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International Component for Unicode for Java (ICU4J) is a mature, widely used Java library providing Unicode and Globalization support

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// © 2016 and later: Unicode, Inc. and others.
// License & terms of use: http://www.unicode.org/copyright.html
/*
 *******************************************************************************
 * Copyright (C) 2009-2010, International Business Machines Corporation and    *
 * others. All Rights Reserved.                                                *
 *******************************************************************************
 */
package com.ibm.icu.impl.locale;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.concurrent.ConcurrentHashMap;

public abstract class LocaleObjectCache {
    private ConcurrentHashMap> _map;
    private ReferenceQueue _queue = new ReferenceQueue();

    public LocaleObjectCache() {
        this(16, 0.75f, 16);
    }

    public LocaleObjectCache(int initialCapacity, float loadFactor, int concurrencyLevel) {
        _map = new ConcurrentHashMap>(initialCapacity, loadFactor, concurrencyLevel);
    }

    public V get(K key) {
        V value = null;

        cleanStaleEntries();
        CacheEntry entry = _map.get(key);
        if (entry != null) {
            value = entry.get();
        }
        if (value == null) {
            key = normalizeKey(key);
            V newVal = createObject(key);
            if (key == null || newVal == null) {
                // subclass must return non-null key/value object
                return null;
            }

            CacheEntry newEntry = new CacheEntry(key, newVal, _queue);

            while (value == null) {
                cleanStaleEntries();
                entry = _map.putIfAbsent(key, newEntry);
                if (entry == null) {
                    value = newVal;
                    break;
                } else {
                    value = entry.get();
                }
            }
        }
        return value;
    }

    @SuppressWarnings("unchecked")
    private void cleanStaleEntries() {
        CacheEntry entry;
        while ((entry = (CacheEntry)_queue.poll()) != null) {
            _map.remove(entry.getKey());
        }
    }

    protected abstract V createObject(K key);

    protected K normalizeKey(K key) {
        return key;
    }

    private static class CacheEntry extends SoftReference {
        private K _key;

        CacheEntry(K key, V value, ReferenceQueue queue) {
            super(value, queue);
            _key = key;
        }

        K getKey() {
            return _key;
        }
    }
}