Java tutorial
/** * 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 org.apache.commons.rdf.api; import java.util.UUID; /** * A <a href= "http://www.w3.org/TR/rdf11-concepts/#dfn-blank-node" >RDF-1.1 * Blank Node</a>, as defined by <a href= * "http://www.w3.org/TR/rdf11-concepts/#section-blank-nodes" >RDF-1.1 Concepts * and Abstract Syntax</a>, a W3C Recommendation published on 25 February 2014.<br> * * Note: <blockquote> <a * href="http://www.w3.org/TR/rdf11-concepts/#dfn-blank-node">Blank nodes</a> * are disjoint from IRIs and literals. Otherwise, the set of possible blank * nodes is arbitrary. RDF makes no reference to any internal structure of blank * nodes. </blockquote> * * Also note that: <blockquote> <a * href="http://www.w3.org/TR/rdf11-concepts/#dfn-blank-node-identifier">Blank * node identifiers</a> are local identifiers that are used in some concrete RDF * syntaxes or RDF store implementations. They are always * <em>locally scoped</em> to the file or RDF store, and are <em>not</em> * persistent or portable identifiers for blank nodes. Blank node identifiers * are <em>not</em> part of the RDF abstract syntax, but are entirely dependent * on the concrete syntax or implementation. The syntactic restrictions on blank * node identifiers, if any, therefore also depend on the concrete RDF syntax or * implementation. * * Implementations that handle blank node identifiers in concrete syntaxes need * to be careful not to create the same blank node from multiple occurrences of * the same blank node identifier except in situations where this is supported * by the syntax. </blockquote> * * A BlankNode SHOULD contain a {@link UUID}-derived string as part of its * universally unique {@link #uniqueReference()}. * * @see RDFTermFactory#createBlankNode() * @see RDFTermFactory#createBlankNode(String) * @see <a href="http://www.w3.org/TR/rdf11-concepts/#dfn-blank-node">RDF-1.1 * Blank Node</a> */ public interface BlankNode extends BlankNodeOrIRI { /** * Return a reference for uniquely identifying the blank node. * <p> * The reference string MUST universally and uniquely identify this blank * node. That is, different blank nodes created separately in different JVMs * or from different {@link RDFTermFactory} instances MUST NOT have the same * reference string. * <p> * The {@link #uniqueReference()} of two <code>BlankNode</code> instances * MUST be equal if and only if the two blank nodes are equal according to * {@link #equals(Object)}. * <p> * Clients should not assume any particular structure of the reference * string, however it is recommended that the reference string contain a * UUID-derived string, e.g. as returned from {@link UUID#toString()}. * <p> * <strong>IMPORTANT:</strong> This is not a <a * href="http://www.w3.org/TR/rdf11-concepts/#dfn-blank-node-identifier"> * blank node identifier</a> nor a serialization/syntax label, and there are * no guarantees that it is a valid identifier in any concrete RDF syntax. * For an N-Triples compatible identifier, use {@link #ntriplesString()}. * * @return A universally unique reference to identify this {@link BlankNode} */ String uniqueReference(); /** * Check it this BlankNode is equal to another BlankNode. Two BlankNodes * MUST be equal if, and only if, they have the same * {@link #uniqueReference()}. * <p> * Implementations MUST also override {@link #hashCode()} so that two equal * Literals produce the same hash code. * * @param other Another object * @return true if other is a BlankNode instance that represent the same blank node * @see Object#equals(Object) */ @Override public boolean equals(Object other); /** * Calculate a hash code for this BlankNode. * <p> * The returned hash code MUST be equal to the * {@link String#hashCode()} of the * {@link #uniqueReference()}. * <p> * This method MUST be implemented in conjunction with * {@link #equals(Object)} so that two equal BlankNodes produce the same * hash code. * * @return a hash code value for this BlankNode. * @see Object#hashCode() */ @Override public int hashCode(); }