added TriplePatternElement as a generic, API-independent interface tp represent elements of triple patterns; plus a generic factory for possible TriplePatternElement objects

Author: Olaf Hartig

src/org/linkeddatafragments/fragments/tpf/TriplePatternElement.java

@@ -0,0 +1,49 @@
+package org.linkeddatafragments.fragments.tpf;
+
+/**
+ * Represents an element of a triple pattern (i.e., subject, predicate, object). 
+ *
+ * @param <TermType> type for representing RDF terms in triple patterns 
+ * @param <VarType> type for representing specific variables in triple patterns
+ *
+ * @author <a href="http://olafhartig.de">Olaf Hartig</a>
+ */
+public interface TriplePatternElement<TermType,VarType>
+{
+    /**
+     * Returns true if this element is a variable (named or unnamed).
+     */
+    boolean isVariable();
+
+    /**
+     * Returns true if this element is a specific variable, and false if either
+     * it is not a variable but an RDF term or it is some variable that is not
+     * specified. The latter (unspecified variables) is possible because when
+     * a client requests a triple pattern fragment, it may omit triple pattern
+     * related parameters.
+     *
+     * If this element is a specific variable (that is, this method returns
+     * true), this specific variable can be obtained by {@link #asVariable()}.
+     */
+    boolean isSpecificVariable();
+
+    /**
+     * Returns a representation of this element as a specific variable (assuming
+     * it is a specific variable).
+     *
+     * @throws UnsupportedOperationException
+     *         If this element is not a specific variable (i.e.,
+     *         if {@link #isSpecificVariable()} returns false).
+     */
+    VarType asVariable() throws UnsupportedOperationException;
+
+    /**
+     * Returns a representation of this element as an RDF term (assuming it is
+     * an RDF term and not a variable).
+     *
+     * @throws UnsupportedOperationException
+     *         If this element is not an RDF term but a variable
+     *         (i.e., if {@link #isVariable()} returns true).
+     */
+    TermType asTerm() throws UnsupportedOperationException;
+}

src/org/linkeddatafragments/fragments/tpf/TriplePatternElementFactory.java

@@ -0,0 +1,63 @@
+package org.linkeddatafragments.fragments.tpf;
+
+/**
+ * A factory for {@link TriplePatternElement}s. 
+ *
+ * @param <TermType> type for representing RDF terms in triple patterns 
+ * @param <VarType> type for representing specific variables in triple patterns
+ *
+ * @author <a href="http://olafhartig.de">Olaf Hartig</a>
+ */
+public class TriplePatternElementFactory<TermType,VarType>
+{
+    public TriplePatternElement<TermType,VarType> createUnspecifiedVariable() {
+        return new UnspecifiedVariable<TermType,VarType>();
+    }
+
+    public TriplePatternElement<TermType,VarType> createSpecificVariable(
+            final VarType variable ) {
+        return new SpecificVariable<TermType,VarType>( variable );
+    }
+    public TriplePatternElement<TermType,VarType> createRDFTerm(
+            final TermType term ) {
+        return new RDFTerm<TermType,VarType>( term );
+    }
+
+    static abstract public class Variable<TermType,VarType>
+        implements TriplePatternElement<TermType,VarType>
+    {
+        public boolean isVariable() { return true; }
+        public TermType asTerm() { throw new UnsupportedOperationException(); }
+    }
+
+    static public class UnspecifiedVariable<TermType,VarType>
+        extends Variable<TermType,VarType>
+    {
+        public boolean isSpecificVariable() { return false; }
+        public VarType asVariable() { throw new UnsupportedOperationException(); }
+        public String toString() { return "UnspecifiedVariable"; }
+    }
+
+    static public class SpecificVariable<TermType,VarType>
+        extends Variable<TermType,VarType>
+    {
+        protected final VarType v;
+        public SpecificVariable( final VarType variable ) { v = variable; }
+        public boolean isSpecificVariable() { return true; }
+        public VarType asVariable() { return v; }
+        public String toString() { return "SpecificVariable(" + v.toString() + ")"; }
+    }
+
+    static public class RDFTerm<TermType,VarType>
+        implements TriplePatternElement<TermType,VarType>
+    {
+        protected final TermType t;
+        public RDFTerm( final TermType term ) { t = term; }
+        public boolean isVariable() { return false; }
+        public boolean isSpecificVariable() { return false; }
+        public VarType asVariable() { throw new UnsupportedOperationException(); }
+        public TermType asTerm() { return t; }
+        public String toString() { return "RDFTerm(" + t.toString() + ")(type: " + t.getClass().getSimpleName() + ")"; }
+    }
+
+}