如何简化对象列表到另一个列表？

Question

我有一个List<report>在Java的,我想将其更改为List<graph>

万一

class report {
   String date;
   String type;
   String count;
}

class graph{
   String date;
   String numberA;
   String numberB;
   String numberC;
}

例如我有:

date    type  count     change it into >  date   numberA  numberB  numberC
03/21   A     8017                        03/21   8017     5019      0
03/21   B     5019                        03/22   5001     0         8870                          
03/22   A     5001                        03/23   0        8305      0  
03/22   C     8870                        03/24   1552     2351      8221  
03/23   B     8305
03/24   A     1552                          
03/24   B     2351                          
03/24   C     8221                         

我知道有一些if(条件)和迭代它是可能的但是有另一种解决方案吗？

任何答案将不胜感激.

Answer 1

您希望Graph每个日期有一个实例.因此,将Graph实例存储到Map中,该映射Graph每个日期保留一个.迭代报告列表并在实例进入时构建Graph(numberA,numberB,numberC)的内容Report,并根据它们携带的类型.最后,List<Graph>根据地图的条目构建一个.

使用Java 8(未经测试),流和collect()假设reports是List<Reports>:

    List<Graph> graphs = new ArrayList<>(reports  // we want a list - so let's create one. The rest of the code is just to give it some initial contents 
                    .stream()    // stream of reports
                    .collect(HashMap<String, Graph>::new, //collect them - start with a map of date -> grap
                    (map, report)->{ // here, for each report:
                         // pick a graph instance for the date, create one if it does not exist yet
                         Graph graph = map.computeIfAbsent(report.date, date -> new Graph(report.date));   
                         // Next, populate the graph instance based on report type
                         if ("A".equals(report.type)) { graph.numberA = report.count; }  
                         else if ("B".equals(report.type)) { graph.numberB = report.count; }
                         else if ("C".equals(report.type)) { graph.numberC = report.count; }
                        },
                    Map::putAll) // see collect() javadoc for a description of why this is here
                    .values());  // last step - get all values from the map (it's a Collection)

编辑:修复编译错误,编辑2:添加代码注释

编辑3:上述代码未在图表中将"0"设置为默认值(对于给定日期/类型组合,报告不存在的情况).我建议在Graph类中处理它(String numberA ="0"等).否则,默认值当然是nulls.