LruCache

Kotlin | Java

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 entryRemoved(boolean, K, V, V) .

If a cache miss should be computed on demand for the corresponding keys, override create(K) . 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 sizeOf(K, V) to size the cache in different units. For example, this cache is limited to 4MiB of bitmaps:

int cacheSize = 4 * 1024 * 1024; // 4MiB LruCache<String, Bitmap> bitmapCache = new LruCache<String, Bitmap>(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:

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 get(K) , put(K, V) or remove(K) 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.

Summary

Public constructors LruCache(int maxSize)

Public methods final int createCount() Returns the number of times create(java.lang.Object) returned a value. final void evictAll() Clear the cache, calling entryRemoved(boolean, K, V, V) on each removed entry. final int evictionCount() Returns the number of values that have been evicted. final V get(K key) Returns the value for key if it exists in the cache or can be created by #create . final int hitCount() Returns the number of times get(K) returned a value that was already present in the cache. final int maxSize() For caches that do not override sizeOf(K, V) , this returns the maximum number of entries in the cache. final int missCount() Returns the number of times get(K) returned null or required a new value to be created. final V put(K key, V value) Caches value for key . final int putCount() Returns the number of times put(K, V) was called. final V remove(K key) Removes the entry for key if it exists. void resize(int maxSize) Sets the size of the cache. final int size() For caches that do not override sizeOf(K, V) , this returns the number of entries in the cache. final Map<K, V> snapshot() Returns a copy of the current contents of the cache, ordered from least recently accessed to most recently accessed. final String toString() Returns a string representation of the object. void trimToSize(int maxSize) Remove the eldest entries until the total of remaining entries is at or below the requested size.

Protected methods V create(K key) Called after a cache miss to compute a value for the corresponding key. void entryRemoved(boolean evicted, K key, V oldValue, V newValue) Called for entries that have been evicted or removed. int sizeOf(K key, V value) Returns the size of the entry for key and value in user-defined units.

Inherited methods java.lang.Object From class Object clone() Creates and returns a copy of this object. boolean equals(Object obj) Indicates whether some other object is "equal to" this one. void finalize() Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. final Class<?> getClass() Returns the runtime class of this Object . int hashCode() Returns a hash code value for the object. final void notify() Wakes up a single thread that is waiting on this object's monitor. final void notifyAll() Wakes up all threads that are waiting on this object's monitor. String toString() Returns a string representation of the object. final void wait(long timeout, int nanos) Causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed. final void wait(long timeout) Causes the current thread to wait until either another thread invokes the notify() method or the notifyAll() method for this object, or a specified amount of time has elapsed. final void wait() Causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object.

Public constructors

LruCache Added in API level 12 public LruCache (int maxSize) Parameters maxSize int : for caches that do not override sizeOf(K, V) , 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.

Public methods

createCount Added in API level 12 public final int createCount () Returns the number of times create(java.lang.Object) returned a value. Returns int

evictionCount Added in API level 12 public final int evictionCount () Returns the number of values that have been evicted. Returns int

get Added in API level 12 public final V get (K key) Returns the value for key if it exists in the cache or can be created by #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. Parameters key K Returns V

hitCount Added in API level 12 public final int hitCount () Returns the number of times get(K) returned a value that was already present in the cache. Returns int

maxSize Added in API level 12 public final int maxSize () For caches that do not override sizeOf(K, V) , 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. Returns int

missCount Added in API level 12 public final int missCount () Returns the number of times get(K) returned null or required a new value to be created. Returns int

put Added in API level 12 public final V put (K key, V value) Caches value for key . The value is moved to the head of the queue. Parameters key K value V Returns V the previous value mapped by key .

putCount Added in API level 12 public final int putCount () Returns the number of times put(K, V) was called. Returns int

remove Added in API level 12 public final V remove (K key) Removes the entry for key if it exists. Parameters key K Returns V the previous value mapped by key .

resize Added in API level 21 public void resize (int maxSize) Sets the size of the cache. Parameters maxSize int : The new maximum size.

size Added in API level 12 public final int size () For caches that do not override sizeOf(K, V) , 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. Returns int

snapshot Added in API level 12 public final Map<K, V> snapshot () Returns a copy of the current contents of the cache, ordered from least recently accessed to most recently accessed. Returns Map<K, V>

toString Added in API level 12 public final String toString () Returns a string representation of the object. In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method. The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character ` @ ', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of: getClass().getName() + '@' + Integer.toHexString(hashCode()) Returns String a string representation of the object.

trimToSize Added in API level 17 public void trimToSize (int maxSize) Remove the eldest entries until the total of remaining entries is at or below the requested size. Parameters maxSize int : the maximum size of the cache before returning. May be -1 to evict even 0-sized elements.

Protected methods

create Added in API level 12 protected V create (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 key exists in the cache when this method returns, the created value will be released with entryRemoved(boolean, K, V, V) 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 put(K, V) while another is creating a value for the same key. Parameters key K Returns V

entryRemoved Added in API level 12 protected void entryRemoved (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 remove(K) , or replaced by a call to put(K, V) . The default implementation does nothing. The method is called without synchronization: other threads may access the cache while this method is executing. Parameters evicted boolean : true if the entry is being removed to make space, false if the removal was caused by a put(K, V) or remove(K) . key K oldValue V newValue V : the new value for key , if it exists. If non-null, this removal was caused by a put(K, V) or a get(K) . Otherwise it was caused by an eviction or a remove(K) .