A decent static type system ensures every object will be compatible with the types of all its references, no matter how the program may manipulate them. Only 10

A decent static type system ensures every object will be compatible with the types of all its references, no matter how the program may manipulate them. Only 100% path coverage could replace that guarantee, and that's generally regarded as infeasible. No project I've worked on in twenty years had test coverage I could call "adequate", though I realize this is partly my fault. Hard-core TDD from day one might get you as far as "mediocre", and the industry average is much worse than that.