Think in java 4th_English_Generics

Generics
Ordinary classes and methods work with specific types: either primitives or class types. If you are writing code that might be used across more types, this rigidity(1) can be overconstraining(2).  (1) 刻板；(2) 超过限制；

One way that object-oriented languages allow generalization is through polymorphism. You can write (for example) a method that takes a base class object as an augument, and then use that method with any class derived from that base calss. Now your method is a little more general and can be used in more places. The same is true within classes--anyplace you use a specific type, a base type provides more flexibility. Of course, anything but final calss can be extended, so this flexibility is automatic much of the time.