java - 如何重构/聚合 Java 中 ArrayList 中的数值？

Question

我有一个如下所示的 POJO:

public class AgentSales {
   private String agent;
   private int month;
   private int year;
   private int salesCount;

   public AgentSales(String agent, int month, int year, int salesCount) {
       // initializers ...
   } 

   // getters & setters
   // ...
}

还有一个像下面这样的集合:

ArrayList<AgentSales> salesList = new ArrayList<AgentSales>();

salesList.add(new AgentSales("Ben", 1, 2013, 3));
salesList.add(new AgentSales("Ben", 1, 2013, 2));
salesList.add(new AgentSales("Ben", 2, 2013, 1));
salesList.add(new AgentSales("Ben", 3, 2013, 1));
salesList.add(new AgentSales("Ben", 2, 2013, 2));
salesList.add(new AgentSales("Tim", 1, 2013, 1));
salesList.add(new AgentSales("Tim", 1, 2013, 1));
salesList.add(new AgentSales("Tim", 2, 2013, 1));
salesList.add(new AgentSales("Tim", 4, 2013, 1));
salesList.add(new AgentSales("Tim", 2, 2013, 5));
salesList.add(new AgentSales("Joe", 2, 2013, 1));
salesList.add(new AgentSales("Joe", 2, 2013, 2));
salesList.add(new AgentSales("Joe", 3, 2013, 1));
salesList.add(new AgentSales("Joe", 3, 2013, 2));
salesList.add(new AgentSales("Joe", 3, 2013, 1));

如何重构/聚合这个 ArrayList 的内容，以便将对应于同一月份和代理的多个项目合并为 1，并将 salesCount 值相加？

Agent | month | year | sales count
----------------------------------
Ben   |   1   | 2013 |      5
Ben   |   2   | 2013 |      3
Ben   |   3   | 2013 |      1 
Tim   |   1   | 2013 |      2
Tim   |   2   | 2013 |      6

Answer 1

使用 HashMap<String,AgentSales>其中key为agent+month+year进行聚合。

有点像这样:

HashMap<String,AgentSales> aggregate = new HashMap<>();
for (AgentSales as : salesList) {
   String key = as.getAgent() + as.getMonth() + as.getYear();
   AgentSales existing = aggregate.get(key);
   if (existing==null) {
      aggregate.put(key,as);
      continue;
   }
   AgentSales combined = new AgentSales(as.getAgent(), as.getMonth(), as.getYear(), as.getSalesCount()+exisgint.getSalesCount());
   aggregate.put(key, combined); 
}

现在您在 map 中有了按代理人+月份+年份的组合值。然后你可以像这样将它们作为 ArrayList 取回:

ArrayList<AgentSales> asList = new ArrayList<>(aggregate.values());

然后您可以对其进行排序(通过编写 Comparator<AgentSales> 或让 AgentSales 实现 Comparable<AgentSales> )并打印出来