java - 将 Pojo 列表转换为嵌套列表

Question

我有 3 个实体:国家、州、城市我将他们的 POJO 写成:

class Country{
String countryName;
List<State> states;
}

class State{
String stateName;
List<City> cities;
}

class City{
String cityName;
}

我的数据库表如下:

Goegraphy
---------
countryName | stateName | cityName

现在，为了从数据库中获取数据，我制作了另一个 POJO:

class Geography{
String countryName;
String stateName;
String cityName;
}

我有一个地理对象列表。但我的要求是现在将此列表转换为早期的 Country-State-City 模型。有人可以帮助实现这一目标。

Answer 1

您正在寻找的是 One to Many关系数据库的关联。所有 JPA 实现都可以很好地为您完成，而无需实现 Geography Pojo。

但如果您坚持手动执行此操作，这里有一种使用 Java 8 流进行操作的优化方法

class Country {

    public Country(String countryName) {
        this.countryName = countryName;
        this.states = new ArrayList<>();
    }

    String countryName;
    List<State> states;

}

class State {

    public State(String stateName) {
        this.stateName = stateName;
        this.cities = new ArrayList<>();
    }


    String stateName;
    List<City> cities;
}

class City {
    public City(String cityName) {
        this.cityName = cityName;
    }

    String cityName;
}

class Geography {
    String countryName;
    String stateName;
    String cityName;
}


List<Country> buildFromGeographies(List<Geography> geos) {

    List<Country> result = new ArrayList<>();

    for (Geography geo : geos) {
        Optional<Country> country1 = result.stream().filter(country -> country.countryName.equals(geo.countryName)).findFirst();
        Country country = country1.orElseGet(() -> {
            Country newOne = new Country(geo.countryName);
            result.add(newOne);
            return newOne;
        });

        Optional<State> state1 = country.states.stream().filter(state -> state.stateName.equals(geo.stateName)).findFirst();
        State state = state1.orElseGet(() -> {
            State newOne = new State(geo.stateName);
            country.states.add(newOne);
            return newOne;
        });

        // taking for granted there is no duplicates in your data set
        state.cities.add(new City(geo.cityName));
    }

    return result;

}