---Open-closed principle of big talk design mode

is open for extensions, and is closed for changes.

　　No matter how "closed" the module is, there will be some changes that cannot be closed to it. Since it is impossible to be completely closed, the designer must choose which of the modules he designed should be closed. He must first guess what kinds of changes are most likely to occur and then construct abstractions to isolate those changes.

When we first wrote the code, we assumed that the change would not happen. When changes occur, we create abstractions to isolate similar changes that occur later.

---Open-closed principle of big talk design mode