1. 自反性. reflexive . for non null value. x.equals(x) always be true.
2. 对称性. symmetric. for non null value x and y. if x.equals(y) = true, y.equals(x) always will be true.
3. 传递性. transitive. x,y,z
4. 一致性.
原文：https://www.cnblogs.com/it-dennis/p/8931045.html