Java 8 lambdas nested Map -


i trying use java-8 lambdas solve following problem:

given list<transaction>, each category.minorcategory require sum of transaction.amount per category.minorcategory , map of transaction.accountnumber sum of transaction.amount per transaction.accountnumber. have working, per code below. have requirement group category.majorcategory, returning map<string, map<string, minorcategorysummary>> keyed on category.majorcategory.

i have working until stage of grouping category.majorcategory struggle see solution; paradigm shift of programming lambdas proving steep learning curve.

transactionbreakdown action happens , i'd return map<string, map<string, minorcategorysummary>>.

public class transaction {      private final string accountnumber;     private final bigdecimal amount;     private final category category;  }  public class category {      private final string majorcategory;     private final string minorcategory;  }  public class minorcategorysummary {      private final bigdecimal sumofamountperminorcategory;     private final map<string, bigdecimal> accountnumbersumofamountmap;     private final category category;  }  public class transactionbreakdown {      function<entry<string, list<transaction>>, minorcategorysummary> transactionsummariser = new transactionsummariser();      public map<object, minorcategorysummary> gettransactionsummaries(list<transaction> transactions) {         return transactions                 .stream()                 .collect(groupingby(t -> t.getcategory().getminorcategory()))                 .entryset()                 .stream()                 .collect(                         tomap(entry::getkey,                                 transactionsummariser));     }  }  public class transactionsummariser implements function<entry<string, list<transaction>>, minorcategorysummary> {      @override     public minorcategorysummary apply(             entry<string, list<transaction>> entry) {          return new minorcategorysummary(                     entry.getvalue()                             .stream()                             .map(transaction::getamount)                             .collect(reducing(bigdecimal.zero, bigdecimal::add)),                     entry.getvalue()                             .stream()                             .collect(                                     groupingby(transaction::getaccountnumber,                                             mapping(transaction::getamount,                                                     reducing(bigdecimal.zero, bigdecimal::add)))),                                                      entry.getvalue().get(0).getcategory());     }  }

your class design seems odd me. why put category summary class have category map key? make more sense have summary class without category in it:

public class transactionsummary {      private final bigdecimal amount;     private final map<string, bigdecimal> accttototal;      transactionsummary(map<string, bigdecimal> accttototal) {         this.accttototal = collections.unmodifiablemap(accttototal);         this.amount = accttototal.values().stream()                 .reduce(bigdecimal.zero, bigdecimal::add);     }      public static collector<transaction, ?, transactionsummary> collector() {         // can static constant         return collectingandthen(             tomap(transaction::getaccountnumber,transaction::getamount,bigdecimal::add),             transactionsummary::new         );     }      // getters }

now 2 problems solved , no redundancy:

map<string, transactionsummary> minorsummary = transactions.stream()         .collect(groupingby(                 t -> t.getcategory().getminorcategory(),                 transactionsummary.collector()         ));  map<string, map<string, transactionsummary>> majorminorsummary = transactions.stream()         .collect(groupingby(                 t -> t.getcategory().getmajorcategory(),                 groupingby(                         t -> t.getcategory().getminorcategory(),                         transactionsummary.collector()                 )         ));

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