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Added in API level 12

LruCache

public class LruCache
extends Object

java.lang.Object
   ↳ android.util.LruCache<K, V>


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 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:

   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(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 containing a concise, human-readable description of this 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

From class java.lang.Object

Public constructors

LruCache

Added in API level 12
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
int createCount ()

Returns the number of times create(Object) returned a value.

Returns
int

evictAll

Added in API level 12
void evictAll ()

Clear the cache, calling entryRemoved(boolean, K, V, V) on each removed entry.

evictionCount

Added in API level 12
int evictionCount ()

Returns the number of values that have been evicted.

Returns
int

get

Added in API level 12
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
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
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
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
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
int putCount ()

Returns the number of times put(K, V) was called.

Returns
int

remove

Added in API level 12
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
void resize (int maxSize)

Sets the size of the cache.

Parameters
maxSize int: The new maximum size.

size

Added in API level 12
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
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
String toString ()

Returns a string containing a concise, human-readable description of this object. Subclasses are encouraged to override this method and provide an implementation that takes into account the object's type and data. The default implementation is equivalent to the following expression:

   getClass().getName() + '@' + Integer.toHexString(hashCode())

See Writing a useful toString method if you intend implementing your own toString method.

Returns
String a printable representation of this object.

trimToSize

Added in API level 17
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
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
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). Otherwise it was caused by an eviction or a remove(K).

sizeOf

Added in API level 12
int sizeOf (K key, 
                V value)

Returns the size of the entry for key and 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.

Parameters
key K
value V
Returns
int
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