The old good Java object serialization is available for you in Android. you can define Serializable classes like:

class Cirle implements Serializable {
  final int radius;
  final String name;

  Circle(int radius, int name) {
    this.radius = radius;
    this.name = name;
  }
}

and then write then to the ObjectOutputStream:

File myFile = new File(getFilesDir(), "myObjects.bin");
FileOutputStream out = new FileOutputStream(myFile);
ObjectOutputStream oout = new ObjectOutputStream(new BufferedOutputStream(out));

oout.writeObject(new Circle(10, "One"));
oout.writeObject(new Circle(12, "Two"));

oout.close()

Java object serialization may be either perfect or really bad choice, depending on what do you want to do with it - outside the scope of this tutorial and sometimes opinion based. Read about the versioning first if you decide to use it.