The Open-Closed Principle aims to make a code entity easy to adapt and reuse without needing to modify the code entity itself.
Open-closed principle (OCP): A module should be open for extension but closed for modification. That is, modules should be written so that they can be extended, without requiring them to be modified. -- proposed by Bertrand Meyer
In object-oriented programming, OCP can be achieved in various ways. This often requires separating the specification (i.e. interface) of a module from its implementation.
In the design given below, the behavior of the CommandQueue class can be altered by adding more concrete Command subclasses. For example, by including a Delete class alongside List, Sort, and Reset, the CommandQueue can now perform delete commands without modifying its code at all. That is, its behavior was extended without having to modify its code. Hence, it is open to extensions, but closed to modification.
The behavior of a Java generic class can be altered by passing it a different class as a parameter. In the code below, the ArrayList class behaves as a container of Students in one instance and as a container of Admin objects in the other instance, without having to change its code. That is, the behavior of the ArrayList class is extended without modifying its code.
ArrayList students = new ArrayList<Student>();
ArrayList admins = new ArrayList<Admin>();