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  • Java: Efficient Equivalent to Removing while Iterating a Collection

    - by Claudiu
    Hello everyone. We all know you can't do this: for (Object i : l) if (condition(i)) l.remove(i); ConcurrentModificationException etc... this apparently works sometimes, but not always. Here's some specific code: public static void main(String[] args) { 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)); } for (Integer i : l) { if (i.intValue() == 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? "Best" here means most time and space efficient (I realize you can't always have both!) I'm also using an arbitrary Collection here, not necessarily an ArrayList, so you can't rely on get.

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  • What's the difference between => , ()=>, and Unit=>

    - by Malvolio
    I'm trying to represent a function that takes no arguments and returns no value (I'm simulating the setTimeout function in JavaScript, if you must know.) case class Scheduled(time : Int, callback : => Unit) doesn't compile, saying " `val' parameters may not be call-by-name" case class Scheduled(time : Int, callback : () => Unit) compiles, but has to be invoked strangely, instead of Scheduled(40, { println("x") } ) I have to do this Scheduled(40, { () => println("x") } ) What also works is class Scheduled(time : Int, callback : Unit => Unit) but is invoked in an even-less-sensible way Scheduled(40, { x : Unit => println("x") } ) (What would a variable of type Unit be?) What I want of course is a constructor that can be invoke the way I would invoke it if it were an ordinary function: Scheduled(40, println("x") ) Give baby his bottle!

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  • Bind to a collection's view and just call ToString()

    - by womp
    I'm binding a GridView to a collection of objects that look like this: public class Transaction { public string PersonName { get; set; } public DateTime TransactionDate { get; set; } public MoneyCollection TransactedMoney { get; set;} } MoneyCollection simply inherits from ObservableCollection<T>, and is a collection of MyMoney type object. In my GridView, I just want to bind a column to the MoneyCollection's ToString() method. However, binding it directly to the TransactedMoney property makes every entry display the text "(Collection)", and the ToString() method is never called. I understand that it is binding to the collection's default view. So my question is - how can I make it bind to the collection in such a way that it calls the ToString() method on it? This is my first WPF project, so I know this might be a bit noobish, but pointers would be very welcome.

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  • is this possible: c# collection of Type with constrains, or collection of generic type?

    - by Jon
    I'm trying to store types in a collection, so that i can later instantiate objects of the types in the collection. But I'm not sure how to do this the best way. What i have so far: List<Type> list = new List<Type>(); list.Add(typeof(MyClass)); var obj = (MyClass)Activator.CreateInstance(list[0]); I would like to have some constrains on the Type, or better yet, just a generic type in the collection instead of an instantiated Type object. Is this possible?

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  • .NET Garbage Collection behavior (with DataTable)

    - by gmac
    I am wonder why after creating a very simple DataTable and then setting it to null why Garbage Collection does not clear out all the memory used by that DataTable. Here is an example. The variable Before should be equal to Removed but it is not. { long Before = 0, After = 0, Removed = 0, Collected = 0; Before = GC.GetTotalMemory(true); DataTable dt = GetSomeDataTableFromSql(); After = GC.GetTotalMemory(true); dt = null; Removed = GC.GetTotalMemory(true); GC.Collect(); Collected = GC.GetTotalMemory(true); } Gives the following results. Before = 388116 After = 731248 Removed = 530176 Collected = 530176

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  • Implicit conversion causes stack overflow

    - by user44242
    The following code snippet worked perfectly, then after some code changes in different files, I've started getting stack overflows resulting from recursive invocation of the implicit conversion. Has this ever happened to anyone, and if so what's the fix. implicit def comparable2ordered[A <: Comparable[_]](x: A): Ordered[A] = new Ordered[A] with Proxy { val self = x def compare(y: A): Int = { self.compareTo(y) } }

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  • Functional equivalent of if (p(f(a), f(b)) a else b

    - by oxbow_lakes
    I'm guessing that there must be a better functional way of expressing the following: def foo(i: Any) : Int if (foo(a) < foo(b)) a else b So in this example f == foo and p == _ < _. There's bound to be some masterful cleverness in scalaz for this! I can see that using BooleanW I can write: p(f(a), f(b)).option(a).getOrElse(b) But I was sure that I would be able to write some code which only referred to a and b once. If this exists it must be on some combination of Function1W and something else but scalaz is a bit of a mystery to me! EDIT: I guess what I'm asking here is not "how do I write this?" but "What is the correct name and signature for such a function and does it have anything to do with FP stuff I do not yet understand like Kleisli, Comonad etc?"

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  • What is the time complexity of LinkedList.getLast() in Java?

    - by i.
    I have a private LinkedList in a Java class & will frequently need to retrieve the last element in the list. The lists need to scale, so I'm trying to decide whether I need to keep a reference to the last element when I make changes (to achieve O(1)) or if the LinkedList class does that already with the getLast() call. What is the big-O cost of LinkedList.getLast() and is it documented? (i.e. can I rely on this answer or should I make no assumptions & cache it even if it's O(1)?)

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  • for (Object object : list) [java] construction

    - by EugeneP
    My question, is, whether the sequence of elements picked from a list will always be the same, is this construction behaviour is deterministic for java "List"s - descendants of java.util.List 2) question, if I use for(Object o: list) construction and inside the loop's body increment a variable, will it be the index of list's elements? So, how it goes through list's elements, from 0 to size()-1 or chaotically? List.get(i) will always return this element? 3) question ( I suppose for the 2-nd question the answer will be negative, so:) for (int i=0; i < list.size(); i++) { } is the best way if I need to save the index of an element and later get it back from a list by its id?

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  • object reference set in java

    - by landon9720
    I need to create a Set of objects. The concern is I do not want to base the hashing or the equality on the objects' hashCode and equals implementation. Instead, I want the hash code and equality to be based only on each object's reference identity (i.e.: the value of the reference pointer). I'm not sure how to do this in Java. The reasoning behind this is my objects do not reliably implement equals or hashCode, and in this case reference identity is good enough.

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  • Using Option all over the place feels a bit awkward. Am I doing something wrong?

    - by Geo
    As a result of articles I read about the Option class which helps you avoid NullPointerException's, I started to use it all over the place. Imagine something like this: var file:Option[File] = None and later when I use it: val actualFile = file.getOrElse(new File("nonexisting")) if(actualFile.getName.equals("nonexisting")) { // instead of null checking } else { // value of file was good } Doing stuff like this doesn't feel all that "right" to me. I also noticed that .get has become deprecated. . Is this sort of stuff what you guys are doing with Option's too, or am I going the wrong way?

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  • C# Generic Collection help

    - by dcmovva
    I have a requirement to implement some kind of dictionary object. Something like MyDict<K,V1, V2). For example if I have a Question as Key(k) then Correct answer is V1 . V2 is user selected answer. Is there a collection that would satisfy this requirement in C#. If I have to design my own type, what interfaces should I implement. ICollection and Ilist ?

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  • List Question in Java

    - by TiNS
    Hello All, I have a following ArrayList, [Title,Data1,Data2,Data3] [A,2,3,4] [B,3,5,7] And I would like to convert this one like this, [Title,A,B] [Data1,2,3] [Data2,3,5] [Data3,4,7] I'm bit confused with the approach. Any hint would be much appreciated. Thanks.

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