/*
 * 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.clerezza;

/**
 * Represents a literal value that can be a node in an RDF Graph.
 * Literals are used to identify values such as numbers and dates by
 * means of a lexical representation. There are two types of literals
 * represented by the subinterfaces {@link PlainLiteral}
 * and {@link TypedLiteral}
 *
 * @author reto
 */
public interface Literal extends RDFTerm {

    /**
     * The lexical form of this literal, represented by a <a
     * href="http://www.unicode.org/versions/latest/">Unicode string</a>.
     *
     * @return The lexical form of this literal.
     * @see <a
     * href="http://www.w3.org/TR/rdf11-concepts/#dfn-lexical-form">RDF-1.1
     * Literal lexical form</a>
     */
    String getLexicalForm();

    /**
     * The IRI identifying the datatype that determines how the lexical form
     * maps to a literal value.
     *
     * @return The datatype IRI for this literal.
     * @see <a
     * href="http://www.w3.org/TR/rdf11-concepts/#dfn-datatype-iri">RDF-1.1
     * Literal datatype IRI</a>
     */
    IRI getDataType();

    /**
     * If and only if the datatype IRI is <a
     * href="http://www.w3.org/1999/02/22-rdf-syntax-ns#langString"
     * >http://www.w3.org/1999/02/22-rdf-syntax-ns#langString</a>, the language
     * tag for this Literal is a language tag as defined by <a
     * href="http://tools.ietf.org/html/bcp47">BCP47</a>.<br>
     * If the datatype IRI is not <a
     * href="http://www.w3.org/1999/02/22-rdf-syntax-ns#langString"
     * >http://www.w3.org/1999/02/22-rdf-syntax-ns#langString</a>, this method
     * must null.
     *
     * @return The language tag of the literal or null if no language tag is defined
     * @see <a
     * href="http://www.w3.org/TR/rdf11-concepts/#dfn-language-tag">RDF-1.1
     * Literal language tag</a>
     */
    public Language getLanguage();

    /**
     * Returns true if <code>obj</code> is an instance of
     * <code>literal</code> that is term-equal with this, false otherwise
     * <p>
     * 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.
     *
     * @return true if obj equals this, false otherwise.
     */
    public boolean equals(Object obj);

    /**
     * Returns the hash code of the lexical form plus the hash code of the
     * datatype plus if the literal has a language the hash code of the
     * language.
     *
     * @return hash code
     */
    public int hashCode();
}