从列表派生密钥时如何迭代guava MultiMap

Question

I have a List of Orders with Order Date and Order Value. How do I group by order date and calculate the total Order Value per Order Date. How can I achieve this in Google Guava? If its complicating things.. how do I achieve this in Java collections?

Order POJO

Date date;
Integer Value;

Util.java

    ListMultimap<Date, Integer> listMultiMap = ArrayListMultimap.create();
    for(Order o : orders){
      listMultiMap.put(o.date, o.value);
   }
   //Now how do I iterate this listMultiMap and calculate the total value?

Answer 1

I don't think Guava is necessarily the best tool here...nor any normal map, for that matter: if you will have a huge amount of Orders, you should think about using Java8 Streams, that will let you parallelise your calculation. It will also have optimization about primitive types (int vs. Integer)...

In any case, for the specific use case you describe and following the starting code you posted, here it is a potential solution (using LocalDate instead of Date just because it's more handy):

@Test
public void test(){

    // Basic test data
    Order today1 = new Order(LocalDate.now(),1);
    Order today2 = new Order(LocalDate.now(),2);
    Order today3 = new Order(LocalDate.now(),5);
    Order tomorrow1 = new Order(LocalDate.now().plusDays(1),2);
    Order yesterday1 = new Order(LocalDate.now().minusDays(1),5);
    Order yesterday2 = new Order(LocalDate.now().minusDays(1),4);
    List<Order> list = Lists.newArrayList(today1,today2,today3,tomorrow1,yesterday1,yesterday2);

    // Setup multimap and fill it with Orders
    ListMultimap<LocalDate, Integer> mm = ArrayListMultimap.create();
    for(Order o : list){
        mm.put(o.date,o.value);
    }

    // At this point, all you need to do is, for each date "bucket", sum up all values.
    Map<LocalDate, Integer> resultMap = Maps.newHashMap();
    for(LocalDate d : mm.keySet()){
        List<Integer> values = mm.get(d);
        int valuesSum = 0;
        for(int i : values){
            valuesSum += i;
        }
        resultMap.put(d,valuesSum);
    }

    /*
    * Result map should contain:
    * today -> 8
    * tomorrow -> 2
    * yesterday -> 9
    * */
    assertThat(resultMap.size(), is(3));
    assertThat(resultMap.get(LocalDate.now()), is(8));
    assertThat(resultMap.get(LocalDate.now().minusDays(1)), is(9));
    assertThat(resultMap.get(LocalDate.now().plusDays(1)), is(2));
}