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  • Problem understanding Inheritance

    - by dhruvbird
    I've been racking my brains over inheritance for a while now, but am still not completely able to get around it. For example, the other day I was thinking about relating an Infallible Human and a Fallible Human. Let's first define the two: Infallible Human: A human that can never make a mistake. It's do_task() method will never throw an exception Fallible Human: A human that will occasionally make a mistakes. It's do_task() method may occasionally throw a ErrorProcessingRequest Exception The question was: IS an infallible human A fallible human OR IS a fallible human AN infallible human? The very nice answer I received was in the form of a question (I love these since it gives me rules to answer future questions I may have). "Can you pass an infallible human where a fallible human is expected OR can you pass a fallible human where an infallible human is expected?" It seems apparent that you can pass an infallible human where a fallible human is expected, but not the other way around. I guess that answered my question. However, it still feels funny saying "An infallible human is a fallible human". Does anyone else feel queasy when they say it? It almost feels as if speaking out inheritance trees is like reading out statements from propositional calculus in plain English (the if/then implication connectives don't mean the same as that in spoken English). Does anyone else feel the same?

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  • Class structure for the proposed data and its containers ?

    - by Prix
    First I would like to wish a happy new year to everyone that may read this :) I am having trouble on how to make a container for some data that I am importing into my application, and I am not sure on how to explain this very well and my english is not really a that good so I hope you can bear with my mistake and help me with some guidance. Currently with a foreach I am importing the follow fields from the data I receive: guid, itemid, name, owner(optional, can be null), zoneid, titleid, subid, heading, x, y, z, typeA, typeB, typeC From the above fields I need to store a Waypoint list of all coords a given item has moved to BUT for each guid I have a new list of waypoints. And from the waypoint list the first entry is also my initial item start location which would be my item initial position (if you notice i have a separate list for it which I was not sure would be better or not) not all items have a waypoint list but all items have the first position. So the first idea I came with to store this data was a list with a class with 2 inner classes with their list: public List<ItemList> myList = new List<ItemList>(); public class ItemList { public int guid { get; set; } public int itemid { get; set; } public string name { get; set; } public int titleid { get; set; } public itemType status { get; set; } public class Waypoint { public float posX { get; set; } public float posY { get; set; } public float posZ { get; set; } } public List<Waypoint> waypoint = new List<Waypoint>(); public class Location { public int zone { get; set; } public int subid { get; set; } public int heading { get; set; } public float posX { get; set; } public float posY { get; set; } public float posZ { get; set; } } public List<Location> position = new List<Location>(); } So here is an example of how I would add a new waypoint to a GUID that exists in the list bool itemExists = myList.Exists(item => item.guid == guid && item.itemid == itemid); if (itemExists) { int lastDistance = 3; ItemList.Waypoint nextWaypoint; ItemList myItem = myList.Find(item => item.guid == guid && item.itemid == itemid); if (myItem.waypoint.Count == 0) { nextWaypoint = new ItemList.Waypoint(); nextWaypoint.posX = PosX; nextWaypoint.posY = PosY; nextWaypoint.posZ = PosZ; } else { ItemList.Waypoint lastWaypoint = myItem.waypoint[myItem.waypoint.Count - 1]; if (lastWaypoint != null) { lastDistance = getDistance(x, y, z, lastWaypoint.posX, lastWaypoint.posY, lastWaypoint.posZ); } if (lastDistance > 2) { nextWaypoint = new ItemList.Waypoint(); nextWaypoint.posX = PosX; nextWaypoint.posY = PosY; nextWaypoint.posZ = PosZ; } } myItem.waypoint.Add(nextWaypoint); } Then to register a new item I would take advantage of the itemExist above so I won't register the same GUID again: ItemList newItem = new ItemList(); newItem.guid = guid; newItem.itemid = itemid; newItem.name = name; newItem.status = status; newItem.titleid = titleid; // Item location ItemList.Location itemLocation = new ItemList.Location(); itemLocation.subid = 0; itemLocation.zone= zone; itemLocation.heading = convertHeading(Heading); itemLocation.posX = PosX; itemLocation.posY = PosY; itemLocation.posZ = PosZ; newItem.position.Add(itemLocation); myList.Add(newItem); Could you help me with advices on how my class structure and lists should look like ? Are there better ways to interate with the lists to get lastWaypoint of a GUID or verify wether an item exist or not ? What else would you advise me in general ? PS: If you have any questions or if there is something I missed to post please let me know and I will update it.

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  • Publish/Subscribe paradigm: Why must message classes not know about their subscribers?

    - by carleeto
    From Wikipedia: "Publish/subscribe (or pub/sub) is a messaging paradigm where senders (publishers) of messages are not programmed to send their messages to specific receivers (subscribers). Rather, published messages are characterized into classes, without knowledge of what (if any) subscribers there may be" I can understand why a sender must not be programmed to send its message to a specific receiver. But why must published messages be classes that do not have knowledge of their subscribers? It would seem that once the messaging system itself is in place, what typically changes as software evolves is the messages sent, the publishers and the receivers. Keeping the messages separate from the subscribers seems to imply that the subscription model might also change. Is this the reason? Also, does this occur in the real world? I realize this may be a basic question, but I'm trying to understand this paradigm and your replies are very much appreciated.

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  • Ensuring uniqueness on a varchar greater than 255 in MYSQL/InnoDB

    - by Vijay Boyapati
    I have a table which contains HTML entries for news pages. When I initially designed it I used URL as the primary key. I've learned the error of my ways because left-joining is super slow. So I want to redesign the table with an integer (id) primary key, but still keep the rows unique based on the URL. The problem is that I've found URLs longer than 255 characters, and MySQL isn't letting my create a key on the URL. I'm using an InnoDB/UTF8 table. From what I understand it's using multiple bytes per character with a limit of 766 bytes for the key (in InnoDB). I would really love suggestions on an elegant way of keeping the rows unique based on URL, while using an integer primary key. Thanks!

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  • Should I use a class in this: Reading a XML file using lxml.

    - by PulpFiction
    Hi everyone. This question is in continuation to my previous question, in which I asked about passing around an ElementTree. I need to read the XML files only and to solve this, I decided to create a global ElementTree and then parse it wherever required. My question is: Is this an acceptable practice? I heard global variables are bad. If I don't make it global, I was suggested to make a class. But do I really need to create a class? What benefits would I have from that approach. Note that I would be handling only one ElementTree instance per run, the operations are read-only. If I don't use a class, how and where do I declare that ElementTree so that it available globally? (Note that I would be importing this module) Please answer this question in the respect that I am a beginner to development, and at this stage I can't figure out whether to use a class or just go with the functional style programming approach.

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  • Why Is Java Missing Access Specifiers?

    - by Tom Tresansky
    Does anyone understand why Java is missing: An access specifier which allows access by the class and all subclasses, but NOT by other classes in the same package? (Protected-minus) An access specifier which allows access by the class, all classes in the same package, AND all classes in any sub-package? (Default-plus) An access specifier which adds classes in sub-packages to the entities currently allowed access by protected? (Protected-plus) I wish I had more choices than protected and default. In particular, I'm interested in the Protected-plus option. Say I want to use a Builder/Factory patterned class to produce an object with many links to other objects. The constructors on the objects are all default, because I want to force you to use the factory class to produce instances, in order to make sure the linking is done correctly. I want to group the factories in a sub-package to keep them all together and distinct from the objects they are instantiating---this just seems like a cleaner package structure to me. No can do, currently. I have to put the builders in the same package as the objects they are constructing, in order to gain the access to defaults. But separating project.area.objects from project.area.objects.builders would be so nice. So why is Java lacking these options? And, is there anyway to fake it?

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  • How to structure data... Sequential or Hierarchical?

    - by Ryan
    I'm going through the exercise of building a CMS that will organize a lot of the common documents that my employer generates each time we get a new sales order. Each new sales order gets a 5 digit number (12222,12223,122224, etc...) but internally we have applied a hierarchy to these numbers: + 121XX |--01 |--02 + 122XX |--22 |--23 |--24 In my table for sales orders, is it better to use the 5 digital number as an ID and populate up or would it be better to use the hierarchical structure that we use when referring to jobs in regular conversation? The only benefit to not populating sequentially seems to be formatting the data later on in my view, but that doesn't sound like a good enough reason to go through the extra work. Thanks

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  • Correct approach to validate attributes of an instance of class

    - by systempuntoout
    Having a simple Python class like this: class Spam(object): __init__(self, description, value): self.description = description self.value = value Which is the correct approach to check these constraints: "description cannot be empty" "value must be greater than zero" Should i: 1.validate data before creating spam object ? 2.check data on __init__ method ? 3.create an is_valid method on Spam class and call it with spam.isValid() ? 4.create an is_valid static method on Spam class and call it with Spam.isValid(description, value) ? 5.check data on setters declaration ? 6.... Could you recommend a well designed\Pythonic\not verbose (on class with many attributes)\elegant approach?

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  • Creating our own APIs

    - by Markii
    We need a bunch of APIs for our service and are looking at the job as being quite tedious for starting from scratch because we need strict user restrictions such as max queries and usage statistics. Are there any pre-made services or scripts for this already?

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  • What exactly is GRASP's Controller about?

    - by devoured elysium
    What is the idea behind Grasp's Controller pattern? My current interpretation is that sometimes you want to achieve something that needs to use a couple of classes but none of those classes could or has access to the information needed to do it, so you create a new class that does the job, having references to all the needed classes(this is, could be the information expert). Is this a correct view of what Grasp's Controller is about? Generally when googling or SO'ing controller, I just get results about MVC's (and whatnot) which are topics that I don't understand about, so I'd like answers that don't assume I know ASP.NET's MVC or something :( Thanks

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  • Setting the type of a field in a superclass from a subclass (Java)

    - by Ibolit
    Hi. I am writing a project on Google App Engine, within it I have a number of abstract classes that I hope I will be able to use in my future projects, and a number of concrete classes inheriting from them. Among other abstract classes I have an abstract servlet that does user management, and I hava an abstract user. The AbstractUser has all the necessary fields and methods for storing it in the datastore and telling whether the user is registered with my service or not. It does not implement any project specific functionality. The abstract servlet that manages users, refers only to the methods declared in the AbstractUser class, which allows it to generate links for logging in, logging out and registering (for unregistered users). In order to implement the project-specific user functionality I need to subclass the Abstract user. The servlets I use in my project are all indirect descendants from that abstract user management servlet, and the user is a protected field in it, so the descendant servlets can use it as their own field. However, whenever i want to access any project specific method of the concrete user, i need to cast it to that type. i.e. (abstract user managing servlet) ... AbstractUser user = getUser(); ... abstract protected AbstractUser getUser(); (project-specific abstract servlet) @Override protected AbstractUser getUser() { return MyUserFactory.getUser(); } any other project specific servlet: int a = ((ConcreteUser) user).getA(); Well, what i'd like to do is to somehow make the type of “user” in the superclass depend on something in the project-specific abstract class. Is it at all possible? And i don't want to move all the user-management stuff into a project-specific layer, for i would like to have it for my future projects already written :) Thank you for your help.

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  • Suggested improvements?

    - by J Harley
    Hello, I have been coding a site in pure HTML/CSS - using no server-side language. I was wondering if anyone had any feedback - is there anything that you would change etc...? Many Thanks, J View Site

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  • What is the definition of a Service object ?

    - by Maskime
    I've been working a lot with PHP. But recently i was sent on a work wich use Java. In PHP i used to do a lot of Singleton object but this pattern has not the same signification in Java that it has in PHP. So i wanted to go for an utility class (a class with static method) but my chief doesn't like this kind of classes and ask me to go for services object. So my guess was that a service object is just a class with a constructor that implement some public methods... Am i right ?

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  • C# language questions

    - by Water Cooler v2
    1) What is int? Is it any different from the struct System.Int32? I understand that the former is a C# alias (typedef or #define equivalant) for the CLR type System.Int32. Is this understanding correct? 2) When we say: IComparable x = 10; Is that like saying: IComparable x = new System.Int32(); But we can't new a struct, right? or in C like syntax: struct System.In32 *x; x=>someThing = 10; 3) What is String with a capitalized S? I see in Reflector that it is the sealed String class, which, of course, is a reference type, unlike the System.Int32 above, which is a value type. What is string, with an uncapitalized s, though? Is that also the C# alias for this class? Why can I not see the alias definitions in Reflector? 4) Try to follow me down this subtle train of thought, if you please. We know that a storage location of a particular type can only access properties and members on its interface. That means: Person p = new Customer(); p.Name = "Water Cooler v2"; // legal because as Name is defined on Person. but // illegal without an explicit cast even though the backing // store is a Customer, the storage location is of type // Person, which doesn't support the member/method being // accessed/called. p.GetTotalValueOfOrdersMade(); Now, with that inference, consider this scenario: int i = 10; // obvious System.object defines no member to // store an integer value or any other value in. // So, my question really is, when the integer is // boxed, what is the *type* it is actually boxed to. // In other words, what is the type that forms the // backing store on the heap, for this operation? object x = i;

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  • Parallel programming, are we not learning from history again?

    - by mezmo
    I started programming because I was a hardware guy that got bored, I thought the problems being solved in the software side of things were much more interesting than those in hardware. At that time, most of the electrical buses I dealt with were serial, some moving data as fast as 1.5 megabit!! ;) Over the years these evolved into parallel buses in order to speed communication up, after all, transferring 8/16/32/64, whatever bits at a time incredibly speeds up the transfer. Well, our ability to create and detect state changes got faster and faster, to the point where we could push data so fast that interference between parallel traces or cable wires made cleaning the signal too expensive to continue, and we still got reasonable performance from serial interfaces, heck some graphics interfaces are even happening over USB for a while now. I think I'm seeing a like trend in software now, our processors were getting faster and faster, so we got good at building "serial" software. Now we've hit a speed bump in raw processor speed, so we're adding cores, or "traces" to the mix, and spending a lot of time and effort on learning how to properly use those. But I'm also seeing what I feel are advances in things like optical switching and even quantum computing that could take us far more quickly that I was expecting back to the point where "serial programming" again makes the most sense. What are your thoughts?

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  • How to implement the disposable pattern in a class that inherits from another disposable class?

    - by TheRHCP
    Hi, I often used the disposable pattern in simple classes that referenced small amount of resources, but I never had to implement this pattern on a class that inherits from another disposable class and I am starting to be a bit confused in how to free the whole resources. I start with a little sample code: public class Tracer : IDisposable { bool disposed; FileStream fileStream; public Tracer() { //Some fileStream initialization } public void Dispose() { this.Dispose(true); GC.SuppressFinalize(this); } protected virtual void Dispose(bool disposing) { if (!disposed) { if (disposing) { if (fileStream != null) { fileStream.Dispose(); } } disposed = true; } } } public class ServiceWrapper : Tracer { bool disposed; ServiceHost serviceHost; //Some properties public ServiceWrapper () { //Some serviceHost initialization } //protected override void Dispose(bool disposing) //{ // if (!disposed) // { // if (disposing) // { // if (serviceHost != null) // { // serviceHost.Close(); // } // } // disposed = true; // } //} } My real question is: how to implement the disposable pattern inside my ServiceWrapper class to be sure that when I will dispose an instance of it, it will dispose resources in both inherited and base class? Thanks.

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  • Static and default constuctor

    - by Ram
    A non static class can have static as well as default constructor at the same time. What is the difference between these two constructors? When shall I go for only static or static with default constructor?

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  • Makefile: expand dependencies

    - by Danyel
    First off, the title is very generic because there are just tons of ways of how to possibly solve this. However, I'm looking for a clean and neat way. Situation: I have two equal object files foo.o and foo-pi.o, the latter of which is position-independent (compiled with -fPIC). Both depend on foo.h and bar.h. Problem: How do I, without code duplication, declare dependency of all foo*.o to bar.h? Solutions so far: $(shell bash -c 'echo -ne foo{-pi,}.o'}: bar.h $(addsuffix .o, $(addprefix fo, o-pi o)): bar.h The first solution is not portable on systems that don't support bash, the second is a dirty solution since I could not figure out how to use empty strings in addprefix.

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  • correct technical term for this pattern

    - by Oliver A.
    sometimes I use a pattern which is very similar to the singleton pattern: There is one default instance which and a static get method to aces it. But you may create other instances and pass it as optional parameter and if you want to and you can even replace the default instance with a instance from a child class. So it is NO SINGLETON at all but it is used like one singleton in most cases. Anyone got an idea who to call something like this ? Maybe half*** singleton? domiton?

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  • Need recommendation for a table structure

    - by yapiskan
    I have an entity which has 4 different types of property that could have only one value for each case which are boolean, decimal, string or text. I don't want to define the table with 4 boolean, decimal, nvarchar and ntext columns. What would you recommend to cover this case?

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