How to think about OO

I'm interested in your claim that extending classes from the outside makes for code that is easier to test. I've often heard that the experience from languages where this practice is prevalent (Ruby and JavaScript, specifically) that this makes code harder to understand and sometimes causes unfortunate complications, especially when you mix several libraries that extend core classes in subtly inco

[msakamoto-sf]

staticメソッドの良し悪しや、メソッドの引数の設計など