// This object retains references to collections returned by keySet() and value(). This saves
    // bytes when the both the map and its keySet or value collection are written to the same
    // instance of ObjectOutputStream.

    // TODO(b/160980469): remove support for the old serialization format after some time
    private static final boolean USE_LEGACY_SERIALIZATION = true;

    private final Object keys;

   *
   * @serialData The length of the array is emitted (int), followed by all of its elements (each a
   *     {@code double}) in the proper order.
   */
  private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
    s.defaultWriteObject();

    // Write out array length
    int length = length();
    s.writeInt(length);

    // Write out all elements in the proper order.

import com.google.common.primitives.Ints;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.AbstractSet;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.ConcurrentModificationException;

import com.google.errorprone.annotations.concurrent.LazyInit;
import com.google.j2objc.annotations.WeakOuter;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.AbstractMap;
import java.util.Arrays;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;

   * locking is not needed on deserialization, so readObject is not defined, just relying on
   * default.
   */
  private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
    monitor.enter();
    try {
      s.defaultWriteObject();
    } finally {
      monitor.leave();
    }
  }