Open/Closed Principle (OCP)

SOLID principles:

• Single responsibility principle (SRP)
• Liskov substitution principle (LSP)
• Interface segregation principle (ISP)
• Dependency inversion principle (DIP)

Classes should be easy to extend and alter without changing their code.

Let's consider the calculator example once again:

class CalculatingMachine {
  public static void add(int x, int y) {
    System.out.println(x + y);
  }
}

How do I change its behavior? What if want it to use something else but System.out for printing? Looks like OCP is violated here, there's no explicit way I can do it. One of the possible solutions here is to make this class abstract.

(not for production, illustrative purposes only)

abstract class AbstractCalculatingMachine {
  public void add(int x, int y) { // cant use static anymore
    getPrintStream().print(x + y);
  }
  
  protected abstract PrintStream getPrintStream();
}

class ConsoleCalculatingMachine extends AbstractCalculatingMachine {
  @Override
  protected PrintStream getPrintStream() {
    return System.out;
  }
}

I have all the calculator machine-specific behavior implemented in abstract class AbstractCalculatingMachine. I also have ConsoleCalculatingMachine that uses System.out for printing the results. As soon as I need a way to print results to file, I'll just subclass AbstractCalculatingMachine once again and will return file-based PrintStream in getPrintStream(). This can be done without altering AbstractCalculatingMachine class' code.