10.4 内部类与向上转型

//只有parcel4 及其子类 和 同包的类可以访问能访问PDestination，其他类都不能访问PDestination
class Parcel4 {
   class PContents implements Contents {
    private int i = 11;
    public int value() { return i; }
  }
  protected class PDestination implements Destination {
    private String label;
     PDestination(String whereTo) {
      label = whereTo;
    }
    @Override
    public String realLabel() {
      return label;
    }
  }
  protected PDestination getPD(String s){   //19-20是为了下面的例子
    return new PDestination(s);
  }
    // destination方法的返回类型是接口类型，new出来的PDestination(s)不知道他的确切类型
  public Destination destination(String s) {
    return new PDestination(s);
  }
  public Contents contents() {
    return new PContents();
  }
}
public class TestParcel {
  public static void main(String[] args) {
    Parcel4 p = new Parcel4();
    Contents c = p.contents();
    //note--第四行的private去掉下面这行代码就不会报错: class PContents implements Contents
      //! Parcel4.PContents c1 = (Parcel4.PContents) c;
    Destination d = p.destination("Tasmania");
    // Illegal -- can't access private class:
    // Parcel4.PContents pc = p.new PContents();
   // Parcel4.PDestination pd = p.new PDestination("sasa");
  }
}
class Test extends Parcel4{
  public static void main(String[] args) {
    Test t = new Test();
    Parcel4 p4 = new Parcel4();
    Contents c = p4.contents();
    Parcel4.PDestination pd = p4.getPD("sss");
    pd.realLabel();
  }
}
class Test2{
  public static void main(String[] args) {
    Parcel4 p = new Parcel4();
    Parcel4.PDestination pd = p.getPD("sssas");
  }
}