JAVA抽象类及接口使用方法解析

public abstract class Door {
  String color;
  abstract void open();
  abstract void close();
  void keep(){
    System.out.println("I am keeping!");
  }
}
public interface Alarm {
  static final String VOICE = "Call the police";
  abstract void alarm();
}

public class AlarmDoor extends Door implements Alarm {
  @Override
  public void alarm() { }
  @Override
  void open() { }
  @Override
  void close() {}
}

public interface Fold {
  void fold();
}
public class AlarmFoldDoor extends Door implements Alarm , Fold{
  @Override
  public void alarm() {}
  @Override
  void open() {}
  @Override
  void close() {}
  @Override
  public void fold() {}
}