根据某些字段对对象进行分组

Question

我有一个对象列表，具有以下特征：

Class myObject{
  String gender;
  String state;
  int quantity;
  int Salary;
}

List<myObject> myList=new ArrayList<Object>;

作为List的输入，我有以下内容：

并且作为输出，我想只保留具有相同性别和相同状态的对象的一次出现，同时总结数量和salsary correspanding，如下所示：

我的问题是我如何循环浏览myList，查找具有相同性别和相同状态的对象，只保留一次，并将数量与工资相加？

换句话说，对于第一行和第二行（同性别，相同的州），只保留一行并将相应的数量和工资相加

Answer 1

private Collection<MyObject> aggregate(List<MyObject> objects) {
    Map<String, MyObject> map = new HashMap<String, MyObject>();
    for (MyObject current : objects) {
       String key = String.format("%s:%s", current.gender, current.state);
       MyObject aggregated = map.get(key);
       if (aggregated == null) {
           aggregated = new MyObject();
           aggregated.gender = current.gender;
           aggregated.state = current.state;
           map.put(key, aggregated);
       }
       aggregated.quantity += current.quantity;
       aggregated.salary += current.salary;
    }
    return map.values();
}

Answer 2

与Java 8等效：

private static Collection<myObject> aggregate(List<myObject> objects) {
        return objects.stream()
                .collect(groupingBy(myObject::genderAndState, reducing(new myObject(), myObject::merge)))
                .values();
    }

private static myObject merge(myObject o1, myObject o2) {
    myObject tmp = new myObject();
    tmp.gender = o2.gender;
    tmp.state =  o2.state;
    tmp.quantity= o1.quantity + o2.quantity;
    tmp.salary = o1.salary + o2.salary;
    return tmp;
}

private static String genderAndState(myObject o) {
    return o.gender.concat(o.state);
}