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Java implementation of a concurrent trie hash map from Scala collections library

There is a newer version: 1.3.2

/*
 * (C) Copyright 2016 PANTHEON.tech, s.r.o. and others.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package tech.pantheon.triemap;

import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;

abstract sealed class MainNode extends BasicNode permits CNode, FailedNode, LNode, TNode {
    static final int NO_SIZE = -1;

    private static final VarHandle PREV;

    static {
        try {
            PREV = MethodHandles.lookup().findVarHandle(MainNode.class, "prev", MainNode.class);
        } catch (NoSuchFieldException | IllegalAccessException e) {
            throw new ExceptionInInitializerError(e);
        }
    }

    private volatile MainNode prev;

    MainNode() {
        prev = null;
    }

    MainNode(final MainNode prev) {
        this.prev = prev;
    }

    /**
     * Return the number of entries in this node, or {@link #NO_SIZE} if it is not known.
     */
    abstract int trySize();

    /**
     * Return the number of entries in this node, traversing it if need be. This method should be invoked only
     * on immutable snapshots.
     *
     * @param ct TrieMap reference
     * @return The actual number of entries.
     */
    abstract int size(ImmutableTrieMap ct);

    final boolean casPrev(final MainNode oldval, final MainNode nval) {
        return PREV.compareAndSet(this, oldval, nval);
    }

    final void writePrev(final MainNode nval) {
        prev = nval;
    }

    final MainNode readPrev() {
        return prev;
    }
}