Connection.java

class Connection
{
  /* generic code ; private */
  
  /* the interface is public */
  public Connection() {
  }

  public int connectTo() {
  }
}  

class OracleConnection extends Connection
{ 
  /* implementation specific code ; private */

  /* public interface */
  public OracleConnection() {
  }

  public int connectTo() {
  }
} 

class Tester
{
  public static void main(String[] a) {
    /* coding to the interface */
    Connection c = new OracleConnection();
}