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LruCache.javaAPI DocAndroid 5.1 API12408Thu Mar 12 22:22:10 GMT 2015android.util

LruCache

public class LruCache extends Object
A cache that holds strong references to a limited number of values. Each time a value is accessed, it is moved to the head of a queue. When a value is added to a full cache, the value at the end of that queue is evicted and may become eligible for garbage collection.

If your cached values hold resources that need to be explicitly released, override {@link #entryRemoved}.

If a cache miss should be computed on demand for the corresponding keys, override {@link #create}. This simplifies the calling code, allowing it to assume a value will always be returned, even when there's a cache miss.

By default, the cache size is measured in the number of entries. Override {@link #sizeOf} to size the cache in different units. For example, this cache is limited to 4MiB of bitmaps:

 {@code
int cacheSize = 4 * 1024 * 1024; // 4MiB
LruCache bitmapCache = new LruCache(cacheSize) {
protected int sizeOf(String key, Bitmap value) {
return value.getByteCount();
}
}}

This class is thread-safe. Perform multiple cache operations atomically by synchronizing on the cache:

 {@code
synchronized (cache) {
if (cache.get(key) == null) {
cache.put(key, value);
}
}}

This class does not allow null to be used as a key or value. A return value of null from {@link #get}, {@link #put} or {@link #remove} is unambiguous: the key was not in the cache.

This class appeared in Android 3.1 (Honeycomb MR1); it's available as part of Android's Support Package for earlier releases.

Fields Summary
private final LinkedHashMap
map
private int
size
Size of this cache in units. Not necessarily the number of elements.
private int
maxSize
private int
putCount
private int
createCount
private int
evictionCount
private int
hitCount
private int
missCount
Constructors Summary
public LruCache(int maxSize)

param
maxSize for caches that do not override {@link #sizeOf}, this is the maximum number of entries in the cache. For all other caches, this is the maximum sum of the sizes of the entries in this cache.

        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }
        this.maxSize = maxSize;
        this.map = new LinkedHashMap<K, V>(0, 0.75f, true);
    
Methods Summary
protected Vcreate(K key)
Called after a cache miss to compute a value for the corresponding key. Returns the computed value or null if no value can be computed. The default implementation returns null.

The method is called without synchronization: other threads may access the cache while this method is executing.

If a value for {@code key} exists in the cache when this method returns, the created value will be released with {@link #entryRemoved} and discarded. This can occur when multiple threads request the same key at the same time (causing multiple values to be created), or when one thread calls {@link #put} while another is creating a value for the same key.

        return null;
    
public final synchronized intcreateCount()
Returns the number of times {@link #create(Object)} returned a value.

        return createCount;
    
protected voidentryRemoved(boolean evicted, K key, V oldValue, V newValue)
Called for entries that have been evicted or removed. This method is invoked when a value is evicted to make space, removed by a call to {@link #remove}, or replaced by a call to {@link #put}. The default implementation does nothing.

The method is called without synchronization: other threads may access the cache while this method is executing.

param
evicted true if the entry is being removed to make space, false if the removal was caused by a {@link #put} or {@link #remove}.
param
newValue the new value for {@code key}, if it exists. If non-null, this removal was caused by a {@link #put}. Otherwise it was caused by an eviction or a {@link #remove}.

public final voidevictAll()
Clear the cache, calling {@link #entryRemoved} on each removed entry.

        trimToSize(-1); // -1 will evict 0-sized elements
    
public final synchronized intevictionCount()
Returns the number of values that have been evicted.

        return evictionCount;
    
public final Vget(K key)
Returns the value for {@code key} if it exists in the cache or can be created by {@code #create}. If a value was returned, it is moved to the head of the queue. This returns null if a value is not cached and cannot be created.

        if (key == null) {
            throw new NullPointerException("key == null");
        }

        V mapValue;
        synchronized (this) {
            mapValue = map.get(key);
            if (mapValue != null) {
                hitCount++;
                return mapValue;
            }
            missCount++;
        }

        /*
         * Attempt to create a value. This may take a long time, and the map
         * may be different when create() returns. If a conflicting value was
         * added to the map while create() was working, we leave that value in
         * the map and release the created value.
         */

        V createdValue = create(key);
        if (createdValue == null) {
            return null;
        }

        synchronized (this) {
            createCount++;
            mapValue = map.put(key, createdValue);

            if (mapValue != null) {
                // There was a conflict so undo that last put
                map.put(key, mapValue);
            } else {
                size += safeSizeOf(key, createdValue);
            }
        }

        if (mapValue != null) {
            entryRemoved(false, key, createdValue, mapValue);
            return mapValue;
        } else {
            trimToSize(maxSize);
            return createdValue;
        }
    
public final synchronized inthitCount()
Returns the number of times {@link #get} returned a value that was already present in the cache.

        return hitCount;
    
public final synchronized intmaxSize()
For caches that do not override {@link #sizeOf}, this returns the maximum number of entries in the cache. For all other caches, this returns the maximum sum of the sizes of the entries in this cache.

        return maxSize;
    
public final synchronized intmissCount()
Returns the number of times {@link #get} returned null or required a new value to be created.

        return missCount;
    
public final Vput(K key, V value)
Caches {@code value} for {@code key}. The value is moved to the head of the queue.

return
the previous value mapped by {@code key}.

        if (key == null || value == null) {
            throw new NullPointerException("key == null || value == null");
        }

        V previous;
        synchronized (this) {
            putCount++;
            size += safeSizeOf(key, value);
            previous = map.put(key, value);
            if (previous != null) {
                size -= safeSizeOf(key, previous);
            }
        }

        if (previous != null) {
            entryRemoved(false, key, previous, value);
        }

        trimToSize(maxSize);
        return previous;
    
public final synchronized intputCount()
Returns the number of times {@link #put} was called.

        return putCount;
    
public final Vremove(K key)
Removes the entry for {@code key} if it exists.

return
the previous value mapped by {@code key}.

        if (key == null) {
            throw new NullPointerException("key == null");
        }

        V previous;
        synchronized (this) {
            previous = map.remove(key);
            if (previous != null) {
                size -= safeSizeOf(key, previous);
            }
        }

        if (previous != null) {
            entryRemoved(false, key, previous, null);
        }

        return previous;
    
public voidresize(int maxSize)
Sets the size of the cache.

param
maxSize The new maximum size.

        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }

        synchronized (this) {
            this.maxSize = maxSize;
        }
        trimToSize(maxSize);
    
private intsafeSizeOf(K key, V value)

        int result = sizeOf(key, value);
        if (result < 0) {
            throw new IllegalStateException("Negative size: " + key + "=" + value);
        }
        return result;
    
public final synchronized intsize()
For caches that do not override {@link #sizeOf}, this returns the number of entries in the cache. For all other caches, this returns the sum of the sizes of the entries in this cache.

        return size;
    
protected intsizeOf(K key, V value)
Returns the size of the entry for {@code key} and {@code value} in user-defined units. The default implementation returns 1 so that size is the number of entries and max size is the maximum number of entries.

An entry's size must not change while it is in the cache.

        return 1;
    
public final synchronized java.util.Mapsnapshot()
Returns a copy of the current contents of the cache, ordered from least recently accessed to most recently accessed.

        return new LinkedHashMap<K, V>(map);
    
public final synchronized java.lang.StringtoString()

        int accesses = hitCount + missCount;
        int hitPercent = accesses != 0 ? (100 * hitCount / accesses) : 0;
        return String.format("LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]",
                maxSize, hitCount, missCount, hitPercent);
    
public voidtrimToSize(int maxSize)
Remove the eldest entries until the total of remaining entries is at or below the requested size.

param
maxSize the maximum size of the cache before returning. May be -1 to evict even 0-sized elements.

        while (true) {
            K key;
            V value;
            synchronized (this) {
                if (size < 0 || (map.isEmpty() && size != 0)) {
                    throw new IllegalStateException(getClass().getName()
                            + ".sizeOf() is reporting inconsistent results!");
                }

                if (size <= maxSize) {
                    break;
                }

                Map.Entry<K, V> toEvict = map.eldest();
                if (toEvict == null) {
                    break;
                }

                key = toEvict.getKey();
                value = toEvict.getValue();
                map.remove(key);
                size -= safeSizeOf(key, value);
                evictionCount++;
            }

            entryRemoved(true, key, value, null);
        }