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Posted to commits@commonsrdf.apache.org by st...@apache.org on 2016/11/21 11:16:07 UTC
incubator-commonsrdf git commit: Introduction to RDF using Commons RDF
Repository: incubator-commonsrdf
Updated Branches:
refs/heads/master da91cb73a -> 567b775be
Introduction to RDF using Commons RDF
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Project: http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/repo
Commit: http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/commit/567b775b
Tree: http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/tree/567b775b
Diff: http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/diff/567b775b
Branch: refs/heads/master
Commit: 567b775be8eac714d557a2c3cc171895f7c08c6c
Parents: da91cb7
Author: Stian Soiland-Reyes <st...@apache.org>
Authored: Mon Nov 21 11:06:45 2016 +0000
Committer: Stian Soiland-Reyes <st...@apache.org>
Committed: Mon Nov 21 11:15:46 2016 +0000
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examples/src/example/IntroToRDF.java | 79 ++++++
examples/src/example/IntroToRDFTest.java | 30 ++
src/site/markdown/introduction.md | 383 ++++++++++++++++++++++++++
src/site/resources/images/rdf-01.svg | 30 ++
src/site/resources/images/rdf-02.svg | 31 +++
5 files changed, 553 insertions(+)
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http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/blob/567b775b/examples/src/example/IntroToRDF.java
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diff --git a/examples/src/example/IntroToRDF.java b/examples/src/example/IntroToRDF.java
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+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package example;
+
+import org.apache.commons.rdf.api.*;
+import org.apache.commons.rdf.simple.SimpleRDF;
+
+public class IntroToRDF {
+ public static void main(String[] args) {
+ RDF rdf = new SimpleRDF();
+
+ IRI alice = rdf.createIRI("Alice");
+ System.out.println(alice.ntriplesString());
+
+ IRI knows = rdf.createIRI("knows");
+ IRI bob = rdf.createIRI("Bob");
+
+ Triple aliceKnowsBob = rdf.createTriple(alice, knows, bob);
+ System.out.println(aliceKnowsBob.getSubject().ntriplesString());
+
+ System.out.println(aliceKnowsBob);
+
+ Graph graph = rdf.createGraph();
+ graph.add(aliceKnowsBob);
+
+ IRI charlie = rdf.createIRI("Charlie");
+
+ IRI plays = rdf.createIRI("plays");
+
+ IRI football = rdf.createIRI("Football");
+ IRI tennis = rdf.createIRI("Tennis");
+
+ graph.add(alice, knows, charlie);
+ graph.add(alice, plays, tennis);
+ graph.add(bob, knows, charlie);
+ graph.add(bob, plays, football);
+ graph.add(charlie, plays, tennis);
+
+ System.out.println("Who plays Tennis?");
+ for (Triple triple : graph.iterate(null, plays, tennis)) {
+ System.out.println(triple.getSubject());
+ System.out.println(plays.equals(triple.getPredicate()));
+ System.out.println(tennis.equals(triple.getObject()));
+ }
+
+ System.out.println("Who does Alice know?");
+ for (Triple triple : graph.iterate(alice, knows, null)) {
+ System.out.println(triple.getObject());
+ }
+
+
+ System.out.println("Does Alice anyone that plays Football?");
+ for (Triple triple : graph.iterate(alice, knows, null)) {
+ RDFTerm aliceFriend = triple.getObject();
+ if (! (aliceFriend instanceof BlankNodeOrIRI)) {
+ continue;
+ }
+ if (graph.contains( (BlankNodeOrIRI)aliceFriend, plays, football)) {
+ System.out.println("Yes, " + aliceFriend);
+ }
+ }
+
+ }
+}
http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/blob/567b775b/examples/src/example/IntroToRDFTest.java
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diff --git a/examples/src/example/IntroToRDFTest.java b/examples/src/example/IntroToRDFTest.java
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@@ -0,0 +1,30 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package example;
+
+import org.junit.Before;
+import org.junit.Test;
+
+public class IntroToRDFTest {
+
+ @Test
+ public void runIntroToRDF() {
+ IntroToRDF.main(new String[0]);
+ }
+
+}
http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/blob/567b775b/src/site/markdown/introduction.md
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diff --git a/src/site/markdown/introduction.md b/src/site/markdown/introduction.md
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+# Introduction to RDF using Commons RDF
+
+This page is a tutorial to introduce programming with the
+[Resource Description Framework (RDF)](https://www.w3.org/TR/rdf11-concepts/)
+using Java and [Apache Commons RDF](index.md).
+If you already know RDF, you may instead jump ahead to the
+[Commons RDF user guide](userguide.html).
+
+This is not meant as an extensive RDF tutorial, for that please consult the
+[W3C RDF 1.1 Primer](https://www.w3.org/TR/rdf11-primer/). You may also like
+the
+[Apache Jena introduction to RDF](https://jena.apache.org/tutorials/rdf_api.html)
+which uses the [Apache Jena](https://jena.apache.org/) implementation directly.
+
+This tutorial attempts to show the basic concepts of RDF and how you can
+work with RDF programmatically using the
+[Apache Commons RDF API](index.md) in a simple Java program.
+
+
+## Getting started with Commons RDF
+
+This tutorial will assume you already know
+a bit of [Java programming](http://docs.oracle.com/javase/tutorial/)
+and that you use an _IDE_ like
+[Eclipse](http://www.eclipse.org/) or
+[Netbeans](https://netbeans.org/). Note that Commons RDF requires
+Open JDK 8, [Java 8](https://www.java.com/) or equivalent.
+
+The Commons RDF JARs are [available from Maven Central](download.html).
+While there are multiple [Commons RDF implementations](implementations.html),
+this tutorial will use the built-in _simple_ implementation as it requires no
+additional dependencies.
+
+First, create a new Java project for this tutorial, say `rdftutorial`.
+
+**Tip**: Check that your IDE project is using the **Java 8** syntax and compiler.
+
+We'll create the package name
+`org.example`, but you can use whatever you prefer. Then create `RdfTutorial.java`
+with a static `main()` method we can run:
+
+```java
+package org.example;
+
+import org.apache.commons.rdf.api.*;
+import org.apache.commons.rdf.simple.SimpleRDF;
+
+public class RdfTutorial {
+ public static void main(String[] args) {
+ // ...
+ }
+}
+```
+
+
+### Adding Commons RDF to the class path
+
+Above we added the `import` for the Commons RDF API, but the library
+is not yet on your class path.
+
+**Note**: If you are already familiar with
+_Maven_, then see instead
+[how to use Commons RDF from Maven](userguide.html#Using_Commons_RDF_from_Maven) and add
+the `commons-rdf-simple` dependency to your project. This will make it easier later
+to share your project or to use newer versions of Commons RDF.
+
+This tutorial assumes a classic Java project with local `.jar` files (say in your project's `lib/` folder), so download and add to your project's class path:
+
+* [commons-rdf-api-0.3.0-incubating.jar](https://repo.maven.apache.org/maven2/org/apache/commons/commons-rdf-api/0.3.0-incubating/commons-rdf-api-0.3.0-incubating.jar)
+([signature](https://repo.maven.apache.org/maven2/org/apache/commons/commons-rdf-api/0.3.0-incubating/commons-rdf-api-0.3.0-incubating.jar.asc))
+* [commons-rdf-simple-0.3.0-incubating.jar](https://repo.maven.apache.org/maven2/org/apache/commons/commons-rdf-simple/0.3.0-incubating/commons-rdf-simple-0.3.0-incubating.jar) ([signature](https://repo.maven.apache.org/maven2/org/apache/commons/commons-rdf-simple/0.3.0-incubating/commons-rdf-simple-0.3.0-incubating.jar.asc))
+
+_Tip: If you prefer you can [verify the signatures](https://www.apache.org/info/verification.html) using the Commons RDF [KEYS](http://www.apache.org/dist/incubator/commonsrdf/KEYS)._
+
+As there are [multiple Commons RDF implementations](implementations.html),
+we have to say which one we want to use. Add to your `RdfTutorial` class:
+
+```java
+RDF rdf = new SimpleRDF();
+```
+
+If you have the classpath set up correctly, you should now
+be able to compile `RdfTutorial` without warnings.
+
+
+## RDF resources
+
+"The clue is in the name"; the _Resource Description Framework_ (RDF) is
+for describing **resources**. But what is a resource?
+
+Anything can be a resource, it is just a concept we want to describe,
+like computer _files_ (text document, image, database),
+_physical things_ (person, place, cat),
+_locations_ (city, point on a map), or more _abstract concepts_ (organization,
+disease, theatre play).
+
+To know which concept we mean, in RDF the resource needs to either:
+
+* have a global _identifier_; we call this an **IRI**
+* be used _indirectly_ in a statement; we call this a **blank node**
+* be a _value_, we call this a **literal**
+
+In this tutorial we'll use the _IRI_ syntax `<identifier>` to indicate an identified resource,
+the _blank node_ syntax `_:it` to indicate an indirectly referenced
+resource, and the _literal_ syntax `"Hello"` to indicate a value.
+
+Don't worry about this syntax, RDF is a **model** with several
+ways to represent it when saved to a file; the [Commons RDF API](apidocs/index.html?org/apache/commons/rdf/api/package-summary.html) directly
+reflects the RDF model in a syntax-neutral way.
+
+Let's create our first identified resource, an `IRI` instance:
+
+```java
+IRI alice = rdf.createIRI("Alice");
+System.out.println(alice.ntriplesString());
+```
+
+This should print out:
+
+> `<Alice>`
+
+
+**Note**: For simplicity this tutorial use _relative IRI references_ which
+are not really global identifiers. While this is supported by
+`SimpleRDF`, some implementations will require
+_absolute IRIs_ like `<http://example.com/Alice>`.
+
+### Triples
+
+
+To describe a resource in RDF we provide one or more statements,
+which are called _triples_ of 3 resources
+(_subject_, _predicate_, _object_):
+
+```turtle
+<Alice> <knows> <Bob> .
+```
+
+
+
+This RDF statement is a relationship between the **subject** `<Alice>`
+and the **object** `<Bob>`, not dissimilar from the subject and direct object
+of the similar English sentence _"Alice knows Bob"_.
+
+What kind of relationship? Well, that is
+identified with the **predicate** `<knows>`.
+The relationship is _directional_, from the subject to the object;
+although _Alice knows Bob_, we don't know if Bob really knows Alice!
+In RDF the predicate is also called a _property_ as it is
+describing the subject.
+
+You may have noticed that properties are also resources -
+to understand
+the kind of relationship we also need a description of it's concept.
+More about this later!
+
+Let's try to create the above statement
+in Commons RDF; first we'll create the remaining
+resources `<knows>` and `<Bob>`:
+
+```java
+IRI knows = rdf.createIRI("knows");
+IRI bob = rdf.createIRI("Bob");
+```
+
+Note that the Java variable names `alice`, `knows` and `bob`
+are not important to Commons RDF,
+we could as well have called these `a`, `k`, `b`,
+but to not confuse yourself it's good to keep the variable names
+somewhat related to the captured identifiers.
+
+Next we'll create a `Triple`:
+
+```java
+Triple aliceKnowsBob = rdf.createTriple(alice, knows, bob);
+```
+
+We can access `.getSubject()`, `.getPredicate()` and `.getObject()` from a `Triple`:
+
+```java
+System.out.println(aliceKnowsBob.getSubject().ntriplesString());
+System.out.println(aliceKnowsBob.getPredicate().ntriplesString());
+System.out.println(aliceKnowsBob.getObject().ntriplesString());
+```
+
+> `<Alice>` <br>
+> `<knows>` <br>
+> `<Bob>`
+
+_**Tip**: Instances from `SimpleRDF` can be printed directly, as
+`System.out` would use their `.toString()`,
+but for consistent behaviour across implementations we use `.ntriplesString()` above._
+
+With `SimpleRDF` we can also print the `Triple` for debugging:
+
+```java
+System.out.println(aliceKnowsBob);
+```
+
+> `<Alice> <knows> <Bob> .`
+
+
+### Graph
+
+By using the same identified resources in multiple triples, you can
+create a _graph_. For instance, this graph shows multiple relations
+of `<knows>` and `<plays>`:
+
+```turtle
+<Alice> <knows> <Bob> .
+<Alice> <knows> <Charlie> .
+<Alice> <plays> <Tennis> .
+<Bob> <knows> <Charlie> .
+<Bob> <plays> <Football> .
+<Charlie> <plays> <Tennis> .
+```
+
+The power of a graph as a data structure is that you don't have to decide a
+hierarchy. The statements of an RDF graph can be listed in any order, and so
+we should not consider the `<Alice>` resource as anything more special
+than `<Bob>` or `<Tennis>`.
+
+
+
+It is therefore possible to _query_ the graph, such as _"Who plays Tennis?_ or
+_"Who does Alice know?"_, but also more complex, like
+_"Does Alice anyone that plays Football?"_.
+
+Let's try that now using Commons RDF. To keep the triples we'll need a `Graph`:
+
+```java
+Graph graph = rdf.createGraph();
+```
+
+We already have the first triple, so we'll `.add()` it to the `graph`:
+
+```java
+graph.add(aliceKnowsBob);
+```
+
+Before adding the remaining statements we need a few more resources:
+
+```java
+IRI charlie = rdf.createIRI("Charlie");
+
+IRI plays = rdf.createIRI("plays");
+
+IRI football = rdf.createIRI("Football");
+IRI tennis = rdf.createIRI("Tennis");
+```
+
+Now we use the `graph.add(subj,pred,obj)` shorthand which creates the
+`Triple` instances and add them to the graph.
+
+```java
+graph.add(alice, knows, charlie);
+graph.add(alice, plays, tennis);
+graph.add(bob, knows, charlie);
+graph.add(bob, plays, football);
+graph.add(charlie, plays, tennis);
+```
+
+Next we'll ask the graph those questions using `.iterate(s,p,o)` and
+`null` as the wildcard.
+
+```java
+System.out.println("Who plays Tennis?");
+for (Triple triple : graph.iterate(null, plays, tennis)) {
+ System.out.println(triple.getSubject());
+}
+```
+
+> `Who plays Tennis?` <br>
+> `<Alice>` <br>
+> `<Charlie>`
+
+
+Notice how we only print out the `.getSubject()` (our wildcard), if you
+check `.getPredicate()` or `.getObject()` you will find they are equal to
+`plays` and `tennis`:
+
+```java
+System.out.println("Who plays Tennis?");
+for (Triple triple : graph.iterate(null, plays, tennis)) {
+ System.out.println(triple.getSubject());
+ System.out.println(plays.equals(triple.getPredicate()));
+ System.out.println(tennis.equals(triple.getObject()));
+}
+```
+
+We can query with wildcards in any positions, for instance for
+the _object_:
+
+```java
+System.out.println("Who does Alice know?");
+for (Triple triple : graph.iterate(alice, knows, null)) {
+ System.out.println(triple.getObject());
+}
+```
+
+> `Who does Alice know?` <br>
+> `<Bob>` <br>
+> `<Charlie>`
+
+Let's try to look up which of those friends play football:
+
+```java
+System.out.println("Does Alice anyone that plays Football?");
+for (Triple triple : graph.iterate(alice, knows, null)) {
+ RDFTerm aliceFriend = triple.getObject();
+ if (graph.contains(aliceFriend, plays, football)) {
+ System.out.println("Yes, " + aliceFriend);
+ }
+}
+```
+
+You will get a compiler error:
+
+> `RDFTerm` cannot be converted to `BlankNodeOrIRI`
+
+This is because in an RDF triple, not all kind of resources can be used in all
+positions, and the kind of resource in Commons RDF is indicated by the
+interfaces:
+
+* [IRI](apidocs/org/apache/commons/rdf/api/IRI.html) (identified)
+* [BlankNode](apidocs/org/apache/commons/rdf/api/BlankNode.html) (indirect)
+* [Literal](apidocs/org/apache/commons/rdf/api/Literal.html) (value)
+
+Look at the method signature of [graph.contains(s,p,o)](http://commonsrdf.incubator.apache.org/apidocs/org/apache/commons/rdf/api/Graph.html#contains-org.apache.commons.rdf.api.BlankNodeOrIRI-org.apache.commons.rdf.api.IRI-org.apache.commons.rdf.api.RDFTerm-):
+
+```java
+boolean contains(BlankNodeOrIRI subject,
+ IRI predicate,
+ RDFTerm object)
+```
+
+In short, for any RDF triple:
+
+* The **subject** must be a [BlankNodeOrIRI](apidocs/org/apache/commons/rdf/api/BlankNodeOrIRI.html), that is either a `BlankNode` or `IRI`
+* The **predicate** must be a [IRI](apidocs/org/apache/commons/rdf/api/IRI.html) (so we can look up what it means)
+* The **object** must be a [RDFTerm](apidocs/org/apache/commons/rdf/api/RDFTerm.html), that is either a `BlankNode`, `IRI` or `Literal`
+
+As we are retrieving triples from the graph, the `triple.getObject()` is only known
+to be an RDFTerm if we use it as a Java variable - there could in theory be triples
+in the graph with `Literal` and `BlankNode` objects:
+
+```
+<Alice> <knows> "Santa Claus".
+<Alice> <knows> _:someone.
+```
+
+
+In this case we could have done a naive casting like `(IRI)aliceFriend`; we
+inserted her `IRI`-represented friends right before, but this is a toy
+example - there's no need to use RDF if you already know the answer!
+
+So unless you know for sure in your graph that `<knows>` is never used with a
+literal value as object, this would not be safe. So we'll do an `instanceof`
+check (skipping any literals) and cast to `BlankNodeOrIRI`:
+
+
+```java
+System.out.println("Does Alice anyone that plays Football?");
+for (Triple triple : graph.iterate(alice, knows, null)) {
+ RDFTerm aliceFriend = triple.getObject();
+ if (! (aliceFriend instanceof BlankNodeOrIRI)) {
+ continue;
+ }
+ if (graph.contains( (BlankNodeOrIRI)aliceFriend, plays, football)) {
+ System.out.println("Yes, " + aliceFriend);
+ }
+}
+```
+
+> `Does Alice anyone that plays Football?`
+> `Yes, <Bob>`
+
+## Literal values
+
+We talked briefly about literals above as a way to represent values in RDF.
+What is a value? In a way you could a value is when we no longer want to
+stay in graph land and just want to use primitive types like `long`,
+`int` or `String`.
http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/blob/567b775b/src/site/resources/images/rdf-01.svg
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diff --git a/src/site/resources/images/rdf-01.svg b/src/site/resources/images/rdf-01.svg
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+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+ Licensed to the Apache Software Foundation (ASF) under one
+ or more contributor license agreements. See the NOTICE file
+ distributed with this work for additional information
+ regarding copyright ownership. The ASF licenses this file
+ to you under the Apache License, Version 2.0 (the
+ "License"); you may not use this file except in compliance
+ with the License. You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+
+-->
+<!--
+To edit, use https://www.draw.io/
+When saving, use
+File -> Export as -> SVG
+ Zoom: 100%
+ (x) Transparent background
+ (x) Include a copy of the diagram
+-->
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http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/blob/567b775b/src/site/resources/images/rdf-02.svg
----------------------------------------------------------------------
diff --git a/src/site/resources/images/rdf-02.svg b/src/site/resources/images/rdf-02.svg
new file mode 100644
index 0000000..0d62cbf
--- /dev/null
+++ b/src/site/resources/images/rdf-02.svg
@@ -0,0 +1,31 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+ Licensed to the Apache Software Foundation (ASF) under one
+ or more contributor license agreements. See the NOTICE file
+ distributed with this work for additional information
+ regarding copyright ownership. The ASF licenses this file
+ to you under the Apache License, Version 2.0 (the
+ "License"); you may not use this file except in compliance
+ with the License. You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+
+-->
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+ Zoom: 100%
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