Question

Which design pattern have you used in your production code? apart from Singleton?

Answer 1

Design patterns are recurring solutions to software design problems you find again and again in real-world application development.  

Using of design patterns shouldn't decided during code writing.  

The decision of using such design pattern should be taken during building the architecture & detailed design of the required system.  

I used to consider Singleton design pattern when I want to ensure that a specific class has only one instance and provide a global point of access to it.  

For example, the PrintSpooler class and LoadBalancer class are typical examples for singleton.