SubObjA in your example is a single object but in your class it is a collection, so JSON should look like
[{"ObjA":"FOO",
"SubObjA":[{
"A":0,
"B":true,
"C":2,
"D":0.2
}],
"ObjB":false,
"ObjC":295,
}]
Note brackets around SubObjA.