/*

 * Copyright (c) 2008, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

package com.sun.beans;

import java.lang.ref.Reference;

import java.lang.ref.WeakReference;

import java.util.Map;

import java.util.WeakHashMap;

/**

 * A hashtable-based cache with weak keys and weak values.

 * An entry in the map will be automatically removed

 * when its key is no longer in the ordinary use.

 * A value will be automatically removed as well

 * when it is no longer in the ordinary use.

 *

 * @since 1.7

 *

 * @author Sergey A. Malenkov

 */

public final class WeakCache<K, V> {

    private final Map<K, Reference<V>> map = new WeakHashMap<K, Reference<V>>();

    /**

     * Returns a value to which the specified {@code key} is mapped,

     * or {@code null} if this map contains no mapping for the {@code key}.

     *

     * @param key  the key whose associated value is returned

     * @return a value to which the specified {@code key} is mapped

     */

    public V get(K key) {

        Reference<V> reference = this.map.get(key);

        if (reference == null) {

            return null;

        }

        V value = reference.get();

        if (value == null) {

            this.map.remove(key);

        }

        return value;

    }

    /**

     * Associates the specified {@code value} with the specified {@code key}.

     * Removes the mapping for the specified {@code key} from this cache

     * if it is present and the specified {@code value} is {@code null}.

     * If the cache previously contained a mapping for the {@code key},

     * the old value is replaced by the specified {@code value}.

     *

     * @param key    the key with which the specified value is associated

     * @param value  the value to be associated with the specified key

     */

    public void put(K key, V value) {

        if (value != null) {

            this.map.put(key, new WeakReference<V>(value));

        }

        else {

            this.map.remove(key);

        }

    }

    /**

     * Removes all of the mappings from this cache.

     */

    public void clear() {

        this.map.clear();

    }

}