Key notes on Java programming ideas (java development must see)

The Java programming idea is a must read classic for java learning. It is worth reading for both beginners and Daniel. Here is a summary of the key knowledge in the book. This knowledge often appears not only in the written examination and interview process of major well-known companies, but also in the development of large-scale projects, There are not only simple concept understanding problems (such as the difference between is-a relationship and has-a relationship), but also in-depth knowledge related to RTTI and JVM underlying decompilation.

1. Understanding polymorphism in Java (pay attention to distinguish it from C + +)

Why is this output? This requires a clear understanding of the calling order of constructors in Java:

2. Is-a relationship and is-like-a relationship