require(LEGAL_KEY_PATTERN.matches(key)) { "keys must match regex [a-z0-9_-]{1,120}: \"$key\"" }
  }

  /**
   * Returns an iterator over the cache's current entries. This iterator doesn't throw
   * `ConcurrentModificationException`, but if new entries are added while iterating, those new
   * entries will not be returned by the iterator. If existing entries are removed during iteration,
   * they will be absent (unless they were already returned).

import com.google.j2objc.annotations.J2ObjCIncompatible;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.time.Duration;
import java.util.ConcurrentModificationException;
import java.util.IdentityHashMap;
import java.util.Map;
import java.util.concurrent.TimeUnit;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.annotation.CheckForNull;

         * element anymore. Otherwise, when we remove from the old iterator, we may be invalidating
         * the new one. The result is a ConcurrentModificationException or other bad behavior.
         *
         * (If we decide that we really, really hate allocating two Iterators per cycle instead of