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General purpose utilities for Android and Java projects (aka java-common)
There is a newer version: 2.3.1
/*
 * Copyright (C) 2014 Markus Junginger, greenrobot (http://greenrobot.de)
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package de.greenrobot.common;

import java.lang.ref.Reference;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

/**
 * A simple in-memory object "cache" based on a ConcurrentHashMap and soft/weak references.
 * 
 * In contrast to Java's WeakHashMap, it does not weakly reference keys but values.
 * 
 * Not thread-safe.
 *
 * @author markus
 */
public class ObjectCache {
    private final Map> cache;
    private final boolean useWeakReferences;

    /** Android note: Weak references are a bad cache but the work with external allocations like bitmaps. */
    public static  ObjectCache createUsingWeakReferences() {
        return new ObjectCache(true);
    }

    /**
     * Android note: Be careful with this one before Android 4.0.  External allocations like bitmaps are not GCed
     * (tested under Android 2.2)!
     */
    public static  ObjectCache createUsingSoftReferences() {
        return new ObjectCache(false);
    }

    private ObjectCache(boolean useWeakReferences) {
        this.useWeakReferences = useWeakReferences;
        cache = new ConcurrentHashMap>();
    }

    /** Stores an new entry in the cache. */
    public T put(KEY key, T object) {
        Reference ref;
        if (useWeakReferences) {
            ref = new WeakReference(object);
        } else {
            ref = new SoftReference(object);
        }
        Reference oldRef = cache.put(key, ref);
        return oldRef != null ? oldRef.get() : null;
    }

    /** Stores all entries contained in the given map in the cache. */
    public void putAll(Map mapDataToPut) {
        Set> entries = mapDataToPut.entrySet();
        for (Entry entry : entries) {
            put(entry.getKey(), entry.getValue());
        }
    }

    /** Get the cached entry or null if no valid cached entry is found. */
    public T get(KEY key) {
        Reference ref = cache.get(key);
        return ref != null ? ref.get() : null;
    }

    /** Clears all cached entries. */
    public void clear() {
        cache.clear();
    }

    /** Removes an entry from the cache. @return The removed entry */
    public T remove(KEY key) {
        Reference oldRef = cache.remove(key);
        return oldRef != null ? oldRef.get() : null;
    }

    /** Removes zombie entries (entries whose reference was set to null). */
    public void cleanUp() {
        Set keySet = cache.keySet();
        for (KEY key : keySet) {
            Reference ref = cache.get(key);
            if (ref != null && ref.get() == null) {
                cache.remove(key);
            }
        }
    }

    /** Returns true if an entry was found with no value: mainly for testing purposes. */
    public boolean isValueExpired(KEY key) {
        Reference ref = cache.get(key);
        return ref != null && ref.get() == null;
    }

    public boolean containsKey(KEY key) {
        return cache.containsKey(key);
    }

    public boolean containsKeyWithValue(KEY key) {
        return get(key) != null;
    }

    public Set keySet() {
        return cache.keySet();
    }

    public int size() {
        return cache.size();
    }

}