This is a data structure which likes the pivot table in Excel. It could be used to hierarchically accumulate value in Java, Especially 'Group By' operations.

In many object-oriented languages, some times it is hard to do 'Group By' like operations to get the value we want just once. We may use a lot of map structures or loops to calculate different values in different layers, which is annoying.

For instance, Hibernate, a Java persistence framework for relational mapping and query databases. We are used to manipulating data in tables likes manipulate objects in Java.

But for 'Group By' operations, we could execute them both in Java and in databases by query(SQL, HQL...etc). In Java implementation, it's very complicate and difficult because of its object-oriented feature.

You may query data like this:

SELECT a.company, a.department, a.team, COUNT(*) FROM
Employee a 
Group By a.company, a.department, a.team

and in Java:

....
Query qry = EntityManager.createQuery(queryString);
....	
return qry.getResultList();

query.getResultList() will return a list of Object Arrays:

[...
Object[]{"CompanyA", "DepartmentA", "TeamA", 3},
Object[]{"CompanyA", "DepartmentA", "TeamB", 2},
Object[]{"CompanyB", "DepartmentB", "TeamB", 1},
...]

you might use these result as key to build a Hierarchical Statistic Tree:

IHierarchicalStatisticTree HST = new SumTree();
HST.buildTree(list);

then you cound get the accumulated value by calling:

HST.getResult(new Object[]{"CompanyB", "DepartmentB"});
//1
HST.getResult(new Object[]{"CompanyA"});
//5

The difference between SumTree.java and CountTree.java is that former treats the last element in object array as calculating value, but latter just count every element.

• See SumTreeDemo.java, CountTreeDemo.java.