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  • How to implement a list fold in Java

    - by Peter Kofler
    I have a List and want to reduce it to a single value (functional programing term "fold", Ruby term "inject"), like Arrays.asList("a", "b", "c") ... fold ... "a,b,c" As I am infected with functional programing ideas (Scala), I am looking for an easier/shorter way to code it than sb = new StringBuilder for ... { append ... } sb.toString

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  • Accessing a struct collection property from within another collection

    - by paddyb
    I have a struct that I need to store in a collection. The struct has a property that returns a Dictionary. public struct Item { private IDictionary<string, string> values; public IDictionary<string, string> Values { get { return this.values ?? (this.values = new Dictionary<string, string>()); } } } public class ItemCollection : Collection<Item> {} When testing I've found that if I add the item to the collection and then try to access the dictionary the structs values property is never updated. var collection = new ItemCollection { new Item() }; // pre-loaded with an item collection[0].Values.Add("myKey", "myValue"); Trace.WriteLine(collection[0].Values["myKey"]); // KeyNotFoundException here However if I load up the item first and then add it to a collection the values field is maintained. var collection = new ItemCollection(); var item = new Item(); item.Values.Add("myKey", "myValue"); collection.Add(item); Trace.WriteLine(collection[0].Values["myKey"]); // ok I've already decided that a struct is the wrong option for this type, and when using a class the issue doesn't occur, but I'm curious what's different between the two methods. Can anybody explain what's happening?

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  • How to detect if collection contain instance of specific type?

    - by KentZhou
    Suppose I create collection like Collection<IMyType> coll; Then I have many implelentations of IMyTypem like, T1, T2, T3... Then I want know if the collection coll contains a instance of type T1. So I want to write a method like public bool ContainType( <T>){...} here the param should be class type, not class instance. How to write code for this kind of issue?

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  • Hibernate inserting into join table

    - by Karl
    I got several entities. Two of them got a many-to-many relation. When I do a bigger operation on these entities it fails with this exception: org.hibernate.exception.ConstraintViolationException: could not insert collection rows: I execute the operation i a @Transactional context. I don't do any explicit flushing i my daos. The flush is triggered by a query. In the queue are 15 elements (all of the same structure). one of them always fails (but it's always a different one (I checked) and always at a different position). Does anybody have a hint for me for what I might do wrong? My Mapping: @ManyToMany(targetEntity = CategoryImpl.class) protected Set<Category> categories = new HashSet<Category>();

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  • Java 7 API design best practice - return Array or return Collection

    - by Shengjie
    I know this question has be asked before generic comes out. Array does win out a bit given Array enforces the return type, it's more type-safe. But now, with latest JDK 7, every time when I design this type of APIs: public String[] getElements(String type) vs public List<String> getElements(String type) I am always struggling to think of some good reasons to return A Collection over An Array or another way around. What's the best practice when it comes to the case of choosing String[] or List as the API's return type? Or it's courses for horses. I don't have a special case in my mind, I am more looking for a generic pros/cons comparison.

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  • How to delete duplicate/aggregate rows faster in a file using Java (no DB)

    - by S. Singh
    I have a 2GB big text file, it has 5 columns delimited by tab. A row will be called duplicate only if 4 out of 5 columns matches. Right now, I am doing dduping by first loading each coloumn in separate List , then iterating through lists, deleting the duplicate rows as it encountered and aggregating. The problem: it is taking more than 20 hours to process one file. I have 25 such files to process. Can anyone please share their experience, how they would go about doing such dduping? This dduping will be a throw away code. So, I was looking for some quick/dirty solution, to get job done as soon as possible. Here is my pseudo code (roughly) Iterate over the rows i=current_row_no. Iterate over the row no. i+1 to last_row if(col1 matches //find duplicate && col2 matches && col3 matches && col4 matches) { col5List.set(i,get col5); //aggregate } Duplicate example A and B will be duplicate A=(1,1,1,1,1), B=(1,1,1,1,2), C=(2,1,1,1,1) and output would be A=(1,1,1,1,1+2) C=(2,1,1,1,1) [notice that B has been kicked out]

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  • NHibernate : Root collection with an root object

    - by Daniel
    Hi, I want to track a list of root objects which are not contained by any element. I want the following pseudo code to work: IList<FavoriteItem> list = session.Linq<FavoriteItem>().ToList(); list.Add(item1); list.Add(item2); list.Remove(item3); list.Remove(item4); var item5 = list.First(i => i.Name = "Foo"); item5.Name = "Bar"; session.Save(list); This should automatically insert item1 and item2, delete item3 and item3 and update item5 (i.e. I don't want to call sesssion.SaveOrUpdate() for all items separately. Is it possible to define a pseudo entity that is not associated with a table? For example I want to define the class Favorites and map 2 collection properties of it and than I want to write code like this: var favs = session.Linq<Favorites>(); favs.FavoriteColors.Add(new FavoriteColor(...)); favs.FavoriteMovies.Add(new FavoriteMovie(...)); session.SaveOrUpdate(favs); FavoriteColors and FavoriteMovies are the only properties of the Favorites class and are of type IList and IList. I do only want to persist the these two collection properties but not the Favorites class. Any help is much appreciated.

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  • Inheritance of TCollectionItem

    - by JamesB
    I'm planning to have collection of items stored in a TCollection. Each item will derive from TBaseItem which in turn derives from TCollectionItem, With this in mind the Collection will return TBaseItem when an item is requested. Now each TBaseItem will have a Calculate function, in the the TBaseItem this will just return an internal variable, but in each of the derivations of TBaseItem the Calculate function requires a different set of parameters. The Collection will have a Calculate All function which iterates through the collection items and calls each Calculate function, obviously it would need to pass the correct parameters to each function I can think of three ways of doing this: Create a virtual/abstract method for each calculate function in the base class and override it in the derrived class, This would mean no type casting was required when using the object but it would also mean I have to create lots of virtual methods and have a large if...else statement detecting the type and calling the correct "calculate" method, it also means that calling the calculate method is prone to error as you would have to know when writing the code which one to call for which type with the correct parameters to avoid an Error/EAbstractError. Create a record structure with all the possible parameters in and use this as the parameter for the "calculate" function. This has the added benefit of passing this to the "calculate all" function as it can contain all the parameters required and avoid a potentially very long parameter list. Just type casting the TBaseItem to access the correct calculate method. This would tidy up the TBaseItem quite alot compared to the first method. What would be the best way to handle this collection?

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  • how to selectively filter items in a collection

    - by Samuel
    I use the following snippet to filter the list of selected users, where isSelected is a boolean variable. Is there a simpler way (helper function) to populate the selectedUsers collection instead of writing the following lines of code. List<User> selectedUsers = new ArrayList<User>(0); for (User user : this.getUsers()) { if (user.isSelected()) { selectedUsers.add(user.getId()); } }

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  • Is HashMap in Java collision safe

    - by changed
    Hi I am developing a parser that needs to put key value pairs in hashmap. But a key can have multiple values which i can do in this way HashMap<String,ArrayList<String>> . But what happens if number of keys are very large and it start matching with other key's hashcode. Will that rewrite previous key's value ? thanks -devSunday

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  • Are upper bounds of indexed ranges always assumed to be exclusive?

    - by polygenelubricants
    So in Java, whenever an indexed range is given, the upper bound is almost always exclusive. From java.lang.String: substring(int beginIndex, int endIndex) Returns a new string that is a substring of this string. The substring begins at the specified beginIndex and extends to the character at index endIndex - 1 From java.util.Arrays: copyOfRange(T[] original, int from, int to) from - the initial index of the range to be copied, inclusive to - the final index of the range to be copied, exclusive. From java.util.BitSet: set(int fromIndex, int toIndex) fromIndex - index of the first bit to be set. toIndex - index after the last bit to be set. As you can see, it does look like Java tries to make it a consistent convention that upper bounds are exclusive. My questions are: Is this the official authoritative recommendation? Are there notable violations that we should be wary of? Is there a name for this system? (ala "0-based" vs "1-based")

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  • Use LINQ, to Sort and Filter items in a List<ReturnItem> collection, based on the values within a Li

    - by Daniel McPherson
    This is tricky to explain. We have a DataTable that contains a user configurable selection of columns, which are not known at compile time. Every column in the DataTable is of type String. We need to convert this DataTable into a strongly typed Collection of "ReturnItem" objects so that we can then sort and filter using LINQ for use in our application. We have made some progress as follows: We started with the basic DataTable. We then process the DataTable, creating a new "ReturnItem" object for each row This "ReturnItem" object has just two properties: ID ( string ) and Columns( List(object) ). The properties collection contains one entry for each column, representing a single DataRow. Each property is made Strongly Typed (int, string, datetime, etc). For example it would add a new "DateTime" object to the "ReturnItem" Columns List containing the value of the "Created" Datatable Column. The result is a List(ReturnItem) that we would then like to be able to Sort and Filter using LINQ based on the value in one of the properties, for example, sort on "Created" date. We have been using the LINQ Dynamic Query Library, which gets us so far, but it doesn't look like the way forward because we are using it over a List Collection of objects. Basically, my question boils down to: How can I use LINQ, to Sort and Filter items in a List(ReturnItem) collection, based on the values within a List(object) property which is part of the ReturnItem class?

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  • C# type safe and developer friendly list/collection technique

    - by Agile Noob
    I am populating a "Dictionary" with the results of an sp call. The key is the field name and the value is whatever value the sp returns for the field. This is all well and good but I'd like developers to have a predefined list of keys to access this list, for safety and documentation reasons. What I'd like to do is have something like an enum as a key for the dictionary so developers can safely access the list, but still have the ability to access the dictionary with a string key value. I am hoping to have a list of string values that I can access with an enum key AND a string key. Please make sure any suggestions are simple to implement, this is not the kind of thing I'm willing to build a lot of overhead to implement.

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  • What is the most popular generic collection data structure library for C?

    - by Tom Dalling
    I'm looking for a C library that provides generic collection data structures such as lists, associative arrays, sets, etc. The library should be stable and well tested. I'm basically looking for something better than the crappy C standard library. What C libraries fit this description? EDIT: I'd prefer that the library was cross-platform, but failing that, anything that works on Mac/Linux.

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  • Custom ASP.net UserControl List<T> Property, having trouble setting declaratively

    - by Chris McCall
    I'm developing a custom UserControl to inject JQuery hotkeys into a page declaratively on the server side. Here's the control (the important parts anyway): [AspNetHostingPermission(SecurityAction.Demand, Level = AspNetHostingPermissionLevel.Minimal), AspNetHostingPermission(SecurityAction.InheritanceDemand, Level = AspNetHostingPermissionLevel.Minimal), DefaultProperty("HotKeys"), ParseChildren(true, "HotKeys"), ToolboxData("<{0}:HotKeysControl runat=\"server\"> </{0}:HotKeysControl>")] public partial class HotKeysControl : System.Web.UI.WebControls.WebControl { private string crlf = Environment.NewLine; public List<HotKey> _HotKeys; public HotKeysControl() { if (_HotKeys == null) { _HotKeys = new List<HotKey>(); } // if I uncomment this line, script is injected into the page // _HotKeys.Add(new HotKey("ctrl+r","thisControl")); } [ Category("Behavior"), Description("The hotkeys collection"), DesignerSerializationVisibility( DesignerSerializationVisibility.Content), Editor(typeof(HotKeyCollectionEditor), typeof(UITypeEditor)), PersistenceMode(PersistenceMode.InnerDefaultProperty) ] public List<HotKey> HotKeys { set { _HotKeys = value; } get { return _HotKeys; } } Here's the .aspx code: <%@ Register Assembly="MyCompany.ProductName.WebControls" Namespace="MyCompany.ProductName.WebControls" TagPrefix="uc" %> ... <uc:HotKeysControl ID="theHotkeys" runat="server" Visible="false"> <uc:HotKey ControlName="firstControl" KeyCode="ctrl+1" /> <uc:HotKey ControlName="thirdControl" KeyCode="ctrl+2" /> </uc:HotKeysControl> Nothing happens, as if no HotKeys objects are being added to the property collection. What Am I doing wrong? If I uncomment out the line above and "manually" add items, it works. It's something about how I'm declaratively adding hotkeys to the page. Any ideas?

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  • Why doesn't Java Map extends Collection?

    - by polygenelubricants
    I was surprised by the fact that Map<?,?> is not a Collection<?>. I thought it'd make a LOT of sense if it was declared as such: public interface Map<K,V> extends Collection<Map.Entry<K,V>> After all, a Map<K,V> is a collection of Map.Entry<K,V>, isn't it? So is there a good reason why it's not implemented as such?

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

    - 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. Note that I do not want to bind to the items in MoneyCollection, I want to bind directly to the property itself and just call ToString() on it. 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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  • Collection type generated by for with yield

    - by Jesper
    When I evaluate a for in Scala, I get an immutable IndexedSeq (a collection with array-like performance characteristics, such as efficient random access): scala> val s = for (i <- 0 to 9) yield math.random + i s: scala.collection.immutable.IndexedSeq[Double] = Vector(0.6127056766832756, 1.7137598183155291, ... Does a for with a yield always return an IndexedSeq, or can it also return some other type of collection class (a LinearSeq, for example)? If it can also return something else, then what determines the return type, and how can I influence it? I'm using Scala 2.8.0.RC3.

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  • Connection and Collection Interfaces in Java

    - by Bhupi
    Which class implements all the Connection Interfaces which are in javax.microedition.io package and how? And in the same way which class implements the some of Collection interfaces like Iterator interface. I saw a code: - Iterator it; ArrayList list = new ArrayList(); it = list.iterator(); The iterator() return type is "Iterator" which is an interface. Please tell me what this code is doing is it returning an object of type Iterator? but as far as I know, interface can't be initialized.

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  • How to create collection object in vbscript?

    - by Onnesh
    what should be the parameter for create object the following code dim a set a=CreateObject("Collection") //getting a runtime error saying ActiveX //component can't create object: 'Collection a.add(CreateObject("Collection")) a.Items(0).Add(1) MsgBox(a.Items(0).count) MsgBox(a.Items(0).Item(0))

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