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• Wildcards vs. generic methods

  - by FredOverflow

  Is there any practical difference between the following approaches to print all elements in a range? public static void printA(Iterable<?> range) { for (Object o : range) { System.out.println(o); } } public static <T> void printB(Iterable<T> range) { for (T x : range) { System.out.println(x); } } Apparently, printB involves an additional checked cast to Object (see line 16), which seems rather stupid to me -- isn't everything an Object anyway? public static void printA(java.lang.Iterable); Code: 0: aload_0 1: invokeinterface #18, 1; //InterfaceMethod java/lang/Iterable.iterator:()Ljava/util/Iterator; 6: astore_2 7: goto 24 10: aload_2 11: invokeinterface #24, 1; //InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object; 16: astore_1 17: getstatic #30; //Field java/lang/System.out:Ljava/io/PrintStream; 20: aload_1 21: invokevirtual #36; //Method java/io/PrintStream.println:(Ljava/lang/Object;)V 24: aload_2 25: invokeinterface #42, 1; //InterfaceMethod java/util/Iterator.hasNext:()Z 30: ifne 10 33: return public static void printB(java.lang.Iterable); Code: 0: aload_0 1: invokeinterface #18, 1; //InterfaceMethod java/lang/Iterable.iterator:()Ljava/util/Iterator; 6: astore_2 7: goto 27 10: aload_2 11: invokeinterface #24, 1; //InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object; 16: checkcast #3; //class java/lang/Object 19: astore_1 20: getstatic #30; //Field java/lang/System.out:Ljava/io/PrintStream; 23: aload_1 24: invokevirtual #36; //Method java/io/PrintStream.println:(Ljava/lang/Object;)V 27: aload_2 28: invokeinterface #42, 1; //InterfaceMethod java/util/Iterator.hasNext:()Z 33: ifne 10 36: return

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• ClassCastException in iterating list returned by Query using Hibernate Query Language

  - by Tushar Paliwal

  I'm beginner in hibernate.I'm trying a simplest example using HQL but it generates exception at line 25 ClassCastException when i try to iterate list.When i try to cast the object returned by next() methode of iterator it generates the same problem.I could not identify the problem.Kindly give me solution of the problem. Employee.java package one; import javax.persistence.Entity; import javax.persistence.Id; @Entity public class Employee { @Id private Long id; private String name; public Long getId() { return id; } public void setId(Long id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Employee(Long id, String name) { super(); this.id = id; this.name = name; } public Employee() { } } Main2.java package one; import java.util.Iterator; import java.util.List; import org.hibernate.Query; import org.hibernate.Session; import org.hibernate.SessionFactory; import org.hibernate.Transaction; import org.hibernate.cfg.Configuration; public class Main2 { public static void main(String[] args) { SessionFactory sf=new Configuration().configure().buildSessionFactory(); Session s1=sf.openSession(); Query q=s1.createQuery("from Employee "); Transaction tx=s1.beginTransaction(); List l=q.list(); Iterator itr=l.iterator(); while(itr.hasNext()) { Object obj[]=(Object[])itr.next();//Line 25 for(Object temp:obj) { System.out.println(temp); } } tx.commit(); s1.close(); sf.close(); } }

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• Type classe, generic memoization

  - by nicolas

  Something quite odd is happening with y types and I quite dont understand if this is justified or not. I would tend to think not. This code works fine : type DictionarySingleton private () = static let mutable instance = Dictionary<string*obj, obj>() static member Instance = instance let memoize (f:'a -> 'b) = fun (x:'a) -> let key = f.ToString(), (x :> obj) if (DictionarySingleton.Instance).ContainsKey(key) then let r = (DictionarySingleton.Instance).[key] r :?> 'b else let res = f x (DictionarySingleton.Instance).[key] <- (res :> obj) res And this ones complains type DictionarySingleton private () = static let mutable instance = Dictionary<string*obj, _>() static member Instance = instance let memoize (f:'a -> 'b) = fun (x:'a) -> let key = f.ToString(), (x :> obj) if (DictionarySingleton.Instance).ContainsKey(key) then let r = (DictionarySingleton.Instance).[key] r :?> 'b else let res = f x (DictionarySingleton.Instance).[key] <- (res :> obj) res The difference is only the underscore in the dictionary definition. The infered types are the same, but the dynamic cast from r to type 'b exhibits an error. 'this runtime coercition ... runtime type tests are not allowed on some types, etc..' Am I missing something or is it a rough edge ?

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• Java downcasting dilemma

  - by Shades88

  please have a look at this code here. class Vehicle { public void printSound() { System.out.print("vehicle"); } } class Car extends Vehicle { public void printSound() { System.out.print("car"); } } class Bike extends Vehicle{ public void printSound() { System.out.print("bike"); } } public class Test { public static void main(String[] args) { Vehicle v = new Car(); Bike b = (Bike)v; v.printSound(); b.printSound(); Object myObj = new String[]{"one", "two", "three"}; for (String s : (String[])myObj) System.out.print(s + "."); } } Executing this code will give ClassCastException saying inheritance.Car cannot be cast to inheritance.Bike. Now look at the line Object myObj = new String[]{"one", "two", "three"};. This line is same as Vehicle v = new Car(); right? In both lines we are assigning sub class object to super class reference variable. But downcasting String[]myObj is allowed but (Bike)v is not. Please help me understand what is going on around here.

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• doesn't parse xml when it's a single node

  - by tag

  my script.php returns this XML <all> <item> <field1>value1</field1> <field2>value2</field2> </item> <item> <field1>value1</field1> <field2>value2</field2> </item> </all> The HTTPService uses the default resultFormat="object" but I don't declare it since it's the default. Then I bind it to a List dataProvider="{getDataHTTP.lastResult.all.item}" I get no problems when the number of item returned is more than 1. But when it's only 1 item I get an error cannot convert XMLList to mx.collections.IList. I tried different solutions including trying to cast it as XMLListCollection but it still gives an error for single items. Does anyone know of a way to possibly solve this?

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• Slope requires a real as parameter 2?

  - by Dave Jarvis

  Question How do you pass the correct value to udf_slope's second parameter type? Attempts CAST(Y.YEAR AS FLOAT), but that failed (SQL error). Y.YEAR + 0.0, but that failed, too (see error message). slope(D.AMOUNT, 1.0), failed as well Error Message Using udf_slope fails due to: Can't initialize function 'slope'; slope() requires a real as parameter 2 Code SELECT D.AMOUNT, Y.YEAR, slope(D.AMOUNT, Y.YEAR + 0.0) as SLOPE, intercept(D.AMOUNT, Y.YEAR + 0.0) as INTERCEPT FROM YEAR_REF Y, DAILY D Here, D.AMOUNT is a FLOAT and Y.YEAR is an INTEGER. Create Function The slope function was created as follows: CREATE AGGREGATE FUNCTION slope RETURNS REAL SONAME 'udf_slope.so'; Function Signature From udf_slope.cc: double slope( UDF_INIT* initid, UDF_ARGS* args, char* is_null, char* is_error ) Example Usages Reading the fine manual reveals: UDF intercept() Calculates the intercept of the linear regression of two sets of variables. Function name intercept Input parameter(s) 2 (dependent variable: REAL, independent variable: REAL) Examples SELECT intercept(income,age) FROM customers UDF slope() Calculates the slope of the linear regression of two sets of variables. Function name slope Input parameter(s) 2 (dependent variable: REAL, independent variable: REAL) Examples SELECT slope(income,age) FROM customers Thoughts? Thank you!

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• Upcasting without any added data fields.

  - by Benjamin Manns

  In my project I have a generic Packet class. I would like to be able to upcast to other classes (like LoginPacket or MovePacket). The base class contains a command and arguments (greatly simplified): public class Packet { public String Command; public String[] Arguments; } I would like to have be able to convert from Packet to LoginPacket (or any other) based on a check if Packet.Command == "LOGIN". The login packet would not contain any new data members, but only methods for accessing specific arguments. For example: public class LoginPacket : Packet { public String Username { get { return Arguments[0]; } set { Arguments[0] == value; } } public String Password { get { return Arguments[1]; } set { Arguments[1] == value; } } } It would be great if I could run a simple code that would cast from Packet to LoginPacket with something like LoginPacket _Login = (LoginPacket)_Packet;, but that throws a System.InvalidCastException. It seems like this would be an easy task, as no new data is included, but I can't figure out any other way than copying everything from the Packet class to a new LoginPacket class.

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• Native Endians and Auto Conversion

  - by KnickerKicker

  so the following converts big endians to little ones uint32_t ntoh32(uint32_t v) { return (v << 24) | ((v & 0x0000ff00) << 8) | ((v & 0x00ff0000) >> 8) | (v >> 24); } works. like a charm. I read 4 bytes from a big endian file into char v[4] and pass it into the above function as ntoh32 (* reinterpret_cast<uint32_t *> (v)) that doesn't work - because my compiler (VS 2005) automatically converts the big endian char[4] into a little endian uint32_t when I do the cast. AFAIK, this automatic conversion will not be portable, so I use uint32_t ntoh_4b(char v[]) { uint32_t a = 0; a |= (unsigned char)v[0]; a <<= 8; a |= (unsigned char)v[1]; a <<= 8; a |= (unsigned char)v[2]; a <<= 8; a |= (unsigned char)v[3]; return a; } yes the (unsigned char) is necessary. yes it is dog slow. there must be a better way. anyone ?

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• Using reflection to find all linq2sql tables and ensure they match the database

  - by Jake Stevenson

  I'm trying to use reflection to automatically test that all my linq2sql entities match the test database. I thought I'd do this by getting all the classes that inherit from DataContext from my assembly: var contexttypes = Assembly.GetAssembly(typeof (BaseRepository<,>)).GetTypes().Where( t => t.IsSubclassOf(typeof(DataContext))); foreach (var contexttype in contexttypes) { var context = Activator.CreateInstance(contexttype); var tableProperties = type.GetProperties().Where(t=> t.PropertyType.Name == typeof(ITable<>).Name); foreach (var propertyInfo in tableProperties) { var table = (propertyInfo.GetValue(context, null)); } } So far so good, this loops through each ITable< in each datacontext in the project. If I debug the code, "table" is properly instantiated, and if I expand the results view in the debugger I can see actual data. BUT, I can't figure out how to get my code to actually query that table. I'd really like to just be able to do table.FirstOrDefault() to get the top row out of each table and make sure the SQL fetch doesn't fail. But I cant cast that table to be anything I can query. Any suggestions on how I can make this queryable? Just the ability to call .Count() would be enough for me to ensure the entities don't have anything that doesn't match the table columns.

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• Serializable object in intent returning as String

  - by B_

  In my application, I am trying to pass a serializable object through an intent to another activity. The intent is not entirely created by me, it is created and passed through a search suggestion. In the content provider for the search suggestion, the object is created and placed in the SUGGEST_COLUMN_INTENT_EXTRA_DATA column of the MatrixCursor. However, when in the receiving activity I call getIntent().getSerializableExtra(SearchManager.EXTRA_DATA_KEY), the returned object is of type String and I cannot cast it into the original object class. I tried making a parcelable wrapper for my object that calls out.writeSerializable(...) and use that instead but the same thing happened. The string that is returned is like a generic Object toString(), i.e. com.foo.yak.MyAwesomeClass@4350058, so I'm assuming that toString() is being called somewhere where I have no control. Hopefully I'm just missing something simple. Thanks for the help! Edit: Some of my code This is in the content provider that acts as the search authority: //These are the search suggestion columns private static final String[] COLUMNS = { "_id", // mandatory column SearchManager.SUGGEST_COLUMN_TEXT_1, SearchManager.SUGGEST_COLUMN_INTENT_EXTRA_DATA }; //This places the serializable or parcelable object (and other info) into the search suggestion private Cursor getSuggestions(String query, String[] projection) { List<Widget> widgets = WidgetLoader.getMatches(query); MatrixCursor cursor = new MatrixCursor(COLUMNS); for (Widget w : widgets) { cursor.addRow(new Object[] { w.id w.name w.data //This is the MyAwesomeClass object I'm trying to pass }); } return cursor; } This is in the activity that receives the search suggestion: public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Object extra = getIntent().getSerializableExtra(SearchManager.EXTRA_DATA_KEY); //extra.getClass() returns String, when it should return MyAwesomeClass, so this next line throws a ClassCastException and causes a crash MyAwesomeClass mac = (MyAwesomeClass)extra; ... }

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• Can't get KnownType to work with WCF

  - by Kelly Cline

  I have an interface and a class defined in separate assemblies, like this: namespace DataInterfaces { public interface IPerson { string Name { get; set; } } } namespace DataObjects { [DataContract] [KnownType( typeof( IPerson ) ) ] public class Person : IPerson { [DataMember] public string Name { get; set; } } } This is my Service Interface: public interface ICalculator { [OperationContract] IPerson GetPerson ( ); } When I update my Service Reference for my Client, I get this in the Reference.cs: public object GetPerson() { return base.Channel.GetPerson(); I was hoping that KnownType would give me IPerson instead of "object" here. I have also tried [KnownType( typeof( Person ) ) ] with the same result. I have control of both client and server, so I have my DataObjects (where Person is defined) and DataInterfaces (where IPerson is defined) assemblies in both places. Is there something obvious I am missing? I thought KnownType was the answer to being able to use interfaces with WCF. ----- FURTHER INFORMATION ----- I removed the KnownType from the Person class and added [ServiceKnownType( typeof( Person ) ) ] to my service interface, as suggested by Richard. The client-side proxy still looks the same, public object GetPerson() { return base.Channel.GetPerson(); , but now it doesn't blow up. The client just has an "object", though, so it has to cast it to IPerson before it is useful. var person = client.GetPerson ( ); Console.WriteLine ( ( ( IPerson ) person ).Name );

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• Adding integer value to a list from XML file

  - by Pramodh

  I've an Xml file like <SampleFile> <Data> <Element Val="8" /> <Element Val="10" /> <Element Val="12" /> <Element Val="14" /> <Element Val="16" /> <Element Val="9" /> <Element Val="11" /> <Element Val="13" /> <Element Val="15" /> <Element Val="17" /> </Data> </SampleFile> i need to read the attribute value of" Val" and convert it to Int32 , then sort and then add to the list now i'm using like: List<Int32> lst = (XDocument.Load("\\Sample.xml").Descendants("Element").Select(l_Temp => l_Temp.Attribute("Val").Value.ToString()).Cast<Int32>().OrderBy(nTemp => nTemp)).ToList(); but its not working properly please give me a better solution

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• How do I check if a variable has been initialized

  - by Javier Parra

  First of all, I'm fairly new to Java, so sorry if this question is utterly simple. The thing is: I have a String[] s made by splitting a String in which every item is a number. I want to cast the items of s into a int[] n. s[0] contains the number of items that n will hold, effectively s.length-1. I'm trying to do this using a foreach loop: int[] n; for(String num: s){ //if(n is not initialized){ n = new int[(int) num]; continue; } n[n.length] = (int) num; } Now, I realize that I could use something like this: int[] n = new int[(int) s[0]]; for(int i=0; i < s.length; i++){ n[n.length] = (int) s[i]; } But I'm sure that I will be faced with that "if n is not initialized initialize it" problem in the future.

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• Help with Java Generics: Cannot use "Object" as argument for "? extends Object"

  - by AniDev

  Hello, I have the following code: import java.util.*; public class SellTransaction extends Transaction { private Map<String,? extends Object> origValueMap; public SellTransaction(Map<String,? extends Object> valueMap) { super(Transaction.Type.Sell); assignValues(valueMap); this.origValueMap=valueMap; } public SellTransaction[] splitTransaction(double splitAtQuantity) { Map<String,? extends Object> valueMapPart1=origValueMap; valueMapPart1.put(nameMappings[3],(Object)new Double(splitAtQuantity)); Map<String,? extends Object> valueMapPart2=origValueMap; valueMapPart2.put(nameMappings[3],((Double)origValueMap.get(nameMappings[3]))-splitAtQuantity); return new SellTransaction[] {new SellTransaction(valueMapPart1),new SellTransaction(valueMapPart2)}; } } The code fails to compile when I call valueMapPart1.put and valueMapPart2.put, with the error: The method put(String, capture#5-of ? extends Object) in the type Map is not applicable for the arguments (String, Object) I have read on the Internet about generics and wildcards and captures, but I still don't understand what is going wrong. My understanding is that the value of the Map's can be any class that extends Object, which I think might be redundant, because all classes extend Object. And I cannot change the generics to something like ? super Object, because the Map is supplied by some library. So why is this not compiling? Also, if I try to cast valueMap to Map<String,Object>, the compiler gives me that 'Unchecked conversion' warning. Thanks!

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• Linq to xml, retreaving generic interface-based list

  - by Rita

  I have an xml document that looks like this <Elements> <Element> <DisplayName /> <Type /> </Element> </Elements> I have an interface, interface IElement { string DisplayName {get;} } and a couple of derived classes: public class AElement: IElement public class BElement: IElement What I want to do, is to write the most efficient query to iterate through the xml and create a list of IElement, containing AElement or BElement, based on the 'Type' property in the xml. So far I have this: IEnumerable<AElement> elements = from xmlElement in XElement.Load(path).Elements("Element") where xmlElement.Element("type").Value == "AElement" select new AElement(xmlElement.Element("DisplayName").Value); return elements.Cast<IElement>().ToList(); But this is only for AElement, is there a way to add BElement in the same query, and also make it generic IEnumerable? Or would I have to run this query once for each derived type?

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• Need opinions on LaTeX and ever upgrading

  - by yCalleecharan

  Hi, I've been using LaTeX since 2005 with the TeXLive distribution and I've been upgrading as each new TeXLive distribution comes out. In the recent years I noticed an increase in new packages, updated packages and in one instance a new package bearing a different name replacing an old one by the same package author. A LaTeX document which relies heavily on packages and which has been produced a few years back may start to get some warnings and error messages on present-day LaTeX compilation. The primary reason I switched to LaTeX is because of its reliability and robustness to create big documents easily, not to mention the adorable typographic quality. With LaTeX one doesn't have to worry about how to open a docx in an old program supporting only doc for instance. Now, when there are so much continual changes in the packages in a LaTeX distribution, I tend to wonder when will this madness end. Not that having enhanced and new features are bad in packages, but not all updated packages are backward compatible. Eventually one would like to be able to compile a LaTeX file in 10 years time that he/she is working on at present and not get any compilation warnings/error messages due to some unpredictable behavior of updated packages or due to a package that has been cast-off from a LaTeX distribution. If I understand correctly CTAN do keep a database with all packages from different versions. I would like to know how you LaTeX users handle this issue. Thanks a lot...

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• Some questions about the .NET Entity Framework and Stored Procedures

  - by Bara

  Hey everyone, I had a couple of questions relating to the .NET Entity Framework and using stored procedures. Here goes: I know that we can right click a stored procedure and choose Function Import to be able to use with code. Is there a way to do this for many stored procedures at once? When doing a Function Import, I can create a new Complex type or use an existing Complex type. Well, how can I access Complex types/objects that are outside of the edmx file? That is, if I have a class in my project, is it possible to access it while doing a Function Import? When calling the stored procedure from code, it returns an IEnumerable of the Complex type I set it as. However, sometimes these complex types do not have all of the properties that I need, so I create a new class in my project that inherits from the complex type used in the stored procedure. Problem is, I can't seem to cast the complex type returned from the stored procedure to the new class I created. Any reason why I can't do this? What I ended up doing is looping through the IEnumerable and adding each item to a new list of the class that I created. But this feels and looks messy. Bara

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• FileHelpers cannot map converted field into destination array

  - by jaffa

  I have the following record (reduced for brevity): [DelimitedRecord(",")] [IgnoreFirst] [IgnoreEmptyLines()] public class ImportRecord { [FieldQuoted] [FieldTrim(TrimMode.Both)] public string FirstName; [FieldQuoted] [FieldTrim(TrimMode.Both)] public string LastName; [FieldQuoted] [FieldTrim(TrimMode.Both)] [FieldOptional] [FieldConverter(typeof(TestPropertyConverter))] public int[] TestProperty; } Converter code: public class TestPropertyConverter : ConverterBase { public override object StringToField(string from) { var ret = from.Split('|').Select(x => Convert.ToInt32(x)).ToArray(); return ret; } } So an example record could be: John, Smith, 1|2|3|4 It would expect the values 1,2,3,4 to expand and fill the TestProperty array. However, I'm getting the following exception: At least one element in the source array could not be cast down to the destination array type. I've tried to debug into the code and it seems to blow-up in the ExtractFieldValue() function inside FieldBase.cs where it tries to return out of the function. The following line seems to be the culprit: res.ToArray(ArrayType); It seems to expect the 'res' variable to be the destination type array, but it contains 1 element of the array itself. Can anyone suggest if I'm doing this wrong or a possible fix?

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• OrderBy Linq.Expression as parameter = (Of Func(Of T,IComparable)) to perform LinqToEntity is not working

  - by NicoJuicy

  I'd like to get this working: Call: (Count & Page are used for pagination, so Count = 20 and Page = 1 for example, for the first 20 values). Sorting should be by name LeverancierService.GetLeveranciers(Function(el) el.Name, Count, Page) Equivalent in c#: LeverancierService.GetLeveranciers(el= el.Name, Count, Page) Method that gives an error (parameters shown above): Public Overridable Function GetAllPaged(orderby As Expression(Of Func(Of T, IComparable)), ByVal Count As Integer, ByVal Page As Integer) As IEnumerable(Of T) Return dbset.OrderBy(orderby).Skip((Page - 1) * Count).Take(Count).ToList() End Function Already tried changing it to this, but it gives the same error: Public Overridable Function GetAllPaged(Of TOrderBy)(orderby As Expression(Of Func(Of T, TOrderBy)), ByVal Count As Integer, ByVal Page As Integer) As IEnumerable(Of T) Return dbset.OrderBy(orderby).Skip((Page - 1) * Count).Take(Count).ToList() End Function Error: Unable to cast the type 'System.String' to type 'System.IComparable'. LINQ to Entities only supports casting Entity Data Model primitive types. Any idea how to do this? Extra info: I'm in a DDD-layered application, so the parameter should stay the same as the called method is an overridden interface (eg. if i change this, i have to do this for 200 times or so, because it's in VB.Net and not in C# (= 1 change) ) I know there is a way to change the expression to a string and then use DLinq (= Dynamic Linq), but that's not how it should be.

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• How Optimize sql query make it faster

  - by user502083

  Hello every one : I have a very simple small database, 2 of tables are: Node (Node_ID, Node_name, Node_Date) : Node_ID is primary key Citation (Origin_Id, Target_Id) : PRIMARY KEY (Origin_Id, Target_Id) each is FK in Node Now I write a query that first find all citations that their Origin_Id has a specific date and then I want to know what are the target dates of these records. I'm using sqlite in python the Node table has 3000 record and Citation has 9000 records, and my query is like this in a function: def cited_years_list(self, date): c=self.cur try: c.execute("""select n.Node_Date,count(*) from Node n INNER JOIN (select c.Origin_Id AS Origin_Id, c.Target_Id AS Target_Id, n.Node_Date AS Date from CITATION c INNER JOIN NODE n ON c.Origin_Id=n.Node_Id where CAST(n.Node_Date as INT)={0}) VW ON VW.Target_Id=n.Node_Id GROUP BY n.Node_Date;""".format(date)) cited_years=c.fetchall() self.conn.commit() print('Cited Years are : \n ',str(cited_years)) except Exception as e: print('Cited Years retrival failed ',e) return cited_years Then I call this function for some specific years, But it's crazy slowwwwwwwww :( (around 1 min for a specific year) Although my query works fine, it is slow. would you please give me a suggestion to make it faster? I'd appreciate any idea about optimizing this query :) I also should mention that I have indices on Origin_Id and Target_Id, so the inner join should be pretty fast, but it's not!!!

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• Weird compile-time behavior when trying to use primitive type in generics

  - by polygenelubricants

  import java.lang.reflect.Array; public class PrimitiveArrayGeneric { static <T> T[] genericArrayNewInstance(Class<T> componentType) { return (T[]) Array.newInstance(componentType, 0); } public static void main(String args[]) { int[] intArray; Integer[] integerArray; intArray = (int[]) Array.newInstance(int.class, 0); // Okay! integerArray = genericArrayNewInstance(Integer.class); // Okay! intArray = genericArrayNewInstance(int.class); // Compile time error: // cannot convert from Integer[] to int[] integerArray = genericArrayNewInstance(int.class); // Run time error: // ClassCastException: [I cannot be cast to [Ljava.lang.Object; } } I'm trying to fully understand how generics works in Java. Things get a bit weird for me in the 3rd assignment in the above snippet: the compiler is complaining that Integer[] cannot be converted to int[]. The statement is 100% true, of course, but I'm wondering WHY the compiler is making this complaint. If you comment that line, and follow the compiler's "suggestion" as in the 4th assignment, the compiler is actually satisfied!!! NOW the code compiles just fine! Which is crazy, of course, since like the run time behavior suggests, int[] cannot be converted to Object[] (which is what T[] is type-erased into at run time). So my question is: why is the compiler "suggesting" that I assign to Integer[] instead for the 3rd assignment? How does the compiler reason to arrive to that (erroneous!) conclusion?

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• Howto use predicates in LINQ to Entities for Entity Framework objects

  - by user274947

  I'm using LINQ to Entities for Entity Framework objects in my Data Access Layer. My goal is to filter as much as I can from the database, without applying filtering logic on in-memory results. For that purpose Business Logic Layer passes a predicate to Data Access Layer. I mean Func<MyEntity, bool> So, if I use this predicate directly, like public IQueryable<MyEntity> GetAllMatchedEntities(Func<MyEntity, Boolean> isMatched) { return qry = _Context.MyEntities.Where(x => isMatched(x)); } I'm getting the exception [System.NotSupportedException] --- {"The LINQ expression node type 'Invoke' is not supported in LINQ to Entities."} Solution in that question suggests to use AsExpandable() method from LINQKit library. But again, using public IQueryable<MyEntity> GetAllMatchedEntities(Func<MyEntity, Boolean> isMatched) { return qry = _Context.MyEntities.AsExpandable().Where(x => isMatched(x)); } I'm getting the exception Unable to cast object of type 'System.Linq.Expressions.FieldExpression' to type 'System.Linq.Expressions.LambdaExpression' Is there way to use predicate in LINQ to Entities query for Entity Framework objects, so that it is correctly transformed it into a SQL statement. Thank you.

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• OOP design issue: Polymorphism

  - by Graham Phillips

  I'm trying to solve a design issue using inheritance based polymorphism and dynamic binding. I have an abstract superclass and two subclasses. The superclass contains common behaviour. SubClassA and SubClassB define some different methods: SubClassA defines a method performTransform(), but SubClassB does not. So the following example 1 var v:SuperClass; 2 var b:SubClassB = new SubClassB(); 3 v = b; 4 v.performTransform(); would cause a compile error on line 4 as performTransform() is not defined in the superclass. We can get it to compile by casting... (v as SubClassA).performTransform(); however, this will cause a runtime exception to be thrown as v is actually an instance of SubClassB, which also does not define performTransform() So we can get around that by testing the type of an object before casting it: if( typeof v == SubClassA) { (cast v to SubClassA).performTransform(); } That will ensure that we only call performTransform() on v's that are instances of SubClassA. That's a pretty inelegant solution to my eyes, but at least its safe. I have used interface based polymorphism (interface meaning a type that can't be instantiated and defines the API of classes that implement it) in the past, but that also feels clunky. For the above case, if SubClassA and SubClassB implemented ISuperClass that defined performTransform, then they would both have to implement performTransform(). If SubClassB had no real need for a performTransform() you would have to implement an empty function. There must be a design pattern out there that addresses the issue.

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• Java Receive Attachment problem ?

  - by Karthick RM

  Hi to all.I use the following code to download the attachment from the mail .But it gives the ClassCastException on the Multipart declaration Exception in thread "main" java.lang.ClassCastException: com.sun.mail.imap.IMAPInputStream cannot be cast to javax.mail.Multipart at ReadAttachment.main(ReadAttachment.java:52) How do I handle IMAPInputStream? Thanks in advance !!! Message messages[] = inbox.getMessages(); for (int j = 0; j < messages.length; j++) { String mailType = messages[j].getContentType(); System.out.println("------------ Message " + (j + 1) + " ------------"); System.out.println("SentDate : " + messages[j].getSentDate()); System.out.println("From : " + messages[j].getFrom()[0]); System.out.println("Subject : " + messages[j].getSubject()); System.out.println("Type :" + messages[j].getContentType()); System.out.println("Attachment :" + messages[j].getFileName()); Multipart mp = (Multipart) messages[j].getContent(); ........... .............. System.out.println(); }

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• Trying to write priority queue in Java but getting "Exception in thread "main" java.lang.ClassCastEx

  - by Dan

  For my data structure class, I am trying to write a program that simulates a car wash and I want to give fancy cars a higher priority than regular ones using a priority queue. The problem I am having has something to do with Java not being able to type cast "Object" as an "ArrayQueue" (a simple FIFO implementation). What am I doing wrong and how can I fix it? public class PriorityQueue<E> { private ArrayQueue<E>[] queues; private int highest=0; private int manyItems=0; public PriorityQueue(int h) { highest=h; queues = (ArrayQueue[]) new Object[highest+1]; <----problem is here } public void add(E item, int priority) { queues[priority].add(item); manyItems++; } public boolean isEmpty( ) { return (manyItems == 0); } public E remove() { E answer=null; int counter=0; do { if(!queues[highest-counter].isEmpty()) { answer = queues[highest-counter].remove(); counter=highest+1; } else counter++; }while(highest-counter>=0); return answer; } }

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