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Example: Hello World

HyperGraph graph = new HyperGraph("/path/to/workdir/bje");
String hello = graph.get(graph.add("Hello World")); 

Example: Store/retrieve data

String someObject = "Lorem ipsum";
HGHandle handle1 = graph.add(someObject);
System.out.println(((String) graph.get(handle1)).toUpperCase());

Example: Store and retrieve data without knowing the handle - querying basics

Querying is done conveniently by using the static helper class "hg":

import org.hypergraphdb.HGQuery.hg.*;

We stored only one element in our database, so we would immediately find "Lorem ipsum" simply by querying for type String:

System.out.println(hg.getOne(graph, hg.type(String.class)));
-> "Lorem ipsum";

hg.getOne returns you any one (of possibly many) matching result as a ready-to-use object, just as did graph.get. But often we have several items that match a certain criteria. We get can get those packed in a List, with hg.getAll: In order to have two atoms, we just add the same object as above:

HGHandle handle2 = graph.add(someObject);

and then query for all Strings:

for (Object s : hg.getAll(graph, hg.type(String.class)))
-> Lorem ipsum Lorem ipsum

We get two results here, that happen to be distinct duplicate copies of the same data (we prove that later).

As you see, querying is generally used in one of this ways:

hg.getOne (graphInstance, QueryCondition); // ->  any one matching object.
hg.getAll (graphInstance, QueryCondition); // ->  all matching objects as a List.

where "QueryCondition" in our example is hg.type(String.class), but of course there is more.


Sometimes you need handles and also you do not want to dereference and deserialize all results of a query into memory. To query by returning handles is easy. It's the same as with getOne/getAll, but instead of h.getOne you use hg.findOne. Instead of hg.getAll, use hg.findAll.

We use this to confirm that we created actual duplicate atoms in the "Lorem ipsum" example above. We printout the handles, and check for equality with the handles obtained before:

for (Object s : hg.findAll(graph, hg.type(String.class)))
  System.out.println((s.equals(handle1) || s.equals(handle2)));
-> 259b3dbd-4e4f-4566-b850-1029f99e6d1b true dceadb0c-318b-4249-917a-559d2f077fcc true

Store data uniquely

How to make sure that a given data is stored only once, even when -by accident or not- the data is stored twice?

String object2 = "dolor sit amet";
HGHandle noDup1 = hg.assertAtom(graph, object2);
HGHandle noDup2 = hg.assertAtom(graph, object2);  //trying hard to duplicate
System.out.println("Are those two handles duplicates, i.e. two distinct handles? : " + (!noDup1.equals(noDup2)));
-> "Are those two handles duplicates, i.e. two distinct handles? : false"

Note that logically there is a cost associated with checking if a given datum already exists. If you don't need unique atoms, graph.add is faster.

Create Links and query for Links

Till now there was nothing graph, only object-oriented database functionality. We also did not do particularly interesting queries.

Let's make a link and query for it:

HGHandle duplicateLink = graph.add(new HGPlainLink(handle1, handle2));
List<HGHandle> dupsList = hg.findAll(graph,, handle2));
System.out.println("querying for link returned that duplicate Link? :" + dupsList.contains(duplicateLink));
=> querying for link returned that duplicate Link? :true

Tags: storage   graph  

Last modified 01 July 2021