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Get rid of the boilerplate code in Java properties based configuration.
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/*
 * Copyright (c) 2012-2015, Luigi R. Viggiano
 * All rights reserved.
 *
 * This software is distributable under the BSD license.
 * See the terms of the BSD license in the documentation provided with this software.
 */

package org.aeonbits.owner;

import java.util.Collections;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

/**
 * Utility class caching Config instances that can be used as Singletons.
 *
 * This class is designed to be thread safe.
 *
 * @author Luigi R. Viggiano
 * @since 1.0.6
 */
public final class ConfigCache {
    private static final ConcurrentMap CACHE = new ConcurrentHashMap();

    /** Don't let anyone instantiate this class */
    private ConfigCache() {}

    /**
     * Gets from the cache or create, an instance of the given class using the given imports.
     * The factory used to create new instances is the static {@link ConfigFactory#INSTANCE}.
     *
     * @param clazz     the interface extending from {@link Config} that you want to instantiate.
     * @param imports   additional variables to be used to resolve the properties.
     * @param        type of the interface.
     * @return          an object implementing the given interface, that can be taken from the cache,
     *                  which maps methods to property values.
     */
    public static  T getOrCreate(Class clazz, Map... imports) {
        return getOrCreate(ConfigFactory.INSTANCE, clazz, clazz, imports);
    }

    /**
     * Gets from the cache or create, an instance of the given class using the given imports.
     *
     * @param factory   the factory to use to eventually create the instance.
     * @param clazz     the interface extending from {@link Config} that you want to instantiate.
     * @param imports   additional variables to be used to resolve the properties.
     * @param        type of the interface.
     * @return          an object implementing the given interface, that can be taken from the cache,
     *                  which maps methods to property values.
     */
    public static  T getOrCreate(Factory factory, Class clazz, Map... imports) {
        return getOrCreate(factory, clazz, clazz, imports);
    }

    /**
     * Gets from the cache or create, an instance of the given class using the given imports.
     * The factory used to create new instances is the static {@link ConfigFactory#INSTANCE}.
     *
     * @param key       the key object to be used to identify the instance in the cache.
     * @param clazz     the interface extending from {@link Config} that you want to instantiate.
     * @param imports   additional variables to be used to resolve the properties.
     * @param        type of the interface.
     * @return          an object implementing the given interface, that can be taken from the cache,
     *                  which maps methods to property values.
     */
    public static  T getOrCreate(Object key, Class clazz, Map... imports) {
        return getOrCreate(ConfigFactory.INSTANCE, key, clazz, imports);
    }

    /**
     * Gets from the cache or create, an instance of the given class using the given imports.
     *
     * @param factory   the factory to use to eventually create the instance.
     * @param key       the key object to be used to identify the instance in the cache.
     * @param clazz     the interface extending from {@link Config} that you want to instantiate.
     * @param imports   additional variables to be used to resolve the properties.
     * @param        type of the interface.
     * @return          an object implementing the given interface, that can be taken from the cache,
     *                  which maps methods to property values.
     */
    public static  T getOrCreate(Factory factory, Object key,
                                                   Class clazz, Map... imports) {
        T existing = get(key);
        if (existing != null) return existing;
        T created = factory.create(clazz, imports);
        T raced = add(key, created);
        return raced != null ? raced : created;
    }

    /**
     * Gets from the cache the {@link Config} instance identified by the given key.
     *
     * @param key       the key object to be used to identify the instance in the cache.
     * @param        type of the interface.
     * @return          the {@link Config} object from the cache if exists, or null if it doesn't.
     */
    @SuppressWarnings("unchecked")
    public static  T get(Object key) {
        return (T) CACHE.get(key);
    }

    /**
     * Adds a {@link Config} object into the cache.
     *
     * @param key       the key object to be used to identify the instance in the cache.
     * @param instance  the instance of the {@link Config} object to be stored into the cache.
     * @param        type of the interface.
     * @return          the previous value associated with the specified key, or
     *                  null if there was no mapping for the key.
     */
    @SuppressWarnings("unchecked")
    public static  T add(Object key, T instance) {
        return (T) CACHE.putIfAbsent(key, instance);
    }

    /**
     * Lists the key objects for all configuration instances present in the cache.
     *
     * @return a set containing the key objects for all instance in the cache.
     */
    public static Set