For example：

Class Cost{
private in distance = 0;
}

Table<String,String,Cost> endTable = HashBasedTable.create();

Cost cost = new Cost();

    cost.setDistance(111);

    cost.setTime(111);

    endTable.put("1","2",cost);

JSON.toJSONString(endTable)

expert: {1:1:{xxxx}}
but is : {empty:false}