What methods can be used to add functionality to an existing class?

In Android (and in Java/Kotlin in general), functionality can be added to an existing class in the following ways:

1. Inheritance — create a subclass that extends or overrides the behavior of the base class.

2. Composition — include an instance of an existing class in a new class and use its methods, adding new logic.

3. Extension functions — in Kotlin, you can add new functions to existing classes without inheritance or modifying the source code:

fun String.addExclamation() = this + "!"

val greeting = "Hello".addExclamation() // Hello!

4. Delegation — use the delegation pattern to pass method calls to another object.

5. Decorator — wrap an object in another object that adds new functionality.

Each method has its advantages and is used depending on the task and project constraints.