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.AbstractSet;
import java.util.Collection;
import java.util.Iterator;
import java.util.Set;

/**
 * This is a set which cannot be changed and has no entries.
 *
 * @since 2016/04/10
 */
public final class EmptySet
    extends AbstractSet
{
    /** The empty set. */
    private static Reference<Set> _EMPTY_SET;
    
    /**
     * Initializes the empty set.
     *
     * @since 2016/04/10
     */
    private EmptySet()
    {
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/05/12
     */
    @Override
    public boolean contains(Object __o)
    {
        return false;
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/05/12
     */
    @Override
    public boolean containsAll(Collection __c)
    {
        return __c.isEmpty();
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/05/12
     */
    @Override
    public boolean isEmpty()
    {
        return true;
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/04/10
     */
    @Override
    public Iterator iterator()
    {
        return EmptyIterator.empty();
    }
    
    /**
     * {@inheritDoc}
     * @since 2016/04/10
     */
    @Override
    public int size()
    {
        return 0;
    }
    
    /**
     * This returns an empty and unmodifiable set.
     *
     * @param <V> The element type used by the set.
     * @return The unmodifiable and empty set.
     * @since 2016/04/10
     */
    @SuppressWarnings({"unchecked"})
    public static <V> Set<V> empty()
    {
        // Get reference
        Reference<Set> ref = EmptySet._EMPTY_SET;
        Set rv;
        
        // Needs creation?
        if (ref == null || null == (rv = ref.get()))
            EmptySet._EMPTY_SET = new WeakReference<>((rv = new EmptySet()));
        
        // Return it
        return (Set<V>)rv;
    }
}