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Xodus is pure Java transactional schema-less embedded database

There is a newer version: 2.0.1

/**
 * Copyright 2010 - 2018 JetBrains s.r.o.
 *
 * 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 jetbrains.exodus.core.dataStructures;

import org.jetbrains.annotations.NotNull;

import java.util.AbstractSet;
import java.util.Collection;
import java.util.Iterator;

public class NanoSet extends AbstractSet {

    @NotNull
    private final E element;

    public NanoSet(@NotNull final E element) {
        this.element = element;
    }

    @Override
    public Iterator iterator() {
        return new NanoIterator<>(this);
    }

    @Override
    public int size() {
        return 1;
    }

    @Override
    public boolean isEmpty() {
        return false;
    }

    @Override
    public boolean contains(Object o) {
        return element == o || element.equals(o);
    }

    @Override
    public boolean add(E e) {
        throw new UnsupportedOperationException();
    }

    @Override
    public boolean addAll(Collection c) {
        throw new UnsupportedOperationException();
    }

    @Override
    public boolean remove(Object o) {
        throw new UnsupportedOperationException();
    }

    @Override
    public boolean removeAll(Collection c) {
        throw new UnsupportedOperationException();
    }

    @Override
    public void clear() {
        throw new UnsupportedOperationException();
    }

    private static class NanoIterator implements Iterator {

        private E element;

        private NanoIterator(final NanoSet set) {
            element = set.element;
        }

        @Override
        public boolean hasNext() {
            return element != null;
        }

        @Override
        public E next() {
            final E result = element;
            element = null;
            return result;
        }

        @Override
        public void remove() {
            throw new UnsupportedOperationException();
        }
    }
}