how to Simplify List of object to another List?

Question

I have a List<report> in Java and I want to change it to List<graph>

in case that

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

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

for example i have :

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                         

I know that with some if (condition) and iteration it can be possible but is there another solution for this?

any answer will be appreciated.

Answer 1

You want one Graph instance per a date. Thus, store Graph instances into a Map that keeps one Graph per a date. Iterate over the list of reports and build the contents of Graph (numberA, numberB, numberC) as Report instances come in, and based on the type they carry. Last, build a List<Graph> based on the entries of the map.

Using Java 8 (untested), streams and collect(), assuming reports is a 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)

Edit: fixed compile errors, Edit 2: added code comments

Edit 3: The above code does not set "0" as default value in the Graphs (for the case a Report does not exist for a given date / type combination). I would suggest to handle this in the Graph class ( String numberA = "0" etc.). Otherwise, the default values would be nulls of course.