Ошибка типа с картой на основе кеша в Guava

Question

Я пытаюсь использовать Guava для имитации карты LRU.

Map<K, V> map = CacheBuilder.newBuilder() 
     .maximumSize(maxSize) 
     .build() // not using a cache loader 
     .asMap(); 

Но когда я попытаюсь сделать это, я получаю сообщение об ошибке.

Однако, если я создаю карту с временной ссылкой на кеш, она отлично работает.

Cache<K, V> cache = CacheBuilder.newBuilder() 
     .maximumSize(maxSize) 
     .build(); 
Map<K, V> map = cache.asMap(); 

Почему эта работа и первый образец нет?

Answer 1

Это ошибка генераторов Java. В этом случае невозможно определить общие типы. Вы должны добавить <K, V>, чтобы сказать Java, чтобы обработать вновь созданный кеш как Cache<K, V>, а не Cache<Object, Object>.

private static <K, V> Map<K, V> makeMap(final int maxSize) { 
    return CacheBuilder.newBuilder() 
     .maximumSize(maxSize) 
     .<K, V>build() // not using a cache loader 
     .asMap(); 
} 

, а затем она будет работать:

final Map<Long, String> map = makeMap(10); 

См this part of Angelika Langer's Java Generics FAQs.

Answer 2

Вот мой вывод из ответа Xaerxess', использование:

private final Map<String, List<Condition>> waitersByKey=MapUtil.create(5, 1000); 

MapUtil.java:

import java.util.Map; 
import java.util.concurrent.TimeUnit; 

import com.google.common.cache.CacheBuilder; 
import com.google.common.cache.RemovalListener; 

public class MapUtil { 

    /** 
    * Value for items which are unrestricted (-1) 
    */ 
    public static final int UNRESTRICTED=-1; 

    /** 
    * Create a generic map that will not exceed max size 
    * @param maxSize 
    * @return created map 
    */ 
    public static <K,V> Map<K,V> createWithMaxSize(int maxSize) { 
    return create(UNRESTRICTED,maxSize); 
    } 

    /** 
    * Create a generic map that will expire objects if not accessed within a given number of minutes 
    * @param expireInMins 
    * @return created map 
    */ 
    public static <K,V> Map<K,V> createWithExpiration(int expireInMins) { 
    return create(expireInMins,UNRESTRICTED,UNRESTRICTED); 
    } 

    /** 
    * Creates a map with an initial size. Reduces growth performance hit. 
    * @param initialSize 
    * @return created map 
    */ 
    public static <K,V> Map<K,V> createWithInitialSize(int initialSize) { 
    return create(UNRESTRICTED,UNRESTRICTED,initialSize); 
    } 

    /** 
    * Create generic map that expires values with max size 
    * @param expireInMins max age of unaccessed items in minutes, or UNRESTRICTED 
    * @param maxSize max number of items before oldest is removed, or UNRESTRICTED 
    * @return created map 
    */ 
    public static <K,V> Map<K,V> create(int expireInMins, int maxSize) { 
    return create(expireInMins,maxSize,UNRESTRICTED); 
    } 

    /** 
    * Create generic map that expires values with max size 
    * @param expireInMins max age of unaccessed items in minutes, or UNRESTRICTED 
    * @param maxSize max number of items before oldest is removed, or UNRESTRICTED 
    * @param removalListener listener called on item removal, if not null 
    * @return created map 
    */ 
    public static <K,V> Map<K,V> create(int expireInMins, int maxSize, RemovalListener<K, V> removalListener) { 
    return create(expireInMins,maxSize,UNRESTRICTED, removalListener); 
    } 

    /** 
    * Convenience method to build a Guava cache as generic map. 
    * http://stackoverflow.com/a/14730359/303442 
    * @param expireInMins max age of unaccessed items in minutes, or UNRESTRICTED 
    * @param maxSize max number of items before oldest is removed, or UNRESTRICTED 
    * @param initialCapacity initial size in elements, reduces growth performance hit. UNRESTRICTED for default 
    * @return created map 
    */ 
    public static <K,V> Map<K,V> create(int expireInMins, int maxSize, int initialCapacity) { 
    return create(expireInMins,maxSize,initialCapacity,null); 
    } 

    /** 
    * Convenience method to build a Guava cache as generic map. 
    * http://stackoverflow.com/a/14730359/303442 
    * @param expireInMins max age of unaccessed items in minutes, or UNRESTRICTED 
    * @param maxSize max number of items before oldest is removed, or UNRESTRICTED 
    * @param initialCapacity initial size in elements, reduces growth performance hit. UNRESTRICTED for default 
    * @param removalListener listener called on item removal, if not null 
    * @return created map 
    */ 
    public static <K,V> Map<K,V> create(int expireInMins, int maxSize, int initialCapacity,RemovalListener<K, V> removalListener) { 
    CacheBuilder<Object, Object> builder = CacheBuilder.newBuilder(); 
    if (expireInMins>-1) 
     builder.expireAfterWrite(expireInMins, TimeUnit.MINUTES); 
    if (maxSize>-1) 
     builder.maximumSize(1000); 
    if (initialCapacity>-1) 
     builder.initialCapacity(initialCapacity); 
    if (removalListener!=null) 
     builder.removalListener(removalListener); 
    return builder.<K,V>build().asMap(); 
    } 
}