英文:
How to fix Fatal Exception: java.lang.IndexOutOfBoundsException Index: 0, Size: 1
问题
Kotlin程序片段：
fun peek() = if (!elements.isEmpty() && elements.size > 0) elements[0] else null
它运行得很好，但有时我会收到Firebase的崩溃报告：
> 严重异常：java.lang.IndexOutOfBoundsException 索引：0，大小：1
> java.util.ArrayList.get (ArrayList.java:437)
> bigmick.ghu.util.Queue.peek (Queue.kt:17)
对我来说，这听起来很疯狂：列表的大小是1，而且第一个（存在的）元素可以通过索引=0引用。
据我所知，它应该能工作。但有时候却不行。
英文:
Kotlin program snippet:
fun peek() = if (!elements.isEmpty() && elements.size > 0) elements[0] else null
It works great, but sometimes I get a firebase crash report:
> Fatal Exception: java.lang.IndexOutOfBoundsException Index: 0, Size: 1
> java.util.ArrayList.get (ArrayList.java:437)
> bigmick.ghu.util.Queue.peek (Queue.kt:17)
For me it sounds crazy: the list's size is 1, also the first (existing) element can be referred by index=0.
As far as I know it should work. But sometimes it does not.
答案1
得分: 4
ArrayList类不是线程安全的。当一个线程在另一个线程调用您的方法的同时进行更改时，可能会发生意外和看似不可能的事情。
一个可能的修复方法是使两个线程不可能同时调用列表的方法（"互斥"）。这可以通过Collections.synchronizedList轻松实现。
elements = Collections.synchronizedList(new ArrayList<>())
如果您从列表中读取元素的频率远高于写入的频率，另一种选择是使用CopyOnWriteArrayList而不是ArrayList。
我看到您正在从一个名为queue的类中使用这段代码。在这种情况下，您可能希望使用实际的队列数据结构，而不是列表，标准库有许多选项供您选择，具体取决于您需要的行为。可以参考这个问题，例如：https://stackoverflow.com/questions/8203864/choosing-the-best-concurrency-list-in-java
英文:
The ArrayList class is not thread safe. When a thread makes a change at the same time another thread calls your method, unexpected and seemingly impossible things can happen.
A possible fix is to make it impossible for two threads to call methods of the list at the same time ("mutual exclusion"). This is easily achieved with Collections.synchronizedList
elements = Collections.synchronizedList(new ArrayList<>())
If you read elements from the list a lot more than you write, an alternative is to use CopyOnWriteArrayList instead of ArrayList
I see that you're using this code from a class called queue. In that case you may want to use an actual queue data structure instead of a list, and the standard library has many options you can choose from, depending on what behavior you want. See this question for example https://stackoverflow.com/questions/8203864/choosing-the-best-concurrency-list-in-java
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论