How can instances of custom classes be saved on a physical device?

To save instances of custom classes on a physical device in Android, object serialization into a format that can be written to a file or SharedPreferences is usually used. Main methods:

1. Serializable — a standard Java interface for object serialization. Simple but less efficient.

2. Parcelable — an Android-specific interface, faster and optimized for data transfer between components and storage.

3. Saving in JSON or XML — converting the object into a string using libraries (e.g., Gson, Moshi), then writing to a file or SharedPreferences.

Example with Gson:

data class User(val name: String, val age: Int)

val user = User("Ivan", 30)
val gson = Gson()
val json = gson.toJson(user)

// Save json to a file or SharedPreferences

When loading from a file or SharedPreferences, JSON can be deserialized back into an object.