在Java中，object.class有什么作用？

Question

I'm working with Google GSON, and in the docs they mention they following:

 Object Examples class BagOfPrimitives { private int value1 = 1; private String value2 = "abc"; private transient int value3 = 3; BagOfPrimitives() { // no-args constructor } } 

(Serialization)

 BagOfPrimitives obj = new BagOfPrimitives(); Gson gson = new Gson(); String json = gson.toJson(obj); ==> json is {"value1":1,"value2":"abc"} 

Note that you can not serialize objects with circular references since that will result in infinite recursion.

(Deserialization)

 BagOfPrimitives obj2 = gson.fromJson(json, BagOfPrimitives.class); ==> obj2 is just like obj 

At the very bottom, they use BagOfPrimitives.class . What does that do exactly? (I would imagine it might return the class, but in that case I'd expect the code just to omit the '.class').

Answer 1

It's a class literal - it gives a reference to the Class object representing the particular class. See section 15.8.2 of the JLS for more details. For example:

String text = "Hello";
Class<?> classFromObject = text.getClass();
Class<?> classFromLiteral = String.class;
System.out.println(classFromObject == classFromLiteral); // true

In the case of the deserialization, it's to tell the deserializer what type to use to try to deserialize the data as.

Answer 2

You put a type in the method so you don't have to cast it. The fromJson method is generic and resolves the type itself according to the type you write in there...

<T> T fromJson(String json, Class<T> classOfT)