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• How to functionally generate a tree breadth-first. (With Haskell)

  - by Dennetik

  Say I have the following Haskell tree type, where "State" is a simple wrapper: data Tree a = Branch (State a) [Tree a] | Leaf (State a) deriving (Eq, Show) I also have a function "expand :: Tree a - Tree a" which takes a leaf node, and expands it into a branch, or takes a branch and returns it unaltered. This tree type represents an N-ary search-tree. Searching depth-first is a waste, as the search-space is obviously infinite, as I can easily keep on expanding the search-space with the use of expand on all the tree's leaf nodes, and the chances of accidentally missing the goal-state is huge... thus the only solution is a breadth-first search, implemented pretty decent over here, which will find the solution if it's there. What I want to generate, though, is the tree traversed up to finding the solution. This is a problem because I only know how to do this depth-first, which could be done by simply called the "expand" function again and again upon the first child node... until a goal-state is found. (This would really not generate anything other then a really uncomfortable list.) Could anyone give me any hints on how to do this (or an entire algorithm), or a verdict on whether or not it's possible with a decent complexity? (Or any sources on this, because I found rather few.)

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• Eclipse formatter to keep One-Liners.

  - by Dennetik

  Can Eclipse Formatter be configured to keep: public Long getId() { return this.id; } And maybe to format small (one line) definitions as one-liners?

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• Catching constraint violations in JPA 2.0.

  - by Dennetik

  Consider the following entity class, used with, for example, EclipseLink 2.0.2 - where the link attribute is not the primary key, but unique nontheless. @Entity public class Profile { @Id private Long id; @Column(unique = true) private String link; // Some more attributes and getter and setter methods } When I insert records with a duplicate value for the link attribute, EclipseLink does not throw a EntityExistsException, but throws a DatabaseException, with the message explaining that the unique constraint was violated. This doesn't seem very usefull, as there would not be a simple, database independent, way to catch this exception. What would be the advised way to deal with this? A few things that I have considered are: Checking the error code on the DatabaseException - I fear that this error code, though, is the native error code for the database; Checking the existence of a Profile with the specific value for link beforehand - this obviously would result in an enormous amount of superfluous queries.

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• Programmatically loading Entity classes with JPA 2.0?

  - by Dennetik

  With Hibernate you can load your Entity classes as: sessionFactory = new AnnotationConfiguration() .addPackage("test.animals") .addAnnotatedClass(Flight.class) .addAnnotatedClass(Sky.class) .addAnnotatedClass(Person.class) .addAnnotatedClass(Dog.class); Is there a way to do the same thing - programmatically loading your Entity classes - in a JPA 2.0 compliant way?

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• Java/Hibernate using interfaces over the entities.

  - by Dennetik

  I am using annoted Hibernate, and I'm wondering whether the following is possible. I have to set up a series of interfaces representing the objects that can be persisted, and an interface for the main database class containing several operations for persisting these objects (... an API for the database). Below that, I have to implement these interfaces, and persist them with Hibernate. So I'll have, for example: public interface Data { public String getSomeString(); public void setSomeString(String someString); } @Entity public class HbnData implements Data, Serializable { @Column(name = "some_string") private String someString; public String getSomeString() { return this.someString; } public void setSomeString(String someString) { this.someString = someString; } } Now, this works fine, sort of. The trouble comes when I want nested entities. The interface of what I'd want is easy enough: public interface HasData { public Data getSomeData(); public void setSomeData(Data someData); } But when I implement the class, I can follow the interface, as below, and get an error from Hibernate saying it doesn't know the class "Data". @Entity public class HbnHasData implements HasData, Serializable { @OneToOne(cascade = CascadeType.ALL) private Data someData; public Data getSomeData() { return this.someData; } public void setSomeData(Data someData) { this.someData = someData; } } The simple change would be to change the type from "Data" to "HbnData", but that would obviously break the interface implementation, and thus make the abstraction impossible. Can anyone explain to me how to implement this in a way that it will work with Hibernate?

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