Visual Data Flow 9. Apache JENA

Here are three examples of Apache Jena code, along with qualitative and technical explanations for each:

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Example 1: Creating an RDF Model

Qualitative Explanation:

1. Purpose: Create an RDF model to represent data as triples (subject, predicate, object).

2. Use Case: Useful for building semantic web applications or integrating heterogeneous data sources.

3. Outcome: A structured RDF model that can be queried or exported.

Technical Explanation:

1. Create Model: Initialize an in-memory RDF model.

    Model model = ModelFactory.createDefaultModel();
   

2. Define Namespace: Create a namespace for resources.

    String ns = "http://example.org/";
   

3. Add Triples: Add RDF statements to the model.

    Resource john = model.createResource(ns + "John");
    Property livesIn = model.createProperty(ns + "livesIn");
    Resource paris = model.createResource(ns + "Paris");
    model.add(john, livesIn, paris);
   

4. Output Model: Write the model to a file.

    model.write(System.out, "Turtle");
   

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Example 2: Querying an RDF Model with SPARQL

Qualitative Explanation:

1. Purpose: Query an RDF model to extract specific information.

2. Use Case: Retrieve data from a knowledge graph or semantic dataset.

3. Outcome: A result set containing the queried data.

Technical Explanation:

1. Create Model: Initialize an RDF model and add data.

    Model model = ModelFactory.createDefaultModel();
    Resource john = model.createResource(ns + "John");
    Property livesIn = model.createProperty(ns + "livesIn");
    Resource paris = model.createResource(ns + "Paris");
    model.add(john, livesIn, paris);
   

2. Define SPARQL Query: Write a SPARQL query to retrieve data.

    String query = "SELECT ?city WHERE { <http://example.org/John> <http://example.org/livesIn> ?city }";
   

3. Execute Query: Run the query on the model.

    QueryExecution qexec = QueryExecutionFactory.create(query, model);
    ResultSet results = qexec.execSelect();
   

4. Process Results: Iterate through the query results.

    while (results.hasNext()) {
        QuerySolution soln = results.nextSolution();
        System.out.println(soln.get("city"));
    }
   

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Example 3: Reading and Writing RDF Data

Qualitative Explanation:

1. Purpose: Read RDF data from a file and write it in a different format.

2. Use Case: Convert RDF data between formats like Turtle, RDF/XML, or JSON-LD.

3. Outcome: Transformed RDF data in the desired format.

Technical Explanation:

1. Read RDF Data: Load RDF data from a file.

    Model model = ModelFactory.createDefaultModel();
    model.read("input.rdf");
   

2. Write RDF Data: Write the model to a file in Turtle format.

    model.write(new FileOutputStream("output.ttl"), "Turtle");
   

3. Convert Formats: Convert RDF/XML to JSON-LD.

    RDFDataMgr.write(new FileOutputStream("output.jsonld"), model, Lang.JSONLD);
   

4. Handle Errors: Ensure proper error handling for file operations.

    try {
        model.read("input.rdf");
    } catch (RiotException e) {
        System.err.println("Error reading RDF file: " + e.getMessage());
    }
   

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Input and Output Examples

Example 1: Creating an RDF Model

• Input: None (programmatically defined).

• Output:

    @prefix ex: <http://example.org/> .
    ex:John ex:livesIn ex:Paris .
  

Example 2: Querying an RDF Model

• Input:

    @prefix ex: <http://example.org/> .
    ex:John ex:livesIn ex:Paris .
  

• Output:

    http://example.org/Paris
  

Example 3: Reading and Writing RDF Data

• Input (input.rdf):

    <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
             xmlns:ex="http://example.org/">
      <rdf:Description rdf:about="http://example.org/John">
        <ex:livesIn rdf:resource="http://example.org/Paris"/>
      </rdf:Description>
    </rdf:RDF>
  

• Output (output.ttl):

    @prefix ex: <http://example.org/> .
    ex:John ex:livesIn ex:Paris .
  

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