java - 编译器是否能从另一个通用参数列表中找出其中一个？

Question

摘自《Thinking in Java》一书:

One of the complaints about generics is that it adds even more text to your code. Consider this :

Map<Person, List<? extends Pet>> petPeople = 
                                 new HashMap<Person, List<? extends Pet>>();

It appears that you are repeating yourself, and that the compiler should figure out one of the generic argument lists from the other. Alas, it cannot, ...

这是否意味着双方的显式类型规范都是由编译器强制执行的？

相反，它似乎对我来说工作得很好(没有明确):

Map<Person, List<? extends Pet>> petPeople = new HashMap();

Answer 1

你读的一本古书副本，自从 Java-7 和添加了菱形运算符以来，它工作得很好:

Map<Person, List<? extends Pet>> petPeople = new HashMap<>();