[英]Using iterator on a TreeSet
SITUATION: I have a TreeSet of custom Objects and I have also used a custom Comparator. 情况:我有一个自定义对象的TreeSet,我也使用了自定义比较器。 I have created an iterator to use on this TreeSet.
我已经创建了一个在这个TreeSet上使用的迭代器。
TreeSet<Custom> ts=new TreeSet<Custom>();
Iterator<Custom> itr=ts.iterator();
while(itr.hasNext()){
Custom c=itr.next();
//Code to add a new element to the TreeSet ts
}
QUESTION: Well I want to know that if I add a new element to the TreeSet within the while loop, then will that new element get sorted immediately. 问题:我想知道如果我在while循环中向TreeSet添加一个新元素,那么新元素会立即被排序。 In other words, if I add a new element within the while loop and it is less than the one which I am currently holding in c, then in the next iteration will I be getting the same element in c as in the last iteration?(since after sorting, the newly added element will occupy a place somewhere before the current element).
换句话说,如果我在while循环中添加一个新元素并且它小于我当前在c中持有的元素,那么在下一次迭代中我将获得与上一次迭代中相同的元素吗?(因为在排序之后,新添加的元素将占据当前元素之前的某个位置。
If you add an element during your iteration, your next iterator call will likely throw a ConcurrentModificationException
. 如果在迭代期间添加元素,则下一个迭代器调用可能会抛出
ConcurrentModificationException
。 See the fail-fast behavior in TreeSet docs. 请参阅TreeSet文档中的故障快速行为。
To iterate and add elements, you could copy first to another set: 要迭代和添加元素,您可以先复制到另一个集合:
TreeSet<Custom> ts = ...
TreeSet<Custom> tsWithExtra = new TreeSet(ts);
for (Custom c : ts) {
// possibly add to tsWithExtra
}
// continue, using tsWithExtra
or create a separate collection to be merged with ts
after iteration, as Colin suggests. 科林建议,或者在迭代后创建一个单独的集合与
ts
合并。
You will get a java.util.ConcurrentModificationException if you add an element into the TreeSet inside while loop. 如果在while循环中向TreeSet中添加元素,则会得到java.util.ConcurrentModificationException 。
Set<String> ts=new TreeSet<String>();
ts.addAll(Arrays.asList(new String[]{"abb", "abd", "abg"}));
Iterator<String> itr=ts.iterator();
while(itr.hasNext()){
String s = itr.next();
System.out.println("s: " + s);
if (s.equals("abd"))
ts.add("abc");
}
Exception in thread "main" java.util.ConcurrentModificationException
public static void main(String[] args) {
TreeSet<Integer> ts=new TreeSet<Integer>();
ts.add(2);
ts.add(4);
ts.add(0);
Iterator<Integer> itr=ts.iterator();
while(itr.hasNext()){
Integer c=itr.next();
System.out.println(c);
//Code
ts.add(1);
}
}
Exception in thread "main" java.util.ConcurrentModificationException
This will come to all collections like List
, Map
, Set
Because when iterator starts it may be putting some lock on it . 这将到达所有集合,如
List
, Map
, Set
因为当迭代器启动时它可能会对它进行一些锁定。
if you iterate list using iterator then this exception will come. 如果使用迭代器迭代列表,则会出现此异常。 I think otherwise this loop will be infinite as you are adding element whole iterating.
我认为否则这个循环将是无限的,因为你添加元素整个迭代。
Consider without iterator: 考虑没有迭代器:
public static void main(String[] args) {
List<Integer> list=new ArrayList<Integer>();
list.add(2);
list.add(4);
list.add(0);
for (int i = 0; i < 3; i++) {
System.out.println(list.get(i));
list.add(3);
}
System.out.println("Size" +list.size());
}
this will be fine . 这没关系。
In order to avoid the ConcurrentModificationException
you might want to check out my UpdateableTreeSet
. 为了避免
ConcurrentModificationException
您可能想要检查我的UpdateableTreeSet
。 I have even added a new test case showing how to add elements during a loop. 我甚至添加了一个新的测试用例,展示了如何在循环中添加元素。 To be more exact, you mark new elements for a later, deferred update of the set.
更确切地说,您为该集合的延迟更新标记新元素。 This works quite nicely.
这很好用。 Basically you do something like
基本上你做的事情就像
for (MyComparableElement element : myUpdateableTreeSet) {
if (someCondition) {
// Add new element (deferred)
myUpdateableTreeSet.markForUpdate(
new MyComparableElement("foo", "bar", 1, 2)
);
}
}
// Perform bulk update
myUpdateableTreeSet.updateMarked();
I guess this is quite exactly what you need. 我想这正是你所需要的。
:-)
To prevent the ConcurrentModificationException while walking. 在行走时防止ConcurrentModificationException。 Below is my version to allow high frequency insertion into the TreeSet() and allow concurrently iterate on it.
下面是我的版本,允许高频插入TreeSet()并允许同时迭代它。 This class use a extra queue to store the inserting object when the TreeSet is being iterating.
当TreeSet正在迭代时,此类使用额外队列来存储插入对象。
public class UpdatableTransactionSet {
TreeSet <DepKey> transactions = new TreeSet <DepKey> ();
LinkedList <DepKey> queue = new LinkedList <DepKey> ();
boolean busy=false;
/**
* directly call it
* @param e
*/
void add(DepKey e) {
boolean bb = getLock();
if(bb) {
transactions.add(e);
freeLock();
} else {
synchronized(queue) {
queue.add(e);
}
}
}
/**
* must getLock() and freeLock() while call this getIterator function
* @return
*/
Iterator<DepKey> getIterator() {
return null;
}
synchronized boolean getLock() {
if(busy) return false;
busy = true;
return true;
}
synchronized void freeLock() {
synchronized(queue) {
for(DepKey e:queue) {
transactions.add(e);
}
}
busy = false;
}
}
While the question has already been answered, I think the most satisfactory answer lies in javadoc of TreeSet itself 虽然问题已经得到解答,但我认为最令人满意的答案在于TreeSet本身的javadoc
The iterators returned by this class's iterator method are fail-fast: if the set is modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException.
这个类的迭代器方法返回的迭代器是快速失败的:如果在创建迭代器之后的任何时候修改了set,除了通过迭代器自己的remove方法之外,迭代器将抛出ConcurrentModificationException。 Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
因此,在并发修改的情况下,迭代器快速而干净地失败,而不是在未来的未确定时间冒着任意的,非确定性行为的风险。
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, >generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification.
请注意,迭代器的快速失败行为无法得到保证,因为一般来说,在存在非同步并发修改的情况下,不可能做出任何硬性保证。 Fail-fast iterators throw ConcurrentModificationException on a best-effort basis.
失败快速迭代器会尽最大努力抛出ConcurrentModificationException。 Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.
因此,编写依赖于此异常的程序以确保其正确性是错误的:迭代器的快速失败行为应该仅用于检测错误。
为了避免在进行插入时必然发生的并发修改错误,您还可以创建Set的临时副本,反而遍历副本,并修改原始版本。
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