public class

LruCache<K, V>

extends java.lang.Object

 java.lang.Object

↳androidx.collection.LruCache<K, V>

Gradle dependencies

compile group: 'androidx.collection', name: 'collection', version: '1.2.0'

  • groupId: androidx.collection
  • artifactId: collection
  • version: 1.2.0

Artifact androidx.collection:collection:1.2.0 it located at Google repository (https://maven.google.com/)

Androidx artifact mapping:

androidx.collection:collection com.android.support:collections

Androidx class mapping:

androidx.collection.LruCache android.support.v4.util.LruCache

Overview

Static library version of android.util.LruCache. Used to write apps that run on API levels prior to 12. When running on API level 12 or above, this implementation is still used; it does not try to switch to the framework's implementation. See the framework SDK documentation for a class overview.

Summary

Constructors
publicLruCache(int maxSize)

Methods
protected java.lang.Objectcreate(java.lang.Object key)

Called after a cache miss to compute a value for the corresponding key.

public final synchronized intcreateCount()

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

protected voidentryRemoved(boolean evicted, java.lang.Object key, java.lang.Object oldValue, java.lang.Object newValue)

Called for entries that have been evicted or removed.

public final voidevictAll()

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

public final synchronized intevictionCount()

Returns the number of values that have been evicted.

public final java.lang.Objectget(java.lang.Object key)

Returns the value for key if it exists in the cache or can be created by #create.

public final synchronized inthitCount()

Returns the number of times LruCache.get(K) returned a value that was already present in the cache.

public final synchronized intmaxSize()

For caches that do not override LruCache.sizeOf(K, V), this returns the maximum number of entries in the cache.

public final synchronized intmissCount()

Returns the number of times LruCache.get(K) returned null or required a new value to be created.

public final java.lang.Objectput(java.lang.Object key, java.lang.Object value)

Caches value for key.

public final synchronized intputCount()

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

public final java.lang.Objectremove(java.lang.Object key)

Removes the entry for key if it exists.

public voidresize(int maxSize)

Sets the size of the cache.

public final synchronized intsize()

For caches that do not override LruCache.sizeOf(K, V), this returns the number of entries in the cache.

protected intsizeOf(java.lang.Object key, java.lang.Object value)

Returns the size of the entry for key and value in user-defined units.

public final synchronized java.util.Map<java.lang.Object, java.lang.Object>snapshot()

Returns a copy of the current contents of the cache, ordered from least recently accessed to most recently accessed.

public final synchronized java.lang.StringtoString()

public voidtrimToSize(int maxSize)

Remove the eldest entries until the total of remaining entries is at or below the requested size.

from java.lang.Objectclone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructors

public LruCache(int maxSize)

Parameters:

maxSize: for caches that do not override LruCache.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.

Methods

public void resize(int maxSize)

Sets the size of the cache.

Parameters:

maxSize: The new maximum size.

public final java.lang.Object get(java.lang.Object 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.

public final java.lang.Object put(java.lang.Object key, java.lang.Object value)

Caches value for key. The value is moved to the head of the queue.

Returns:

the previous value mapped by key.

public void trimToSize(int maxSize)

Remove the eldest entries until the total of remaining entries is at or below the requested size.

Parameters:

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

public final java.lang.Object remove(java.lang.Object key)

Removes the entry for key if it exists.

Returns:

the previous value mapped by key.

protected void entryRemoved(boolean evicted, java.lang.Object key, java.lang.Object oldValue, java.lang.Object 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 LruCache.remove(K), or replaced by a call to LruCache.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: true if the entry is being removed to make space, false if the removal was caused by a LruCache.put(K, V) or LruCache.remove(K).
newValue: the new value for key, if it exists. If non-null, this removal was caused by a LruCache.put(K, V). Otherwise it was caused by an eviction or a LruCache.remove(K).

protected java.lang.Object create(java.lang.Object 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 LruCache.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 LruCache.put(K, V) while another is creating a value for the same key.

protected int sizeOf(java.lang.Object key, java.lang.Object 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.

public final void evictAll()

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

public final synchronized int size()

For caches that do not override LruCache.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.

public final synchronized int maxSize()

For caches that do not override LruCache.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.

public final synchronized int hitCount()

Returns the number of times LruCache.get(K) returned a value that was already present in the cache.

public final synchronized int missCount()

Returns the number of times LruCache.get(K) returned null or required a new value to be created.

public final synchronized int createCount()

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

public final synchronized int putCount()

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

public final synchronized int evictionCount()

Returns the number of values that have been evicted.

public final synchronized java.util.Map<java.lang.Object, java.lang.Object> snapshot()

Returns a copy of the current contents of the cache, ordered from least recently accessed to most recently accessed.

public final synchronized java.lang.String toString()

Source

/*
 * Copyright 2018 The Android Open Source Project
 *
 * 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 androidx.collection;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;

import java.util.LinkedHashMap;
import java.util.Locale;
import java.util.Map;

/**
 * Static library version of {@code android.util.LruCache}. Used to write apps
 * that run on API levels prior to 12. When running on API level 12 or above,
 * this implementation is still used; it does not try to switch to the
 * framework's implementation. See the framework SDK documentation for a class
 * overview.
 */
public class LruCache<K, V> {
    private final LinkedHashMap<K, V> map;

    /** Size of this cache in units. Not necessarily the number of elements. */
    private int size;
    private int maxSize;

    private int putCount;
    private int createCount;
    private int evictionCount;
    private int hitCount;
    private int missCount;

    /**
     * @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.
     */
    public LruCache(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }
        this.maxSize = maxSize;
        this.map = new LinkedHashMap<K, V>(0, 0.75f, true);
    }

    /**
     * Sets the size of the cache.
     *
     * @param maxSize The new maximum size.
     */
    public void resize(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }

        synchronized (this) {
            this.maxSize = maxSize;
        }
        trimToSize(maxSize);
    }

    /**
     * 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.
     */
    @Nullable
    public final V get(@NonNull K key) {
        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;
        }
    }

    /**
     * Caches {@code value} for {@code key}. The value is moved to the head of
     * the queue.
     *
     * @return the previous value mapped by {@code key}.
     */
    @Nullable
    public final V put(@NonNull K key, @NonNull V value) {
        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;
    }

    /**
     * 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.
     */
    public void trimToSize(int maxSize) {
        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 || map.isEmpty()) {
                    break;
                }

                Map.Entry<K, V> toEvict = map.entrySet().iterator().next();
                key = toEvict.getKey();
                value = toEvict.getValue();
                map.remove(key);
                size -= safeSizeOf(key, value);
                evictionCount++;
            }

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

    /**
     * Removes the entry for {@code key} if it exists.
     *
     * @return the previous value mapped by {@code key}.
     */
    @Nullable
    public final V remove(@NonNull K 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;
    }

    /**
     * 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.
     *
     * <p>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}.
     */
    protected void entryRemoved(boolean evicted, @NonNull K key, @NonNull V oldValue,
            @Nullable V newValue) {
    }

    /**
     * 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.
     *
     * <p>The method is called without synchronization: other threads may
     * access the cache while this method is executing.
     *
     * <p>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.
     */
    @Nullable
    protected V create(@NonNull K key) {
        return null;
    }

    private int safeSizeOf(K key, V value) {
        int result = sizeOf(key, value);
        if (result < 0) {
            throw new IllegalStateException("Negative size: " + key + "=" + value);
        }
        return result;
    }

    /**
     * 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.
     *
     * <p>An entry's size must not change while it is in the cache.
     */
    protected int sizeOf(@NonNull K key, @NonNull V value) {
        return 1;
    }

    /**
     * Clear the cache, calling {@link #entryRemoved} on each removed entry.
     */
    public final void evictAll() {
        trimToSize(-1); // -1 will evict 0-sized elements
    }

    /**
     * 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.
     */
    public synchronized final int size() {
        return size;
    }

    /**
     * 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.
     */
    public synchronized final int maxSize() {
        return maxSize;
    }

    /**
     * Returns the number of times {@link #get} returned a value that was
     * already present in the cache.
     */
    public synchronized final int hitCount() {
        return hitCount;
    }

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

    /**
     * Returns the number of times {@link #create(Object)} returned a value.
     */
    public synchronized final int createCount() {
        return createCount;
    }

    /**
     * Returns the number of times {@link #put} was called.
     */
    public synchronized final int putCount() {
        return putCount;
    }

    /**
     * Returns the number of values that have been evicted.
     */
    public synchronized final int evictionCount() {
        return evictionCount;
    }

    /**
     * Returns a copy of the current contents of the cache, ordered from least
     * recently accessed to most recently accessed.
     */
    public synchronized final Map<K, V> snapshot() {
        return new LinkedHashMap<K, V>(map);
    }

    @Override public synchronized final String toString() {
        int accesses = hitCount + missCount;
        int hitPercent = accesses != 0 ? (100 * hitCount / accesses) : 0;
        return String.format(Locale.US, "LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]",
                maxSize, hitCount, missCount, hitPercent);
    }
}