What does anonymous subclass mean in java?

Question

Recently, I'm learning about JSON, using Google's Gson. And I come across a problem. Here is code:

Type type = new TypeToken<Collection<Data>>(){}.getType();

I can't understand what {} really mean.So I read source code, and get class description like: Constructs a new type literal. Derives represented class from type parameter... Clients create an empty anonymous subclass. anonymous subclass really make me confused? Can anyone explain it concretely?

Are you asking about the type token or about anonymous classes in general? — Savior, Apr 22 '16 at 16:59
Here's an explanation of the type token "hack": http://stackoverflow.com/questions/22271779/is-it-possible-to-use-gson-fromjson-to-get-arraylistarrayliststring — Savior, Apr 22 '16 at 17:11

smac89 · Answer 1 · 2020-03-31T15:38:01.227

The {} is the body of an anonymous class.

If you wanted to make it more readable, this is what it could look like:

class MyTypeToken extends TypeToken<Collection<Data>> { }
TypeToken<Collection<Data>> tcd = new MyTypeToken();
Type type = tcd.getType();

Here is a breakdown of the more succinct shorthand you have used:

Type type =
    new TypeToken<Collection<Data>>() { // this will initialise an anonymous object
        /* in here you can override methods and add your own if you need to */

    }   /* this ends the initialisation of the object. 
           At this point the anonymous class is created and initialised. */

    .getType(); // here you call a method on the object

I prefer using this:

TypeToken<Collection<Data>> tt = new TypeToken<Collection<Data>>(){ };
Type type = tt.getType();

One more thing I want to mention is that anonymous classes do not have the same class type as other anonymous classes which inherit from the same parent.

So if you have the following:

TypeToken<Collection<Data>> tt1 = new TypeToken<Collection<Data>>(){ };
TypeToken<Collection<Data>> tt2 = new TypeToken<Collection<Data>>(){ };

tt1.getClass() == tt2.getClass() will never be true.

Your answer really helps me. Thx. – liaoming Apr 22 '16 at 17:11 — liaoming, Apr 22 '16 at 17:11

What does anonymous subclass mean in java?

1 Answers1