protected V create(K key) {
        return null;
    }
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.
VideoSqlLite/src/main/java/com/yc/videosqllite/cache/SystemLruCache.java