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• How to detect open database connection with Hibernate / JPA?

  - by John K

  I am learning JPA w/Hibernate using a Java SE 6 project. I'd simply like to be able to detect if the connection between Hibernate and my database (MS SQL Server) is open. For example, I'd like to be able to detect this, log it, and try reconnecting again in 60 seconds. This is what I thought would work but isOpen() doesn't appear to be what I want (always is true): EntityManagerFactory emf = Persistence.createEntityManagerFactory("rcc", props); if (emf != null && emf.isOpen()) { EntityManager em = emf.createEntityManager(); if (em == null || !emf.isOpen()) // error connecting to database else ... This seems to me to be a simple problem, but I cannot find an answer!

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• Is lock returned by ReentrantReadWriteLock equivalent to it's read and write locks?

  - by Todd

  Hello, I have been looking around for the answer to this, but no joy. In Java, is using the lock created by ReentrantReadWriteLock equivalent to getting the read and write locks as returned by readLock.lock() and writeLock.lock()? In other words, can I expect the read and write locks associated with the ReentrantReadWriteLock to be requested and held by synchronizing on the ReentrantReadWriteLock? My gut says "no" since any object can be used for synchronization. I wouldn't think that there would be special behavior for ReentrantReadWriteLock. However, special behavior is the corner case of which I may not be aware. Thanks, Todd

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• Verify mail server connection programmatically

  - by Sergii

  I'm using custom SMTP servers and would like to verify the connection when user enters his own server credentials. Exactly the same type of check as Adobe CF and Railo allow to do when adding mail server. Sure, this does not guarantee that delivery will be working, but at least to check that entered server/username/pass are valid. I can see one tricky way: try to send the email with cfmail and check the mail log. But I believe that it can be done with more elegant. Is there any Java library available with standard ACF/Railo distro to help me? How would I use them? Examples are highly appreciated. Thanks in advance.

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• How to count occurrence of an element in a List

  - by MM

  I have an ArrayList a collection class of java as follows. ArrayList<String>animals = new ArrayList<String>(); animals.add("bat"); animals.add("owl"); animals.add("bat"); animals.add("bat"); As you can see the animals ArrayList consists of 3 bat elements and one owl element. I was wondering if there is any API in collection framework that returns the number of bat occurrences or is there a way to determine number of occurrences. I found that google's collection multiset does have an api that returns total number of occurrences of an element. But that is compatible only with jdk1.5. Our product is currently in jdk 1.6. Hence cannot use it.

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• default maven compiler setting

  - by Jeeyoung Kim

  Hello Maven gurus, Right now, I'm writing a small java application by my own, with few maven pom.xml files. I want to make all my maven packages to compile with jdk 1.6, and I can't find a good way to do it without manually setting it on every single POMs - I'm sick of copy-and-pasting <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-compiler-plugin</artifactId> <configuration> <source>1.6</source> <target>1.6</target> </configuration> in every single pom.xml file I generate. Is there a simpler way to resolve this issue?

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• Batch and the for loop

  - by Farid

  Hi everyone, I have a java application launched by a .cmd file. I want to set the classpath of the application through this batch, all the needed jars are into a lib folder. Here is what I tried : set _classpath=. for %%i in (%1/lib/*.*) do ( set _classpath=%_classpath%;%%i ) Surprisingly, it seems that it does not act as expected. Let's say there is 3 jar in the lib folder : pikachu.jar sonic.jar mario.jar Here is what happens : set _classpath=. set _classpath=.;pikachu.jar set _classpath=.;sonic.jar set _classpath=.;mario.jar Obviously, what I am looking to get is set _classpath=.;pikachu.jar;sonic.jar;mario.jar Any idea ? Thanks and regards,

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• Best Practice Guide: Swing

  - by wishi_

  Hi! Does anybody know Swing related GUI guidelines - specifically on how to design Swing apps and which components I should use? I'm not looking for an official standard, but pragmatic tips I can use to set a good standard for my projects. I haven't used too much of Swing by myself. Surely clicking a GUI with a GUI designer isn't a big deal. However I'd like to get some insights from people who have experience with Swing and know what to avoid. Swing lately (in Java 6- 10) got decent changes. So there isn't too much specific standardization out there currently.

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• Android serialization: ImageView

  - by embo

  I have a simple class: public class Ball2 extends ImageView implements Serializable { public Ball2(Context context) { super(context); } } Serialization ok: private void saveState() throws IOException { ObjectOutputStream oos = new ObjectOutputStream(openFileOutput("data", MODE_PRIVATE)); try { Ball2 data = new Ball2(Game2.this); oos.writeObject(data); oos.flush(); } catch (Exception e) { Log.e("write error", e.getMessage(), e); } finally { oos.close(); } } But deserealization private void loadState() throws IOException { ObjectInputStream ois = new ObjectInputStream(openFileInput("data")); try { Ball2 data = (Ball2) ois.readObject(); } catch (Exception e) { Log.e("read error", e.getMessage(), e); } finally { ois.close(); } } fail with error: 03-24 21:52:43.305: ERROR/read error(1948): java.io.InvalidClassException: android.widget.ImageView; IllegalAccessException How deserialize object correctly?

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• Type-safe, generic, empty Collections with static generics

  - by Droo

  I return empty collections vs. null whenever possible. I switch between two methods for doing so using java.util.Collections: return Collections.EMPTY_LIST; return Collections.emptyList(); where emptyList() is supposed to be type-safe. But I recently discovered: return Collections.<ComplexObject> emptyList(); return Collections.<ComplexObject> singletonList(new ComplexObject()); etc. I see this method in Eclipse Package Explorer: <clinit> () : void but I don't see how this is done in the source code (1.5). How is this magic tomfoolerie happening!!

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• Maven eclipse does not add a dependency

  - by Calm Storm

  I have the following snippet in my pom.xml <dependency> <groupId>aspectj</groupId> <artifactId>aspectjrt</artifactId> <version>1.5.3</version> </dependency> and in one of my Java files I refer a class org.aspectj.lang.ProceedingJoinPoint. When I do a "mvn clean install" it compiles and builds fine but when I do an eclipse:eclipse, and import the project in eclipse it gives me an error The import org.aspectj cannot be resolved. I checked the .classpath file that was generated and it does not have an entry to this file. I tried a "mvn dependency:tree" and it lists this fine. Can someone tell me what is going wrong here?

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• Google App Engine - low-level datastore API flag?

  - by Keyur

  In my GAE-Java app, I'm using the low-level datastore API. Hence I don't need the GAE app instance to load any of the higher level data access libraries such as JPA, JDO, Data Nucleus, etc. Is there a flag that I can set to indicate that I don't want these libraries to be loaded? My motivation to do this is to reduce app instance startup time everywhere I can. Now I don't know if these libraries are loaded only on-demand or always. The dev environment logs messages related to data nucleus which seems to indicate that some of these libraries may be pre-loaded? I hope I'm wrong here. Thanks, Keyur

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• Techniques to avoid DeadlineExceededException in GAE/J?

  - by Tahir Akram

  I am developing an Twitter4J web application in Google App Engine/Java. I need to show two lists. One is Twitter friends and other is followers. With photo and screen name. It is working fine for people who have 20-30 followers and friends. But it gave me DeadlineExceededException when I try a user who has 150+ followers and friends. GAE throws this exception if web request take time more than 30 seconds. So what techniques I can adopt to avoid this exception. Should I generate two AJAX calls for each of my list. After page loads. So that every call will have its own 30 secs limit? Or what else you think? I am gone make it. Please help.

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• Simple cross platform GUI app

  - by Joe Cannatti

  I would like to know if there is any way that I could build a very simple GUI app (it doesn't even have to look good) that will run on a fresh install of Windows Vista and OS X with no other installations needed by the user. I would perfer not to use Java (just out of personal programming preference). I will use it though, if it is the only way. Specically, I am wondering if I can write a swing app with Scala or Groovy and run in on windows without them having to install anything. Sorry if this is a silly question, I am a Obj-C developer by trade.

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• What would be the use of accepting itself as type arguments in generics

  - by Newtopian

  I saw some code on an unrelated question but it got me curious as I never saw such construct with Java Generics. What would be the use of creating a generic class that can take as type argument itself or descendants of itself. Here is example : abstract class A<E extends A> { abstract void foo(E x); } the first thing that came to mind would be a list that takes a list as parameter. Using this code feels strange, how do you declare a variable of type A ? Recursive declaration !? Does this even work ? If so did any of you see that in code ? How was it used ?

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• Understanding the workings of equals and hashCode in a HashMap

  - by andandandand

  I have this test code: import java.util.*; class MapEQ { public static void main(String[] args) { Map<ToDos, String> m = new HashMap<ToDos, String>(); ToDos t1 = new ToDos("Monday"); ToDos t2 = new ToDos("Monday"); ToDos t3 = new ToDos("Tuesday"); m.put(t1, "doLaundry"); m.put(t2, "payBills"); m.put(t3, "cleanAttic"); System.out.println(m.size()); } } class ToDos{ String day; ToDos(String d) { day = d; } public boolean equals(Object o) { return ((ToDos)o).day == this.day; } // public int hashCode() { return 9; } } When // public int hashCode() { return 9; } is uncommented m.size() returns 2, when it's left commented it returns three. Why?

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• How to use third party themes in swing application?

  - by swift

  I want to use some third party themes (like synthetica http://www.javasoft.de/synthetica/themes/) in my swing appliaction. i am using eclipse ide, got the jar file of theme and did the following modification(according to the readme file from the theme) in my code try { UIManager.setLookAndFeel(new SyntheticaBlackMoonLookAndFeel()); } catch (Exception e) { e.printStackTrace(); } but after this modification its showing the following error The type de.javasoft.plaf.synthetica.SyntheticaLookAndFeel cannot be resolved. It is indirectly referenced from required .class files what does this mean? i tried searching on net but cant really find any useful answers Contents of Readme file: System Requirements =================== Java SE 5 (JRE 1.5.0) or above Synthetica V2.2.0 or above Integration =========== 1. Ensure that your classpath contains all Synthetica libraries (including Synthetica's core library 'synthetica.jar'). 2. Enable the Synthetica Look and Feel at startup time in your application: import de.javasoft.plaf.synthetica.SyntheticaBlackMoonLookAndFeel; try { UIManager.setLookAndFeel(new SyntheticaBlackMoonLookAndFeel()); } catch (Exception e) { e.printStackTrace(); }

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• Best design for generating code from an AST?

  - by Sam Washburn

  I'm working on a pretty complex DSL that I want to compile down into a few high level languages. The whole process has been a learning experience. The compiler is written in java. I was wondering if anyone knew a best practice for the design of the code generator portion. I currently have everything parsed into an abstract syntax tree. I was thinking of using a template system, but I haven't researched that direction too far yet as I would like to hear some wisdom first from stack overflow. Thanks!

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• Accessing constructor from abstract base class with reflection

  - by craesh

  Hi! I'm playing around with Java's Reflection. I have an abstract class Base with a constructor. abstract class Base { public Base( String foo ) { // do some magic } } I have some further classes extending Base. They don't contain much logic. I want to instantiate them with Base's constructor, without having to write some proxy contructors in those derived classes. And of course, I want to instantiate those derived classes with Reflection. Say: Class cls = SomeDerivedClass.class; Constructor constr; constr = cls.getConstructor( new Class[] { String.class } ); // will return null Class clsBase = Base.class; constr = clsBase.getConstructor( new Class[] { String.class } ); // ok Base obj = (Base) constr.newInstance( new Object[] { "foo" } ); // will throw InstantiationException because it belongs to an abstract class Any ideas, how I can instantiate a derived class with Base's constructor? Or must I declare those dumb proxy constructors?

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• Implements an Undo/Redo in MVC

  - by bnabilos

  Hello, I have a Java application and I want to implement an Undo/Redo option. the value that I want to stock and that I want to be able to recover is an integer. My Class Model implements the interface StateEditable and I have to redefine the 2 functions restoreState(Hashtable<?, ?> state) and storeState(Hashtable<Object, Object> state) but I don't know what to put on them. It will be really great if somebody can help me to do that. These are the first lines of my Model class, the value that I want to do an undo/redo on it is value public class Model extends Observable implements StateEditable { private int value = 5; private UndoManager undoRedo = new UndoManager(); final UndoableEditListener editListener = new UndoableEditListener() { public void undoableEditHappened(UndoableEditEvent evt) { undoRedo.addEdit(evt.getEdit()); } }; @Override public void restoreState(Hashtable<?, ?> state) { } @Override public void storeState(Hashtable<Object, Object> state) { } }

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• Equivalent of public static final fields in Scala

  - by JT

  I'm learning Scala, and I can't figure out how to best express this simple Java class in Scala: public class Color { public static final Color BLACK = new Color(0, 0, 0); public static final Color WHITE = new Color(255, 255, 255); public static final Color GREEN = new Color(0, 0, 255); private static final int red; private static final int blue; private static final int green; public Color(int red, int blue, int green) { this.red = red; this.blue = blue; this.green = green; } // getters, et cetera } The best I have is the following: class Color(val red: Int, val blue: Int, val green: Int) object BLACK extends Color(0, 0, 0) object WHITE extends Color(255, 255, 255) object GREEN extends Color(0, 0, 255) But I lose the advantages of having BLACK, WHITE, and GREEN being tied to the Color namespace.

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• Creating a list of integers in XML for android.

  - by Leif Andersen

  I would like to create a list of Integers in the /res folder of an android project. However, I want those integers to point resources in /res/raw. So for example, I would like something like this: <?xml version="1.0" encoding="utf-8"?> <resources> <integer-array name="built_in_sounds"> <item>@raw/sound</item> </integer-array> </resources> But id doesn't look like I can do that, is there any way to do this? Or should I just create the list in a java class? Thank you

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• Retrieving the type of a Collection

  - by nevets1219

  So I have something like the following in Java: private List<SomeType>variable; // ....variable is instantiated as so ... variable = new ArrayList<SomeType>(); // there's also a getter public List<SomeType> getVariable() { /* code */ } What I would like to be able to do is figure out that variable is a collection of SomeType programmatically. I read here that I can determine that from the method getVariable() but is there any way to tell directly from variable? I have been able to retrieve SomeType from the getter method based on the information in the link. I have also been successful in retrieving all the fields of the surrounding class via SurroundingClass.getClass().getDeclaredFields() but this doesn't tell me that it is List<SomeType>.

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• Riddle: Spot the serious bug in this bubble sort implementation

  - by ripper234

  (No, this isn't a homework assignment, I just found the bug and thought it might be useful to share it here) import java.util.List; public class BubbleSorter { public <T extends Comparable<T>> void sort(List<T> list) { while (true) { boolean didWork = false; for (int i = 0; i < list.size() - 1; ++i) { if (list.get(i).compareTo(list.get(i + 1)) > 0) { swap(list, i, i + 1); didWork = true; break; } } if (!didWork) return; } } private static <T> void swap(List<T> list, int i, int j) { T tmp = list.get(i); list.set(i, list.get(j)); list.set(j, tmp); } }

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• Refresh backend in GWT development

  - by T.K.

  I am developing a GWT application that uses EJB and other Java EE 6 technology as the backend. I am currently using the GWT 2.0 plugin for Safari. When I change my GWT client side code and save in my IDE (NetBeans), all that's required is a simple reload in the browser for the changes to become active. That works great! However, often I work on the server-side (the EJBs, GWT server code, etc) and then something in on the GWT client side. Any changes done to the server-side do not appear to incrementally deploy to the Glassfish V3 server. Currently I close the GWT Development Mode application, and then recompile the EJBs, and then go back into GWT Development mode. That is tedious. Any better way of doing this? I tried the "deploy on save" option in NetBeans but it does not seem to do the trick.

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• Best Tomcat6 JNDI + Hibernate configuration for session/transaction support

  - by EugeneP

  I use tomcat6 as a servlet container, and need to integrate Hibernate 3.2 into the project. I found a document how to configure SessionFactory lookup through JNDI, though it does not work as expected. Quote: Hibernate works in any environment that uses JTA, in fact, we recommend to use JTA whenever possible as it is the standard Java transaction interface. End of quote. In hibernate config I indicate current_session_context_class = jta and now I get an error "No TransactionManagerLookup specified". The problem is that Tomcat does not support JTA, and to get it worked, if I understand it correctly, you need to add JOTM or other library to Tomcat. But according to quote it is recommended to use JTA. What can you recommend in this situation?

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