Java 集合对象出现 UnsupportedOperationException

Question

我在 JDK 1.7 中使用 Java 集合时出错：我在这一行得到了这个异常：proposalStatuses.addAll(getAllSubmittedStatuses())

java.lang.UnsupportedOperationException
        at java.util.AbstractList.add(Unknown Source)
        at java.util.AbstractList.add(Unknown Source)
        at java.util.AbstractCollection.addAll(Unknown Source)

正在尝试将集合添加到列表

/**
     * Gets the all submitted statuses.
     *
     * @return the all submitted statuses
     */
    private Collection<ProposalStatus> getAllSubmittedStatuses() {

        return Arrays.asList(
                  ProposalStatus.SAVED_TO_IOS
                , ProposalStatus.SENDED_TO_IOS_IN_PROGRESS
                );
    }

    /**
     * Gets the all received statuses.
     *
     * @return the all received statuses
     */
    private Collection<ProposalStatus> getAllReceivedStatuses() {

        Collection<ProposalStatus> proposalStatuses =

                Arrays.asList(
                  ProposalStatus.RECEIVED_BY_IOS
                , ProposalStatus.SUBMITTED_TO_IOS
                , ProposalStatus.RECEIVED_IOS
                );

        proposalStatuses.addAll(getAllSubmittedStatuses());

        return proposalStatuses;
    }

Answer 1

为了更实际地解释一下，我正在使用您的代码：

private Collection<ProposalStatus> getAllSubmittedStatuses() {

    // This returns a list that cannot be modified, fixed size
    return Arrays.asList(
              ProposalStatus.SAVED_TO_IOS
            , ProposalStatus.SENDED_TO_IOS_IN_PROGRESS
            );
}

/**
 * Gets the all received statuses.
 *
 * @return the all received statuses
 */
private Collection<ProposalStatus> getAllReceivedStatuses() {

    // proposalStatuses will be a fixed-size list so no changing
    Collection<ProposalStatus> proposalStatuses =

            Arrays.asList(
              ProposalStatus.RECEIVED_BY_IOS
            , ProposalStatus.SUBMITTED_TO_IOS
            , ProposalStatus.RECEIVED_IOS
            );

    // This will not be possible, also you cannot remove anything.
    proposalStatuses.addAll(getAllSubmittedStatuses());

    return proposalStatuses;
}

为了您的目的，我将执行以下操作：

return new ArrayList<ProposalStatus>(Arrays.asList(ProposalStatus.SAVED_TO_IOS,ProposalStatus.SENDED_TO_IOS_IN_PROGRESS)

这应该可以帮助您获取集合对象。

Answer 2

Arrays.asList() returns 一个不可变列表，您无法修改。

更具体地说，add()、addAll() 和 remove() 方法未实现。

Answer 3

来自 javadoc of Arrays.asList()（强调我的）：

Returns a fixed-size list backed by the specified array

简而言之：您不能从这样的列表中 .add*() 或 .remove*()！您必须使用另一个可修改的 List 实现（例如 ArrayList）。

Java 集合对象出现 UnsupportedOperationException

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