java class declaration

问题:

I'm familiar with simple class declaration public class test but I don't understand public class test<T>.

回答1:

< T > refers to a generic type. Generic types are introduced in Java to provide you with compile time, and this is important due to type erasure, type-safety. It's especially useful in Collections because it frees you from manual casting.

It is a good idea to read more on generics, especially the documentation on the topic by Angelika Langer is very good: http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html

回答2:

I assume that HTML ate your <T> (you need to write &lt;T&gt; to display it)

T is a type parameter or "generic" parameter. Say you have a List. Then it is for the structure of the list unimportant what exactly you are storing there. Could be Strings, Dates, Apples, SpaceShips, it doesn't matter for list operations like add, remove etc. So you keep it abstract when defining the class ("this is an abstract list"), but specify it when you instantiate it ("this is a list of Strings")

//in Java, C# etc would be similar

//definition
public class List<T> {
   public void add(T t) { ... }  
   public void remove(T t) { ... }  
   public T get(int index) { ... } 
}

//usage
List<String> list = new List<String>(); 
list.add("x"); //now it's clear that every T needs to be a String
...  

回答3:

You are probably referring to Java Generics:

http://www.oracle.com/technetwork/java/javase/generics-tutorial-159168.pdf

回答4:

This is parametric polymorphism, another important form of polymorphism other than subtyping.

In Java land, they call it Generics (see also Lesson: Generics).