Iterator details in list... Interview pits

List Small details of middle iterator

 Hello everyone   I'm handsome Li  ！ Kim San Yin four    As a programmer who doesn't know the greatness of the world   Of course, we should seize this opportunity to be severely beaten by society 

Look at the problem first Deepen by the problem 、

// What happens to the following code

List list = new XXXList();
for(Object item: list){
    
    list.remove(item);
}

This problem may often be seen, hahaha Very classic however ！ It's very detailed There are many things involved

First see foreach grammar for(T item: list){} This is a Grammatical sugar Bottom In fact, it is traversed by the iterator That is to say Iterator

This syntax is compiled class The file looks like this

Execute the above code See what it looks like Is it the same as you think （ In the imagination _{Very different} Everything is different from later ~）

And then we Change it this time

Delete this Will report ConcurrentModificationException Error of

Why is that? I'm not sure First We can see where the error was reported

Go straight into Iterator next The first line of the method This method of

If modCount != expectModCount It will make this mistake .

The problem is coming.

modCount and expectModCount What is it? ？

1, modCount

Actual number of modifications

be located AbstractList abstract class （ps： English is a good thing ！ ）

		 It involves rapid failure   and   Failure safety   What?  
		 then   When the implementation class wants to use   When you fail quickly 
		 If in   In the process of iterator iteration  modCount  change   Then it will cause the above exception ：**ConcurrentModificationException**

nudges

When This value will change ？

     In execution ArrayList  Of  add   Or is it  remove  Method time   Will trigger the modification of this value 

Na Simple and crude

2, expectModCount

Expected number of revisions

This is arrayList There is an attribute of iterator

He will assign values when the iterator is initialized This is to put modCount Given to He

When will he revise it （ Don't bother to take screenshots = =）

iterator remove Or is it add（） When Note that it's an iterator

in other words After the iterator is initialized
You call next Method If you call list Class add perhaps remove Methods lead to modCount The value of has changed and expectModCount The values are different
Will throw ConcurrentModificationException

ok Now let's discuss why the above situation occurs

First look at the first column

This is normally executed and no error will be reported

Step by step
The first that hasNext Post it ··

	cursor  What is it?   It is an attribute in the iterator   Index of the next element 
	size  That is, the size of the set 
	 Notice that he directly uses it here, which is not equal to making judgments  
	
	teaList  There is 2 Elements    Corresponding subscript  0  Young people  1  Don't talk about military morality  
	 First entry  while  Judge  hasNext 
	 The index of the next element is 0 size =2   Determine the successful entry   stay next  Determine whether the number of modifications is equal to the expected number of modifications   Because it is not done after the iterator is created   Add and delete operations   therefore  
	 This is the time   The number of modifications is the same as the expected number of modifications   Normal execution   Take out item =  Young people  if Judge   success   call list Of remove  Method   Delete the “ Young people ” Be careful ： Delete without iterators 
	 This is the time  size  It becomes 1   Keep going down  
	 Enter for the second time  while  hashNext  The judgement of 
	 The index of the next element is 1  however  size  Turned into 1  therefore   Judgment failed   It will jump out directly   It won't come to next  It will not determine the number of modifications   So there's no mistake 
	（ Two   Or two or more sizes    Delete the penultimate   You can't report an error ）

ok Here is the second example

This direct lv That's it

	 First entry  while  Judge  hasNext   No problem   The normal process is executed downward 
	 Enter for the second time  while  Judge  hasNext  The next element subscript is 1 size  yes 2  Determine the success and enter the following logic  
	 stay next  Determine whether the number of modifications is equal to the expected number of modifications   Because it is not done after the iterator is created   Add and delete operations   therefore  
	 This is the time   The number of modifications is the same as the expected number of modifications   Normal execution    Take out item =  Don't talk about military morality  if Judge   success   call list Of remove  Method   Delete ....
	 Then go to the next   determine   At this time, the subscript of the next element is 2  however  size  yes 1  At this time, it will still be judged to be successful   Get into next Method   because   He did  list  Of remove 
	 therefore   The expected number of modifications is less than the actual number of modifications   So at this point   Will directly throw this error .ok 了 

Finished ！ Actually, it's OK Get to know What is security failure And fast failure . I don't have much here bb 了 People are interested in You can find out for yourself “ The more you know The more you don't know ” Here is forced to develop chic Li See you next time .