What does program to interfaces, not implementations mean?

The principle "program to interfaces, not implementations" is a fundamental concept in software design, particularly in object-oriented programming. It advises developers to depend on abstractions (interfaces or abstract classes) rather than concrete classes. This approach enhances the flexibility and extensibility of code, making it easier to modify or extend without altering existing code.

Understanding the Principle

1. Abstraction Over Implementation:

   • When you program to an interface, you write your code to an abstract type which defines a set of methods that the concrete implementations must provide. This means your code only knows about the interface (the set of methods it can call), not the specific details of how these methods are implemented[1][2][3].

2. Decoupling:

   • This principle helps in decoupling the code dependencies. For instance, if your application needs to switch from one type of database to another, you can do so seamlessly if you have programmed to an interface. The rest of your application interacts with an interface, and you can change the underlying implementation without affecting the dependent code[2].

3. Flexibility and Maintainability:

   • By depending on abstractions, you can change the behavior of your application at runtime by switching the implementations behind these interfaces. This is particularly useful in scenarios where you might not know the specific requirements during the initial phases of development or when those requirements are expected to change over time[3].

4. Example in Practice:

   • Consider a sorting functionality in a software application. By defining a Sorter interface with a sort method, different sorting algorithms like QuickSort or MergeSort can be implemented by classes that implement this Sorter interface. The client code remains unchanged and can operate with any object that conforms to the Sorter interface, thus adhering t...