英文:
Spring boot JPA fetch a list and add an item to it throws error
问题
我正在使用 JPA，我有一个名为 Order 的实体/类。我有一个用于通过 id 获取订单的 rest GET 端点。它工作得非常好。订单实体如下所示：
@Entitypublic class Order {@Idprivate Long id;@Columnprivate List<String> transactionRefs;}
现在，在一个特定的场景中，我需要从数据库中获取订单并向 transactionRefs 添加另一项并保存。所以我按照下面的方式进行：
Order order = orderRepository.findById(1).get();List<String> transactionList = order.getTransactionRefs();transactionList.add("transaction-ref");
当我这样做时，我会得到以下错误：
java.lang.UnsupportedOperationException: null\n\tat java.util.AbstractList.add(AbstractList.java:148)
如果我按照下面的方式做，问题就会得到解决：
Order order = orderRepository.findById(1).get();List<String> transactionList = order.getTransactionRefs();transactionList = new ArrayList<>(transactionList);transactionList.add("transaction-ref");
所以，我需要知道我是否走在正确的方向上，这是否是一个预期的错误场景。
更新：
每当我们向列表中添加项目时，我们有以下条件：
if (transactionRefs == null) {transactionRefs = new ArrayList<>();}
因此，每当第一次保存 transactionref 时，我们会将其转换为 ArrayList。
更新 2：
以下是 transactionRef 的 getter 方法：
public List<String> getTransactionRefs(){if (this.transactionRefs != null) {return Arrays.asList(this.transactionRefs.split(","));}return null;}
英文:
I am using JPA and I have an entity/class named Order. I have a rest GET endpoint to fetch an order by an id. It works perfectly fine. The order entity looks like below:
@Entitypublic class Order {@Idprivate Long id;@Columnprivate List<String> transactionRefs;}
Now, in one particular scenario, I need to fetch the order from the database and add another item to the transactionRefs and save it. So I do as below:
Order order = orderRepository.findById(1).get();List<String> transactionList = order.getTransactionRefs();transactionList.add("transaction-ref");
I get the below error when I do that:
java.lang.UnsupportedOperationException: null\n\tat java.util.AbstractList.add(AbstractList.java:148)
If I do as below, that fixes the problem:
Order order = orderRepository.findById(1).get();List<String> transactionList = order.getTransactionRefs();transactionList = new ArrayList<>(transactionList);transactionList.add("transaction-ref");
So, I need to know if I am in the right direction here and is this an expected error scenario.
Update:
Whenever we are adding an item to the list, we have the below condition :
if (transactionRefs == null) {transactionRefs = new ArrayList<>();}
So, whenever the transactionref is saved for the first time, we cast it to a ArrayList.
Update 2 :
Below is the getter for the transactionRef:
public List<String> getTransactionRefs(){if (this.transactionRefs != null) {return Arrays.asList(this.transactionRefs.split(","));}return null;}
答案1
得分: 2
这是您异常的原因：
return Arrays.asList(this.transactionRefs.split(","));
Arrays.asList 返回由数组支持的集合，无法使用 add 或 addAll 进行修改。您需要像在问题中所做的那样创建一个新的 List：
List<String> transactionList = order.getTransactionRefs();transactionList = new ArrayList<>(transactionList);
更多示例：
-
https://stackoverflow.com/questions/18389012/how-to-add-elements-in-list-when-used-arrays-aslist
-
https://stackoverflow.com/questions/25447502/regarding-immutable-list-created-by-arrays-aslist
英文:
This is the cause of your exception
return Arrays.asList(this.transactionRefs.split(","));
Arrays.asList returns a collection backed by the array and it can't be modified with add or addAll. You need to create the List just like you are doing in the question:
List<String> transactionList = order.getTransactionRefs();transactionList = new ArrayList<>(transactionList);
For more examples:
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