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Commons Java API for RDF 1.1
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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 org.apache.commons.rdf.api;

import java.io.Serializable;
import java.util.Locale;
import java.util.Objects;
import java.util.Optional;

/**
 * An RDF-1.1 Literal, as defined by
 * RDF-1.1 Concepts and Abstract Syntax, a W3C Recommendation published on
 * 25 February 2014.
 *
 * @see RDF#createLiteral(String)
 * @see RDF#createLiteral(String, IRI)
 * @see RDF#createLiteral(String, String)
 */
public interface Literal extends RDFTerm {

    /**
     * The lexical form of this literal, represented by a
     * Unicode string.
     *
     * @return The lexical form of this literal.
     * @see RDF-1.1
     *      Literal lexical form
     */
    String getLexicalForm();

    /**
     * The IRI identifying the datatype that determines how the lexical form
     * maps to a literal value.
     *
     * If the datatype IRI is
     * http://www.w3.org/1999/02/22-rdf-syntax-ns#langString,
     * {@link #getLanguageTag()} must not return {@link Optional#empty()}, and
     * it must return a valid
     * BCP47 language tag.
     *
     * @return The datatype IRI for this literal.
     * @see RDF-1.1
     *      Literal datatype IRI
     */
    IRI getDatatype();

    /**
     * If and only if the datatype IRI is
     * http://www.w3.org/1999/02/22-rdf-syntax-ns#langString, the language
     * tag for this Literal is a non-empty language tag as defined by
     * BCP47.

     * If the datatype IRI is not
     * http://www.w3.org/1999/02/22-rdf-syntax-ns#langString, this method
     * must return {@link Optional#empty()}.
     * 
     * The value space of language tags is always in lower case; although
     * RDF implementations MAY convert all language tags to lower case,
     * safe comparisons of language tags should be done using
     * {@link String#toLowerCase(Locale)} with the locale
     * {@link Locale#ROOT}.
     * 

     * Implementation note: If your application requires {@link Serializable}
     * objects, it is best not to store an {@link Optional} in a field. It is
     * recommended to use {@link Optional#ofNullable(Object)} to create the
     * return value for this method.
     *
     * @return The {@link Optional} language tag for this literal. If
     *         {@link Optional#isPresent()} returns true, the value returned by
     *         {@link Optional#get()} must be a non-empty language tag string
     *         conforming to BCP47.
     * @see RDF-1.1
     *      Literal language tag
     */
    Optional getLanguageTag();

    /**
     * Check it this Literal is equal to another Literal.
     * 

     * Literal
     * term equality:
     * Two literals are term-equal (the same RDF literal) if
     * and only if the two lexical forms, the two datatype IRIs, and the two
     * language tags (if any) compare equal, character by character. Thus, two
     * literals can have the same value without being the same RDF term.
     * 
     * As the value space for language tags is lower-space, if they are present,
     * they MUST be compared character by character
     * using the equivalent of {@link String#toLowerCase(java.util.Locale)} with
     * the locale {@link Locale#ROOT}.
     * 
     * 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 Literal and is equal to this
     * @see Object#equals(Object)
     */
    @Override
    public boolean equals(Object other);

    /**
     * Calculate a hash code for this Literal.
     * 

     * The returned hash code MUST be equal to the result of
     * {@link Objects#hash(Object...)} with the arguments
     * {@link #getLexicalForm()}, {@link #getDatatype()},
     * {@link #getLanguageTag()}.map(s->s.toLowerString(Locale.ROOT)).
     * 
     * This method MUST be implemented in conjunction with
     * {@link #equals(Object)} so that two equal Literals produce the same hash
     * code.
     *
     * @return a hash code value for this Literal.
     * @see Object#hashCode()
     * @see Objects#hash(Object...)
     */
    @Override
    public int hashCode();

}