Java6or7でラウンドロビン

ラウンドロビン的なものが欲しくなったので書いてみた。
一応スレッドセーフにはしてるつもりなんだけど、これでいけるのかは実際に動かしてみないと、わからないです。
Java8の無限ストリーム使うとどうなるかはまだ考え中。

code

package me.masahito;

import java.util.Iterator;
import java.util.List;

public class RoundRobin<T> implements Iterable<T> {
    private final List<T> list;
    private final int listSize;

    private RoundRobin(final Builder<T> builder) {
        this.list = builder.list;
        this.listSize = builder.list.size();
    }

    @Override
    public final Iterator<T> iterator() {
        return new Iterator<T>() {

            private int index = 0;
            private final Object lock = new Object();

            public T next() {
                final T result;
                synchronized (lock) {
                    result = list.get(index);
                    index = (index + 1) % listSize;
                }
                return result;
            }

            public boolean hasNext() {
                return true;
            }

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

        };
    }

    public static class Builder<T> {
        private final List<T> list;

        public Builder(final List<T> list) {
            this.list = list;
        }

        public final Iterator<T> build() {
            return new RoundRobin<>(this).iterator();
        }
    }

}

unittest

package me.masahito;

import org.junit.Test;

import java.util.Arrays;
import java.util.Iterator;
import java.util.List;

import static org.hamcrest.CoreMatchers.equalTo;
import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;

/**
 * RoundRobin Test
 */
public class RoundRobinTest {

    @Test
    public void buildRoundRobinIterator() throws Exception {
        final List<Integer> list = Arrays.asList(1, 2, 3);
        final Iterator<Integer> ite = new RoundRobin.Builder<>(list).build();

        assertThat(ite.next(), is(equalTo(1)));
        assertThat(ite.next(), is(equalTo(2)));
        assertThat(ite.next(), is(equalTo(3)));
        assertThat(ite.next(), is(equalTo(1)));
    }
}