Immutability is a special thing that in my mind deserves a short explanation and praise.
If you're familiar with functional programming you surely recognize the concept of immutability because it's a key ingredient of the paradigm. In the world of object-oriented programming, it's not as used and as easy to use approach but there are ways to incorporate immutability to parts of the code and I strongly suggest you do so too.
Quick Intro to Immutability
The basic idea of immutability is unchangeable data.