001/*
002 * ModeShape (http://www.modeshape.org)
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 *       http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016package org.modeshape.schematic.document;
017
018import org.modeshape.schematic.annotation.Immutable;
019
020/**
021 * An object representation of 'null'. This is sometimes more convenient than dealing with 'null' as values.
022 * 
023 * @author Randall Hauch <rhauch@redhat.com> (C) 2011 Red Hat Inc.
024 */
025@Immutable
026public class Null {
027
028    protected static final Null INSTANCE = new Null();
029
030    public static final Null getInstance() {
031        return INSTANCE;
032    }
033
034    public static final boolean matches( Object value ) {
035        return value == null || value == INSTANCE || value instanceof Null;
036    }
037
038    private Null() {
039        // prevent instantiation
040    }
041
042    @Override
043    public int hashCode() {
044        return INSTANCE.hashCode();
045    }
046
047    @Override
048    public boolean equals( Object obj ) {
049        return obj == null || obj == this || obj instanceof Null;
050    }
051
052    @Override
053    public String toString() {
054        return "null";
055    }
056
057    /**
058     * There should only be on instance of this (though there may be others due to serialization).
059     */
060    @Override
061    protected final Object clone() {
062        return INSTANCE;
063    }
064}