getContactDetails(){
    List<Opportunity> opportunityList;
    this.Id = ApexPages.currentPage().getParameters().get('Id');
    opportunityList= [Select AccountId,ContactId From Opportunity Where Id =: this.Id ];
    List<Contact> contactList = new List<Contact>();
    List<Account> AccountList = new List<Account>();
    List<Contact> ContactAccountList = new List<Contact>();
    for(Opportunity oppObj:opportunityList) {
      contactList = [Select Name, Phone, Zip_code__c,Email From Contact Where Id =:oppObj.ContactId ];
      AccountList = [Select BillingStreet,BillingCity From Account Where Id =:oppObj.AccountId];
      for(Contact contactobj:contactList){
          contName = contactobj.Name;
          contPhone = contactobj.Phone;
          contZipcode = contactobj.Zip_code__c;
          contEmail = contactobj.Email;
        system.debug('name '+contName);
        system.debug('phone '+contPhone);
        system.debug('zip '+contZipcode); 
        system.debug('Email '+contEmail); 
        for(Account accountobj:AccountList){
          acctCity = accountobj.BillingStreet;
          acctStreet = accountobj.BillingCity;
        }
      }
      if(oppObj.ContactId == null){
        ContactAccountList =  [Select Name, Phone, Zip_code__c,Email From Contact Where AccountId =:oppObj.AccountId ];
        for(Contact contactaccountobj:contactaccountList){
          contName = contactaccountobj.Name;
          contPhone = contactaccountobj.Phone;
          contZipcode = contactaccountobj.Zip_code__c;
          contEmail = contactaccountobj.Email;
          system.debug('name '+contName);
          system.debug('phone '+contPhone);
          system.debug('zip '+contZipcode); 
          system.debug('Email '+contEmail); 
          for(Account accountobj:AccountList){
            acctCity = accountobj.BillingStreet;
            acctStreet = accountobj.BillingCity;
          }
        }
      }
    }
}