我正在寻找一种从java中的hashset中检索对象的方法。我对它的元素做了如下迭代：

for (Customer remainingNode : availableNodes) {
remainingNode.setMarginalGain(calculateMarginalGain(
                    remainingNode, seedSet, network, availableNodes,
                    churnNet));
}

不幸的是，由于并发修改异常，我必须将其更改为以下内容：

for(int i=0;i<numberofRemainingNodes;i++){
Customer remainingNode=availableNodes.get(i);
remainingNode.setMarginalGain(calculateMarginalGain(
                        remainingNode, seedSet, network, availableNodes,
                        churnNet));
numberofRemainingNodes=availableNodes.size();
}

但我不能这样做，因为没有任何用于javahashset的get（index）方法。你能帮我处理一下这种情况吗？
p、 s：我使用hashset是因为我想处理并集和交集的情况，并且我不想向其中添加重复的元素。请考虑我的程序的这一部分应该运行数百万次，所以对于整个程序来说，额外的延迟可能是昂贵的。
仅供参考：

private int calculateMarginalGain(Customer remainingNode,
            HashSet<Customer> seedSet,
            DirectedSparseGraph<Customer, Transaction> net,
            Set<Customer> availableNodes, HashSet<Customer> churnNetwork) {
        // Marginal gain for short-term campaign
        HashSet<Customer> tmp = new HashSet<Customer>(); // seedset U
                                                            // {remainingNode}
        tmp.add(remainingNode);
        Set<Customer> tmpAvailableNodes = availableNodes;
        HashSet<Customer> NeighborOfChurn = getNeighbors(churnNetwork, net);
        // sigma function for calculating the expected number of influenced
        // customers- seedSettmp=seedset U {u}
        tmpAvailableNodes.removeAll(NeighborOfChurn);
        Set<Customer> influencedNet = getNeighbors(tmp, net);
        tmpAvailableNodes.retainAll(influencedNet);
        return tmpAvailableNodes.size();
    }

private HashSet<Customer> getNeighbors(HashSet<Customer> churnNetwork,
        DirectedSparseGraph<Customer, Transaction> net) {
    HashSet<Customer> churnNeighbors = churnNetwork;
    Collection<Customer> neighbors = new HashSet<Customer>();
    for (Customer node : churnNetwork) {
        neighbors = net.getNeighbors(node);
        for (Customer neighbor : neighbors) {
            churnNeighbors.add(neighbor);
        }
    }
    return churnNeighbors;
}