Scala prepend +：op不修改ListBufffer

Question

我在listbuffer類型上使用prepend方法並觀察一些奇怪的行爲。 prepend操作返回一個可接受的新列表。但是不應該也修改ListBuffer？預先確定後，我仍然看到ListBuffer的長度沒有改變。我在這裏錯過了什麼嗎？

scala> val buf = new ListBuffer[Int] 
buf: scala.collection.mutable.ListBuffer[Int] = ListBuffer() 

scala> buf += 1     
res47: buf.type = ListBuffer(1) 

scala> buf += 2 
res48: buf.type = ListBuffer(1, 2) 

scala> 3 +: buf 
res49: scala.collection.mutable.ListBuffer[Int] = ListBuffer(3, 1, 2) 

scala> buf.toList 
res50: List[Int] = List(1, 2) 

Answer 1

使用+=:

scala> val buf = new ListBuffer[Int] 
buf: scala.collection.mutable.ListBuffer[Int] = ListBuffer() 

scala> buf += 1 
res0: buf.type = ListBuffer(1) 

scala> buf += 2 
res1: buf.type = ListBuffer(1, 2) 

scala> 3 +=: buf 
res2: buf.type = ListBuffer(3, 1, 2) 

scala> buf.toList 
res3: List[Int] = List(3, 1, 2)