As these questions point out, Guid.NewGuid will return the same value for all rows due to the enforced deterministic nature of U-SQL i.e if it's scaled out if an element (vertex) needs retrying then it should return the same value....
Guid.NewGuid() always return same Guid for all rows
auto_increment in U-SQL
However.... the code example in the officials documentation for a User Defined Extractor purposefully uses Guid.NewGuid().
I'm not querying the validity of the answers for the questions above, as they are from an authoritative source (the programme manager for u-sql, so very authoritative!). However, what I'm wondering if the action of using an Extractor means NewGuid can be used as normal? Is it simply within c# expressions in u-sql and User Defined Functions in which NewGuid is unsafe?
[SqlUserDefinedExtractor(AtomicFileProcessing = true)]
public class FullDescriptionExtractor : IExtractor
{
private Encoding _encoding;
private byte[] _row_delim;
private char _col_delim;
public FullDescriptionExtractor(Encoding encoding, string row_delim = "\r\n", char col_delim = '\t')
{
this._encoding = ((encoding == null) ? Encoding.UTF8 : encoding);
this._row_delim = this._encoding.GetBytes(row_delim);
this._col_delim = col_delim;
}
public override IEnumerable<IRow> Extract(IUnstructuredReader input, IUpdatableRow output)
{
string line;
//Read the input line by line
foreach (Stream current in input.Split(_encoding.GetBytes("\r\n")))
{
using (System.IO.StreamReader streamReader = new StreamReader(current, this._encoding))
{
line = streamReader.ReadToEnd().Trim();
//Split the input by the column delimiter
string[] parts = line.Split(this._col_delim);
int count = 0; // start with first column
foreach (string part in parts)
{
if (count == 0)
{ // for column “guid”, re-generated guid
Guid new_guid = Guid.NewGuid();
output.Set<Guid>(count, new_guid);
}
else if (count == 2)
{
// for column “user”, convert to UPPER case
output.Set<string>(count, part.ToUpper());
}
else
{
// keep the rest of the columns as-is
output.Set<string>(count, part);
}
count += 1;
}
}
yield return output.AsReadOnly();
}
yield break;
}
}
https://learn.microsoft.com/en-us/azure/data-lake-analytics/data-lake-analytics-u-sql-programmability-guide#use-user-defined-extractors
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.
heres my code below...
TableColumn tc = new TableColumn();
TableColumn[] tc2 = new TableColumn[10];
for(int i=0; i<5, i++){
tc.getColumns().add(tc2[i]);
}
and i try to override commit method for editing cells.
public void commit(Object val) {
// Get the table
TableView<MainTable> t = this.getTableView();
// Get the selected row/column
MainTable selectedRow = t.getItems().get(this.getTableRow().getIndex());
TableColumn<MainTable, ?> selectedColumn = t.getColumns().get(t.getColumns().indexOf(this.getTableColumn()));
// Get current property name
String propertyName = ((PropertyValueFactory) selectedColumn.getCellValueFactory()).getProperty();
// Create a method name conforming to java standards ( setProperty )
propertyName = ("" + propertyName.charAt(0)).toUpperCase() + propertyName.substring(1);
// Try to run the update
try {
// Type specific checks - could be done inside each setProperty() method
if(val instanceof Double) {
Method method = selectedRow.getClass().getMethod("set" + propertyName, double.class);
method.invoke(selectedRow, (double) val);
}
if(val instanceof String) {
Method method = selectedRow.getClass().getMethod("set" + propertyName, String.class);
method.invoke(selectedRow, (String) val);
}
if(val instanceof Integer) {
Method method = selectedRow.getClass().getMethod("set" + propertyName, int.class);
method.invoke(selectedRow, (int) val);
}
} catch (Exception e) {
e.printStackTrace();
}
// CommitEdit for good luck
commitEdit((String) val);
}
and i got ArrayIndexOutofBoundsException on console view.
so my question is
how can i select getcolumns added other column???
TableColumn<MainTable, ?> selectedColumn = t.getColumns().get(t.getColumns().indexOf(this.getTableColumn()));
i think this code has to be changed...
anyone got ideas??
Nested columns are not part of the TableView.columns list.
If you need the corresponding TableView column, just go up through the hierarchy until you reach a column without a parentColumn:
TableColumn<MainTable, ?> selectedColumn = this.getTableColumn();
TableColumn<MainTable, ?> c = selectedColumn;
while ((c = selectedColumn.getParentColumn()) != null) {
selectedColumn = c;
}
If you just need the column itself, simply use this.getTableColumn(), instead of finding the index of the column in the columns list and then accessing that index in the same list. (I guess the latter is what you need.)
Furthermore, if PropertyValueFactory returns properties of the item class, you could use this property to set the value instead of using reflection:
ObservableValue obs = selectedColumn.getCellObservableValue(this.getIndex());
if (obs instanceof WritableValue) {
((WritableValue) obs).setValue(val);
} else {
// reflecitive approach
}
Furthermore you shouldn't add null as a nested column, but you're doing it here:
TableColumn[] tc2 = new TableColumn[10];
for(int i=0; i<5, i++){
tc.getColumns().add(tc2[i]);
}
I have a piece of code that opens a HTTP connection to read the data contained on a webpage.
HttpConnection h = new HttpConnection();
InputStream input = h.openInputStream();
int len = (int) h.httpConn.getLength();
StringBuffer raw = new StringBuffer();
if(len > 0)
{
byte[] data = new byte[len];
while( -1 != (len = input.read(data)))
{
raw.append(new String(data, 0, len));
}
}
response = raw.toString();
input.close();
h.httpConn.close();
//System.out.println("Response -----> " + response);
return response;
This code works absolutely fine, but on certain punctuation marks its not reading properly. For example >> ' << an apostrophe comes out as >> â <<.
I'm guessing it might be something to do with encoding but I have tried UTF-8, UTF-16 and ASCII and this didn't fix the problem.
I've had this work for me when I was getting odd characters in place of punctuation:
public String getContents(InputStream stream) {
String contents = null;
try{
contents = new String(IOUtilities.streamToBytes(stream), "UTF-8");
}
catch(Exception e) {
//encoding error
}
return contents;
}
Instead of this:
raw.append(new String(data, 0, len));
you can use
raw.append(new String(data, 0, len, "UTF-8"));
passing in the name of a character enconding, in this case UTF-8.
I am beginner in JavaME. I'd like to make simple dicitionary. The source data is placed on "data.txt" file in "res" directory. The structure is like this:
#apple=kind of fruit;
#spinach=kind of vegetable;
The flow is so simple. User enters word that he want to search in a text field, e.g "apple", system take the user input, read the "data.txt", search the matched word in it, take corresponding word, and display it to another textfield/textbox.
I've managed to read whole "data.txt" using this code..
private String readDataText() {
InputStream is = getClass().getResourceAsStream("data.txt");
try {
StringBuffer sb = new StringBuffer();
int chr, i=0;
while ((chr = is.read()) != -1)
sb.append((char) chr);
return sb.toString();
}
catch (Exception e) {
}
return null;
}
but I still dont know how to split it, find the matched word with the user input and take corresponding word. Hope somebody willing to share his/her knowledge to help me..
Basically you want something like this:
private String readDataText() {
InputStream is = getClass().getResourceAsStream("data.txt");
BufferedReader br = new BufferedReader(new InputStreamReader(is));
String line;
try {
while ((line = br.readLine()) != null)
{
String[] split = line.split("=");
if (split[0].equals("#someFruit"))
return split[1];
}
}
catch (Exception e) {}
return null;
}
Read the line using a BufferedReader, no need to handle single chars.
Split the line by the = token
Check if the key in the dictionary is the wanted key, if so, return the value.