Gson.toJson incorrectly serializes collection of objects with some circumstances (see unit test)

[pkravtsov]

I guess it is something wrong with TypeAdapter cache mechanism for collections.

My Java version:

java version "1.7.0_80"
Java(TM) SE Runtime Environment (build 1.7.0_80-b15)
Java HotSpot(TM) 64-Bit Server VM (build 24.80-b11, mixed mode)

Gson dependency:

<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.5</version>
</dependency>

Just run this unit test:

package pkravtsov;

import static org.junit.Assert.*;

import java.util.ArrayList;
import java.util.List;

import org.junit.Test;

import com.google.gson.Gson;

public class GsonTest {
    class R {
          List<B> blist;
        }

        class B {
          int b = 1;
          List<R> rlist;
        }

        class C extends B {
          int c = 2;
        }

        class A {
          List<B> a;
        }

    @Test
    public void testToJsonObject() {
        Gson gson = new Gson();
        List<Object> source = new ArrayList<Object>();
        A a = new A();
        a.a = new ArrayList<B>();
        a.a.add(new B());
        a.a.add(new C());
        source.add(a);
        String expected = "[{\"a\":[{\"b\":1},{\"c\":2,\"b\":1}]}]";
        String result = gson.toJson(source);
        assertEquals(expected, result);


        gson = new Gson();// This line is essential to reproduce the bug! Do not use previous successful gson instance!
        source = new ArrayList<Object>();
        source.add(new B());
        a = new A();
        a.a = new ArrayList<B>();
        a.a.add(new B());
        a.a.add(new C());
        source.add(a);
        expected = "[{\"b\":1},{\"a\":[{\"b\":1},{\"c\":2,\"b\":1}]}]";
        String wrong = "[{\"b\":1},{\"a\":[{\"b\":1},{\"b\":1}]}]";
        result = gson.toJson(source);
        assertEquals(wrong, result);
        assertEquals(expected, result);
    }

}

This unit test will be successful if you do any of the following:

1. Move line "source.add(new B());" after "source.add(a);" (do not forget to update "expected" value appropriately).
2. Remove R.blist (do not forget to update "expected" value appropriately).

[Marcono1234]

Can still reproduce this in Gson 2.9.0. It looks like this might be related to #1833, or possibly the same bug.