SquirrelJME/SquirrelJME

// -*- Mode: Java; indent-tabs-mode: t; tab-width: 4 -*-
// ---------------------------------------------------------------------------
// SquirrelJME
//     Copyright (C) Stephanie Gawroriski <xer@multiphasicapps.net>
// ---------------------------------------------------------------------------
// SquirrelJME is under the Mozilla Public License Version 2.0.
// See license.mkd for licensing and copyright information.
// ---------------------------------------------------------------------------

package net.multiphasicapps.collections;

import java.lang.ref.Reference;
import java.lang.ref.WeakReference;
import java.util.AbstractMap;
import java.util.Collection;
import java.util.Map;
import java.util.Set;

/**
 * This is a map which contains no entries.
 *
 * @since 2016/05/12
 */
public class EmptyMap
    extends AbstractMap
{
    /** The empty map. */
    private static Reference<Map> _EMPTY_MAP;
    
    /**
     * Initializes the empty map.
     *
     * @since 2016/05/12
     */
    private EmptyMap()
    {
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/05/12
     */
    @Override
    public boolean containsKey(Object __k)
    {
        return false;
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/05/12
     */
    @Override
    public boolean containsValue(Object __v)
    {
        return false;
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/05/12
     */
    @Override
    public Set<Map.Entry> entrySet()
    {
        return EmptySet.<Map.Entry>empty();
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/05/12
     * @param __k
     */
    @Override
    public Object get(Object __k)
    {
        return null;
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/05/12
     */
    @Override
    public Set keySet()
    {
        return EmptySet.<Object>empty();
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/05/12
     */
    @Override
    public int size()
    {
        return 0;
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/05/12
     */
    @Override
    public Collection values()
    {
        return EmptySet.<Object>empty();
    }
    
    /**
     * This returns an empty and unmodifiable map.
     *
     * @param <K> The key type to use.
     * @param <V> The value type to use.
     * @return The unmodifiable and empty map.
     * @since 2016/05/12
     */
    @SuppressWarnings({"unchecked"})
    public static <K, V> Map<K, V> empty()
    {
        // Get reference
        Reference<Map> ref = EmptyMap._EMPTY_MAP;
        Map rv;
        
        // Needs creation?
        if (ref == null || null == (rv = ref.get()))
            EmptyMap._EMPTY_MAP = new WeakReference<>((rv = new EmptyMap()));
        
        // Return it
        return (Map<K, V>)rv;
    }
}