public class DelegateApex {

  public interface SObjectDelegate {
    List<SObject> getObject(String search);
  }

  public class AccountDelegate implements SObjectDelegate {
    public List<SObject> getObject(String searchVar) {
      List<Account> accounts = [SELECT Id, Name
        FROM Account
        WHERE Name LIKE :('%' + searchVar + '%')];
      if (accounts != null && accounts.size() > 0) {
        return (List<SObject>)accounts;
      }
      return null;
    }
  }

  public class ContactDelegate implements SObjectDelegate {
    public List<SObject> getObject(String searchVar) {
      List<Contact> contacts = [SELECT Id, Name
      FROM Contact
      WHERE Name LIKE :('%' + searchVar + '%')];
      if (contacts != null && contacts.size() > 0) {
        return (List<SObject>)contacts;
      }
      return null;
    }
  }

  public class DelegateClient {
    private SObjectDelegate delegate;
    public DelegateClient(SObjectDelegate delegate) {
      this.delegate = delegate;
    }
    public List<SObject> doSearch(String search) {
      return delegate.getObject(search);
    }
  }

  public static void delegateExampleRunner() {
    AccountDelegate ad = new AccountDelegate();
    ContactDelegate cd = new ContactDelegate();
    DelegateClient dc = new DelegateClient(ad);
    System.debug(dc.doSearch('a')); // List of Accounts containing 'a' in the name
    dc = new DelegateClient(cd);
    System.debug(dc.doSearch('a')); // List of Contacts containing 'a' in the name
  }
}

/**
Sample Output:

09:51:12.24 (118080844)|USER_DEBUG|[49]|DEBUG|(Account:{Id=0016300000dmlgMAAQ, Name=San Antonio Spurs})
09:51:12.24 (42083669)|USER_DEBUG|[51]|DEBUG|(Contact:{Id=0036300000T6MJ3AAN, Name=Manu Ginobilli}, Contact:{Id=0036300000T6MI1AAN, Name=Tim Duncan})


**/