code: public Object get(Object key) {
          Object value = mCacheMap.get(key);
          if (value != null || mCacheMap.containsKey(key)) {
              return value;
          }
          value = mBackingMap.get(key);
          if (value != null || mBackingMap.containsKey(key)) {
              mCacheMap.put(key, value);
          }
          return value;
      }
docs: Returns the value from the cache, or if not found, the backing map.
  If the backing map is accessed, the value is saved in the cache for
  future gets.