Okay, I found solution. It was very simple. Just had to use the function fromJson instead of toJson with the same GraphAdapterBuilder structure.
...
GsonBuilder gsonBuilder = new GsonBuilder();
new GraphAdapterBuilder()
.addType(ClassA.class)
.addType(ClassB.class)
.registerOn(gsonBuilder);
gson = gsonBuilder.create();
СlassB B = gson.fromJson(json,ClassB.class);
System.out.println("B " + B.vector);
for(ClassA classA:B.vector){
System.out.println(classA.field + " " + classA.parent);
}
Output is:
B [ClassA@10178f2b, ClassA@7ab8584d, ClassA@5cad662c]
1 ClassB@7c0f023c
2 ClassB@7c0f023c
3 ClassB@7c0f023c