Serializing not as expected when instancing within double braces

[tkzt]

Gson version

2.10

Java / Android version

Java

Used tools

• Maven; version: 3.8.1
• Gradle; version:
• ProGuard (attach the configuration file please); version:
• ...

Description

When creating an instance of a nested class in a normal way:

class Test {
    public String name;
    public Test2 test2;

    public void setName(String name){
        this.name = name;
    }

    public void setTest2(Test2 test2){
        this.test2 = test2;
    }
}
class Test2{
    public String info;
    public void setInfo(String info){
        this.info = info;
    }
}

public class Main {
    public static void main(String[] args) {
        Gson gson = new Gson();
        
        Test test = new Test(){{setName("foo");}};
        test.setTest2(new Test2(){{setInfo("bar");}});
        String result = gson.toJson(test);
    }
}

The result is:

{"name":"foo","test2":{"info":"bar"}}

But when instancing Test2 also in a double braces way:

public class Main {
    public static void main(String[] args) {
        Gson gson = new Gson();
        
        Test test = new Test(){{ setName("foo"); setTest2(new Test2(){{ setInfo("bar"); }}); }};
        String result = gson.toJson(test);
    }
}

test2 part is gone:

{"name":"foo"}

Expected behavior

Nested part will also be serialized when in a double braces way.

Actual behavior

It will not.

[Marcono1234]

Double brace initialization actually creates an anonymous class, and Gson does not allow anonymous classes at the moment. Unfortunately it does not handle them in a very user friendly way and just serializes a JSON null, which is omitted from your output because new Gson() omits null values, see also GsonBuilder.serializeNulls().

There is an open discussion about this in #1510, therefore I am closing your issue. Unfortunately no backward compatible solution for this has been found yet.

[eamonnmcmanus]

Just to add that double-brace initialization is usually best avoided anyway, as detailed here.

[tkzt]

@eamonnmcmanus @Marcono1234
Learned a lot! Thanks~👻