引入泛型类型 : 泛型基础 « 泛型 « Java 教程

1. A generic type can accept parameters.
2. A generic type is often called a parameterized type.
3. You pass reference types in angle brackets to generic types.

E is called a type variable, namely a variable that will be replaced by a type.

1. A generic type that uses a type variable E allows you to pass E when declaring or instantiating the generic type.
2. If E is a class, you may also pass a subclass of E
3. If E is an interface, you may also pass a class that implements E.
4. By convention, you use a single uppercase letter for type variable names.