We all know you can't do the following because of ConcurrentModificationException:
for (Object i : l) {
if (condition(i)) {
l.remove(i);
}
}
But this apparently works sometimes, but not always. Here's some specific code:
public static void main(String[] args) {
Collection<Integer> l = new ArrayList<>();
for (int i = 0; i < 10; ++i) {
l.add(4);
l.add(5);
l.add(6);
}
for (int i : l) {
if (i == 5) {
l.remove(i);
}
}
System.out.println(l);
}
This, of course, results in:
Exception in thread "main" java.util.ConcurrentModificationException
Even though multiple threads aren't doing it. Anyway.
What's the best solution to this problem? How can I remove an item from the collection in a loop without throwing this exception?
I'm also using an arbitrary Collection here, not necessarily an ArrayList, so you can't rely on get.
Iterator.remove() is safe, you can use it like this:
List<String> list = new ArrayList<>();
// This is a clever way to create the iterator and call iterator.hasNext() like
// you would do in a while-loop. It would be the same as doing:
// Iterator<String> iterator = list.iterator();
// while (iterator.hasNext()) {
for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {
String string = iterator.next();
if (string.isEmpty()) {
// Remove the current element from the iterator and the list.
iterator.remove();
}
}
Note that Iterator.remove() is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress.
Source: docs.oracle > The Collection Interface
And similarly, if you have a ListIterator and want to add items, you can use ListIterator#add, for the same reason you can use Iterator#remove — it's designed to allow it.
In your case you tried to remove from a list, but the same restriction applies if trying to put into a Map while iterating its content.
This works:
Iterator<Integer> iter = l.iterator();
while (iter.hasNext()) {
if (iter.next() == 5) {
iter.remove();
}
}
I assumed that since a foreach loop is syntactic sugar for iterating, using an iterator wouldn't help... but it gives you this .remove() functionality.
With Java 8 you can use the new removeIf method. Applied to your example:
Collection<Integer> coll = new ArrayList<>();
//populate
coll.removeIf(i -> i == 5);
Since the question has been already answered i.e. the best way is to use the remove method of the iterator object, I would go into the specifics of the place where the error "java.util.ConcurrentModificationException" is thrown.
Every collection class has a private class which implements the Iterator interface and provides methods like next(), remove() and hasNext().
The code for next looks something like this...
public E next() {
checkForComodification();
try {
E next = get(cursor);
lastRet = cursor++;
return next;
} catch(IndexOutOfBoundsException e) {
checkForComodification();
throw new NoSuchElementException();
}
}
Here the method checkForComodification is implemented as
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
So, as you can see, if you explicitly try to remove an element from the collection. It results in modCount getting different from expectedModCount, resulting in the exception ConcurrentModificationException.
You can either use the iterator directly like you mentioned, or else keep a second collection and add each item you want to remove to the new collection, then removeAll at the end. This allows you to keep using the type-safety of the for-each loop at the cost of increased memory use and cpu time (shouldn't be a huge problem unless you have really, really big lists or a really old computer)
public static void main(String[] args)
{
Collection<Integer> l = new ArrayList<Integer>();
Collection<Integer> itemsToRemove = new ArrayList<>();
for (int i=0; i < 10; i++) {
l.add(Integer.of(4));
l.add(Integer.of(5));
l.add(Integer.of(6));
}
for (Integer i : l)
{
if (i.intValue() == 5) {
itemsToRemove.add(i);
}
}
l.removeAll(itemsToRemove);
System.out.println(l);
}
In such cases a common trick is (was?) to go backwards:
for(int i = l.size() - 1; i >= 0; i --) {
if (l.get(i) == 5) {
l.remove(i);
}
}
That said, I'm more than happy that you have better ways in Java 8, e.g. removeIf or filter on streams.
Same answer as Claudius with a for loop:
for (Iterator<Object> it = objects.iterator(); it.hasNext();) {
Object object = it.next();
if (test) {
it.remove();
}
}
With Eclipse Collections, the method removeIf defined on MutableCollection will work:
MutableList<Integer> list = Lists.mutable.of(1, 2, 3, 4, 5);
list.removeIf(Predicates.lessThan(3));
Assert.assertEquals(Lists.mutable.of(3, 4, 5), list);
With Java 8 Lambda syntax this can be written as follows:
MutableList<Integer> list = Lists.mutable.of(1, 2, 3, 4, 5);
list.removeIf(Predicates.cast(integer -> integer < 3));
Assert.assertEquals(Lists.mutable.of(3, 4, 5), list);
The call to Predicates.cast() is necessary here because a default removeIf method was added on the java.util.Collection interface in Java 8.
Note: I am a committer for Eclipse Collections.
Make a copy of existing list and iterate over new copy.
for (String str : new ArrayList<String>(listOfStr))
{
listOfStr.remove(/* object reference or index */);
}
People are asserting one can't remove from a Collection being iterated by a foreach loop. I just wanted to point out that is technically incorrect and describe exactly (I know the OP's question is so advanced as to obviate knowing this) the code behind that assumption:
for (TouchableObj obj : untouchedSet) { // <--- This is where ConcurrentModificationException strikes
if (obj.isTouched()) {
untouchedSet.remove(obj);
touchedSt.add(obj);
break; // this is key to avoiding returning to the foreach
}
}
It isn't that you can't remove from the iterated Colletion rather that you can't then continue iteration once you do. Hence the break in the code above.
Apologies if this answer is a somewhat specialist use-case and more suited to the original thread I arrived here from, that one is marked as a duplicate (despite this thread appearing more nuanced) of this and locked.
With a traditional for loop
ArrayList<String> myArray = new ArrayList<>();
for (int i = 0; i < myArray.size(); ) {
String text = myArray.get(i);
if (someCondition(text))
myArray.remove(i);
else
i++;
}
ConcurrentHashMap or ConcurrentLinkedQueue or ConcurrentSkipListMap may be another option, because they will never throw any ConcurrentModificationException, even if you remove or add item.
Another way is to use a copy of your arrayList just for iteration:
List<Object> l = ...
List<Object> iterationList = ImmutableList.copyOf(l);
for (Object curr : iterationList) {
if (condition(curr)) {
l.remove(curr);
}
}
A ListIterator allows you to add or remove items in the list. Suppose you have a list of Car objects:
List<Car> cars = ArrayList<>();
// add cars here...
for (ListIterator<Car> carIterator = cars.listIterator(); carIterator.hasNext(); )
{
if (<some-condition>)
{
carIterator().remove()
}
else if (<some-other-condition>)
{
carIterator().add(aNewCar);
}
}
Now, You can remove with the following code
l.removeIf(current -> current == 5);
I know this question is too old to be about Java 8, but for those using Java 8 you can easily use removeIf():
Collection<Integer> l = new ArrayList<Integer>();
for (int i=0; i < 10; ++i) {
l.add(new Integer(4));
l.add(new Integer(5));
l.add(new Integer(6));
}
l.removeIf(i -> i.intValue() == 5);
Java Concurrent Modification Exception
Single thread
Iterator<String> iterator = list.iterator();
while (iterator.hasNext()) {
String value = iter.next()
if (value == "A") {
list.remove(it.next()); //throws ConcurrentModificationException
}
}
Solution: iterator remove() method
Iterator<String> iterator = list.iterator();
while (iterator.hasNext()) {
String value = iter.next()
if (value == "A") {
it.remove()
}
}
Multi thread
copy/convert and iterate over another one collection. For small collections
synchronize[About]
thread safe collection[About]
I have a suggestion for the problem above. No need of secondary list or any extra time. Please find an example which would do the same stuff but in a different way.
//"list" is ArrayList<Object>
//"state" is some boolean variable, which when set to true, Object will be removed from the list
int index = 0;
while(index < list.size()) {
Object r = list.get(index);
if( state ) {
list.remove(index);
index = 0;
continue;
}
index += 1;
}
This would avoid the Concurrency Exception.
for (Integer i : l)
{
if (i.intValue() == 5){
itemsToRemove.add(i);
break;
}
}
The catch is the after removing the element from the list if you skip the internal iterator.next() call. it still works! Though I dont propose to write code like this it helps to understand the concept behind it :-)
Cheers!
Example of thread safe collection modification:
public class Example {
private final List<String> queue = Collections.synchronizedList(new ArrayList<String>());
public void removeFromQueue() {
synchronized (queue) {
Iterator<String> iterator = queue.iterator();
String string = iterator.next();
if (string.isEmpty()) {
iterator.remove();
}
}
}
}
I know this question assumes just a Collection, and not more specifically any List. But for those reading this question who are indeed working with a List reference, you can avoid ConcurrentModificationException with a while-loop (while modifying within it) instead if you want to avoid Iterator (either if you want to avoid it in general, or avoid it specifically to achieve a looping order different from start-to-end stopping at each element [which I believe is the only order Iterator itself can do]):
*Update: See comments below that clarify the analogous is also achievable with the traditional-for-loop.
final List<Integer> list = new ArrayList<>();
for(int i = 0; i < 10; ++i){
list.add(i);
}
int i = 1;
while(i < list.size()){
if(list.get(i) % 2 == 0){
list.remove(i++);
} else {
i += 2;
}
}
No ConcurrentModificationException from that code.
There we see looping not start at the beginning, and not stop at every element (which I believe Iterator itself can't do).
FWIW we also see get being called on list, which could not be done if its reference was just Collection (instead of the more specific List-type of Collection) - List interface includes get, but Collection interface does not. If not for that difference, then the list reference could instead be a Collection [and therefore technically this Answer would then be a direct Answer, instead of a tangential Answer].
FWIWW same code still works after modified to start at beginning at stop at every element (just like Iterator order):
final List<Integer> list = new ArrayList<>();
for(int i = 0; i < 10; ++i){
list.add(i);
}
int i = 0;
while(i < list.size()){
if(list.get(i) % 2 == 0){
list.remove(i);
} else {
++i;
}
}
One solution could be to rotate the list and remove the first element to avoid the ConcurrentModificationException or IndexOutOfBoundsException
int n = list.size();
for(int j=0;j<n;j++){
//you can also put a condition before remove
list.remove(0);
Collections.rotate(list, 1);
}
Collections.rotate(list, -1);
Try this one (removes all elements in the list that equal i):
for (Object i : l) {
if (condition(i)) {
l = (l.stream().filter((a) -> a != i)).collect(Collectors.toList());
}
}
You can use a while loop.
Iterator<Map.Entry<String, String>> iterator = map.entrySet().iterator();
while(iterator.hasNext()){
Map.Entry<String, String> entry = iterator.next();
if(entry.getKey().equals("test")) {
iterator.remove();
}
}
I ended up with this ConcurrentModificationException, while iterating the list using stream().map() method. However the for(:) did not throw the exception while iterating and modifying the the list.
Here is code snippet , if its of help to anyone:
here I'm iterating on a ArrayList<BuildEntity> , and modifying it using the list.remove(obj)
for(BuildEntity build : uniqueBuildEntities){
if(build!=null){
if(isBuildCrashedWithErrors(build)){
log.info("The following build crashed with errors , will not be persisted -> \n{}"
,build.getBuildUrl());
uniqueBuildEntities.remove(build);
if (uniqueBuildEntities.isEmpty()) return EMPTY_LIST;
}
}
}
if(uniqueBuildEntities.size()>0) {
dbEntries.addAll(uniqueBuildEntities);
}
If using HashMap, in newer versions of Java (8+) you can select each of 3 options:
public class UserProfileEntity {
private String Code;
private String mobileNumber;
private LocalDateTime inputDT;
// getters and setters here
}
HashMap<String, UserProfileEntity> upMap = new HashMap<>();
// remove by value
upMap.values().removeIf(value -> !value.getCode().contains("0005"));
// remove by key
upMap.keySet().removeIf(key -> key.contentEquals("testUser"));
// remove by entry / key + value
upMap.entrySet().removeIf(entry -> (entry.getKey().endsWith("admin") || entry.getValue().getInputDT().isBefore(LocalDateTime.now().minusMinutes(3)));
The best way (recommended) is use of java.util.concurrent package. By
using this package you can easily avoid this exception. Refer
Modified Code:
public static void main(String[] args) {
Collection<Integer> l = new CopyOnWriteArrayList<Integer>();
for (int i=0; i < 10; ++i) {
l.add(new Integer(4));
l.add(new Integer(5));
l.add(new Integer(6));
}
for (Integer i : l) {
if (i.intValue() == 5) {
l.remove(i);
}
}
System.out.println(l);
}
Iterators are not always helpful when another thread also modifies the collection. I had tried many ways but then realized traversing the collection manually is much safer (backward for removal):
for (i in myList.size-1 downTo 0) {
myList.getOrNull(i)?.also {
if (it == 5)
myList.remove(it)
}
}
In case ArrayList:remove(int index)- if(index is last element's position) it avoids without System.arraycopy() and takes not time for this.
arraycopy time increases if(index decreases), by the way elements of list also decreases!
the best effective remove way is- removing its elements in descending order:
while(list.size()>0)list.remove(list.size()-1);//takes O(1)
while(list.size()>0)list.remove(0);//takes O(factorial(n))
//region prepare data
ArrayList<Integer> ints = new ArrayList<Integer>();
ArrayList<Integer> toRemove = new ArrayList<Integer>();
Random rdm = new Random();
long millis;
for (int i = 0; i < 100000; i++) {
Integer integer = rdm.nextInt();
ints.add(integer);
}
ArrayList<Integer> intsForIndex = new ArrayList<Integer>(ints);
ArrayList<Integer> intsDescIndex = new ArrayList<Integer>(ints);
ArrayList<Integer> intsIterator = new ArrayList<Integer>(ints);
//endregion
// region for index
millis = System.currentTimeMillis();
for (int i = 0; i < intsForIndex.size(); i++)
if (intsForIndex.get(i) % 2 == 0) intsForIndex.remove(i--);
System.out.println(System.currentTimeMillis() - millis);
// endregion
// region for index desc
millis = System.currentTimeMillis();
for (int i = intsDescIndex.size() - 1; i >= 0; i--)
if (intsDescIndex.get(i) % 2 == 0) intsDescIndex.remove(i);
System.out.println(System.currentTimeMillis() - millis);
//endregion
// region iterator
millis = System.currentTimeMillis();
for (Iterator<Integer> iterator = intsIterator.iterator(); iterator.hasNext(); )
if (iterator.next() % 2 == 0) iterator.remove();
System.out.println(System.currentTimeMillis() - millis);
//endregion
for index loop: 1090 msec
for desc index: 519 msec---the best
for iterator: 1043 msec
you can also use Recursion
Recursion in java is a process in which a method calls itself continuously. A method in java that calls itself is called recursive method.
is it possible to use a Kleene Operator (Kleene Star) for the Formatters?
I want to use a phoneFormatter, which puts a minus after the 5th number and afterwards it should be possible to have a variable number of numbers.
E.g.: 0172-555666999, 0160-44552 etc.
That is how I started, but I don't know which character belongs after the last hash (it is not a star, I already tried it ;-) ):
<fx:Declarations>
<mx:PhoneFormatter id="mPhoneFormat"
formatString="####-#"/>
</fx:Declarations>
The default PhoneFormatter expects the input string to have the same number of characters as the format string. They don't support regular expression patterns (like * to match the element zero or more times).
However, it's pretty easy to make your own formatter. To do this, I extended the PhoneFormatter class and overrode its format() method. I copied and pasted the original format() method and made the following modifications:
comment out the code that compared the length of the source string with the length of the format string
compare the length of the formatted string. If the original string is longer, append the remaining chars from the original string to the formatted string.
This probably won't handle all of your use cases, but it should be pretty straightforward to modify this to your needs.
package
{
import mx.formatters.PhoneFormatter;
import mx.formatters.SwitchSymbolFormatter;
public class CustomPhoneNumberFormatter extends PhoneFormatter
{
public function CustomPhoneNumberFormatter()
{
super();
}
override public function format(value:Object):String
{
// Reset any previous errors.
if (error)
error = null;
// --value--
if (!value || String(value).length == 0 || isNaN(Number(value)))
{
error = defaultInvalidValueError;
return "";
}
// --length--
var fStrLen:int = 0;
var letter:String;
var n:int;
var i:int;
n = formatString.length;
for (i = 0; i < n; i++)
{
letter = formatString.charAt(i);
if (letter == "#")
{
fStrLen++;
}
else if (validPatternChars.indexOf(letter) == -1)
{
error = defaultInvalidFormatError;
return "";
}
}
// if (String(value).length != fStrLen)
// {
// error = defaultInvalidValueError;
// return "";
// }
// --format--
var fStr:String = formatString;
if (fStrLen == 7 && areaCode != -1)
{
var aCodeLen:int = 0;
n = areaCodeFormat.length;
for (i = 0; i < n; i++)
{
if (areaCodeFormat.charAt(i) == "#")
aCodeLen++;
}
if (aCodeLen == 3 && String(areaCode).length == 3)
{
fStr = String(areaCodeFormat).concat(fStr);
value = String(areaCode).concat(value);
}
}
var dataFormatter:SwitchSymbolFormatter = new SwitchSymbolFormatter();
var source:String = String(value);
var returnValue:String = dataFormatter.formatValue(fStr, value);
if (source.length > returnValue.length)
{
returnValue = returnValue + source.substr(returnValue.length-1);
}
return returnValue;
}
}
}
I want to check if an IP address is in a certain range, matching by "*" only. For example, "202.121.189.8" is in "202.121.189.*".
The scenario is that I have a list of banned IPs, some of them contains "*", so I wrote a function, it works fine so far:
static bool IsInRange(string ip, List<string> ipList)
{
if (ipList.Contains(ip))
{
return true;
}
var ipSets = ip.Split('.');
foreach (var item in ipList)
{
var itemSets = item.Split('.');
for (int i = 0; i < 4; i++)
{
if (itemSets[i] == "*")
{
bool isMatch = true;
for (int j = 0; j < i; j++)
{
if (ipSets[i - j - 1] != itemSets[i - j - 1])
{
isMatch = false;
}
}
if (isMatch)
{
return true;
}
}
}
}
return false;
}
Test code:
string ip = "202.121.189.8";
List<string> ipList = new List<string>() { "202.121.168.25", "202.121.189.*" };
Console.WriteLine(IsInRange(ip, ipList));
But I think what i wrote is very stupid, and I want to optimize it, does anyone have an idea how to simplify this function? not to use so many "for....if...".
A good idea would be to represent the banned subnets in a form of a pair: mask + base address. So your check will look like that:
banned = (ip & mask == baseaddress & mask);
For 11.22.33.* the base address will be 11*0x1000000 + 22*0x10000 + 33*0x100, mask will be 0xffffff00.
For single address 55.44.33.22 the address will be 55*0x1000000 + 44*0x10000 * 33*0x100 + 22, mask will be 0xffffffff.
You'll need to convert the address to a 32-bit int as a separate procedure.
After that all, your code will look like that:
int numip = ip2int(ip);
bool isIpBanned = banList.Any(item =>
numip & item.mask == item.baseaddress & item.mask);
By the way, this way you'll be able to represent even bans on smaller subsets.
int ip2int(string ip) // error checking omitted
{
var parts = ip.Split('.');
int result = 0;
foreach (var p in parts)
result = result * 0x100 + int.Parse(p);
}
class BanItem { public int baseaddres; public int mask; }
BanItem ip2banItem(string ip)
{
BanItem bi = new BanItem() { baseaddres = 0, mask = 0 };
var parts = ip.Split('.');
foreach (var p in parts)
{
bi.baseaddress *= 0x100;
bi.mask *= 0x100;
if (p != "*")
{
bi.mask += 0xff;
bi.baseaddress += int.Parse(p);
}
}
return bi;
}
banList = banIps.Select(ip2banItem).ToList();
I think you should keep a separate list for IP with * and those without asterick.
say IpList1 contains IP's without *
and
IpList2 --those contain * ..actually what we will be storing is the part before .* in this list. for e.g. 202.121.189.* would be stored as 202.121.189 only..
Thus for a given IP addrerss you just need to check for that IP address in IpList1,if it is not found over there then
for each Ip in IPList 2 you need to check whether it is a substring of input IP or not.
Thus no requirement of complex for and if loops.
Written In Java (Untested):
static boolean IsInRange(String ip, Vector<String> ipList) {
int indexOfStar = 0;
for (int i=0; i<ipList.size(); i++) {
if (ipList.contains("*")) {
indexOfStar = ipList.indexOf("*");
if ((ip.substring(0, indexOfStar)).equals(ipList.get(i).substring(0, indexOfStar))) {
return true;
}
}
}
return false;
}
I would use a space filling curve like in the xkcd comic: http://xkcd.com/195/. It's the function H(x,y) = (H(x),H(y)) and it reduces the 2 dimension to 1 dimension. It would also show that you are a real b*** coder.
I must display very small values (capacitor) in a Flex-AdvancedDataGrid
I use spark.formatters.NumberFormatter.
If I use 3, 6 or 9 for fractionalDigits, everything is fine.
But if I use 12, because I need 12 digits after decimal separator, then the value is cut after 9 digits!
Is there a way to get more then 9 digits after separator.
Or is there a way to use a formatting like "4.7 E-12" (Must be E-9, E-12, E-15 and so on)
toPrecision and toFixed works fine up to 20 digits. Thats enough.
I will write a function on this base to get results like 4.7 E-12.
Thanks for the help
Jan
I created a custom pattern formatter class that allows you to specify how you want a string/number formatted using whatever symbols or structure you want. Depending on your requirements, this may help you. Feel free to modify this class as needed.
CustomPatternFormatter.as
package
{
import mx.formatters.Formatter;
public class CustomPatternFormatter extends Formatter
{
private static const VALID_PATTERN_CHARACTERS:String = "#,.-";
// Use # as a placeholder for a regular
// character in the input string.
// Then add other characters between the
// # symbol for the desired output format.
// ex. The pattern ##,##,##.## with input 12345678
// will output 12,34,56.78
public var formatPattern:String;
// If True, the input string must match the number
// of # characters in the formatPattern.
public var inputMustMatchPatternLength:Boolean;
//Constructor
public function CustomPatternFormatter()
{
super();
formatPattern = "";
inputMustMatchPatternLength = false;
}
// Override format().
override public function format(value:Object):String
{
// Reset error if it exists.
if (error)
error = null;
// If value is null, or empty String just return ""
// but treat it as an error for consistency.
// Users will ignore it anyway.
if (!value || (value is String && value == ""))
{
error = "Cannot convert an empty value";
return "";
}
// Check to see if the input value must match the format pattern
if (inputMustMatchPatternLength && String(value).length != countOccurrences(formatPattern, "#"))
{
error = "The input value length does not match the format pattern length.";
return "";
}
// If the value is valid, format the string.
var fStrLen:int = 0;
var letter:String;
var n:int;
var i:int;
var v:int;
// Make sure the formatString is valid.
n = formatPattern.length;
for (i = 0; i < n; i++)
{
letter = formatPattern.charAt(i);
if (letter == "#")
{
fStrLen++;
}
else if (VALID_PATTERN_CHARACTERS.indexOf(letter) == -1)
{
error = "You can only use the following symbols in the formatPattern: " + VALID_PATTERN_CHARACTERS;
return "";
}
}
var returnString:String = "";
var vStr:String = String(value).replace(".", "").split("").reverse().join("");
var fArr:Array = formatPattern.split("").reverse();
var fChar:String;
// Format the string
for (v = 0; v < vStr.length; v++) { if (fArr.length > 0)
{
do
{
fChar = fArr.shift();
if (fChar != "#")
returnString += fChar;
} while (fChar != "#" && fArr.length > 0);
}
returnString += vStr.charAt(v);
}
// Return the formatted string
return returnString.split("").reverse().join("");
}
protected function countOccurrences(str:String, char:String):int
{
var count:int = 0;
for (var i:int=0; i < str.length; i++)
{
if (str.charAt(i) == char)
{
count++;
}
}
return count;
}
}
}