当foreach添加元素时，scala MutableList为什么不抛出异常

Question

when I try scala list, I found a question. when I use a MutableList, just want to try append a element in foreach (I know this is a bad operation):

import scala.collection.mutable.MutableList
val tts = MutableList("Hello World~")
tts.foreach { t => 
  tts += "Hello World~"
  println(t)
}

Console Output:

Hello World~
Hello World~

My question is why the above code not throw a ConcurrentModificationException or output normaly?

Answer 1

Throwing the ConcurrentModificationException on modifications during iteration is generally called fail-fast semantics. Scala collection classes normally do not throw ConcurrentModificationException s because:

• fail-fast complicates the implementation of the iterator
• fail-fast usually slows down the iterator implementation
• fail-fast is hard to guarantee in all cases (even Java classes do not guarantee it and do it on a best-effort basis )
• fail-fast is even harder to ensure in multithreaded code

Because of this reasons, Scala just documents that concurrent modifications are not allowed, rather than striving for fail-fast and ending up with a leaky abstraction.