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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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  • programing logic and design plzzzzzzzzz help [closed]

    - by alex
    `*the midvile park maintains records containing info about players on it's soccer teams . each record contain a players first name,last name,and team number . the team are team number team name 1 goal getters 2 the force 3 top gun 4 shooting stars 5 midfield monsters design a proggram that accept player data and creates a report that lists each player along with his or her team number and team name

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  • Alternative to singleton for unique resources

    - by user1320881
    I keep reading over and over again that one should avoid using singletons for various reasons. I'm wondering how to correctly handle a situation where a class represents a unique system resource. For example, a AudioOutput class using SDL. Since SDL_OpenAudio can only be open once at a time it makes no sense having more then one object of this type and it seems to me preventing accidentally making more then one object would actually be good. Just wondering what experienced programmers think about this, am i missing another option ?

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  • How can I achieve this kind of relationship (inheritance, composition, something else)?

    - by Tim
    I would like to set up a foundation of classes for an application, two of which are person and student. A person may or may not be a student and a student is always a person. The fact that a student “is a” person led me to try inheritance, but I can't see how to make it work in the case where I have a DAO that returns an instance of person and I then want to determine if that person is a student and call student related methods for it. class Person { private $_firstName; public function isStudent() { // figure out if this person is a student return true; // (or false) } } class Student extends Person { private $_gpa; public function getGpa() { // do something to retrieve this student's gpa return 4.0; // (or whatever it is) } } class SomeDaoThatReturnsPersonInstances { public function find() { return new Person(); } } $myPerson = SomeDaoThatReturnsPersonInstances::find(); if($myPerson->isStudent()) { echo 'My person\'s GPA is: ', $myPerson->getGpa(); } This obviously doesn't work, but what is the best way to achieve this effect? Composition doesn't sond right in my mind because a person does not “have a” student. I'm not looking for a solution necessarily but maybe just a term or phrase to search for. Since I'm not really sure what to call what I'm trying to do, I haven't had much luck. Thank you!

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  • does anyone see any issues with this thread pattern?

    - by prmatta
    Here is a simple thread pattern that I use when writing a class that needs just one thread, and needs to a specific task. The usual requirements for such a class are that it should be startable, stopable and restartable. Does anyone see any issues with this pattern that I use? public class MyThread implements Runnable { private boolean _exit = false; private Thread _thread = null; public void start () { if (_thread == null) { _thread = new Thread(this, "MyThread"); _thread.start(); } } public void run () { while (_exit) { //do something } } public void stop () { _exit = true; if (_thread != null) { _thread.interrupt(); _thread = null; } } } I am looking for comments around if I am missing something, or if there is a better way to write this.

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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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  • How to implement the API/SPI Pattern in Java?

    - by Adam Tannon
    I am creating a framework that exposes an API for developers to use: public interface MyAPI { public void doSomeStuff(); public int getWidgets(boolean hasRun); } All the developers should have to do is code their projects against these API methods. I also want them to be able to place different "drivers"/"API bindings" on the runtime classpath (the same way JDBC or SLF4J work) and have the API method calls (doSomeStuff(), etc.) operate on different 3rd party resources (files, servers, whatever). Thus the same code and API calls will map to operations on different resources depending on what driver/binding the runtime classpath sees (i.e. myapi-ftp, myapi-ssh, myapi-teleportation). How do I write (and package) an SPI that allows for such runtime binding, and then maps MyAPI calls to the correct (concrete) implementation? In other words, if myapi-ftp allows you to getWidgets(boolean) from an FTP server, how would I could this up (to make use of both the API and SPI)? Bonus points for concrete, working Java code example! Thanks in advance!

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  • Session vs singleton pattern

    - by chobo
    Hi, I have a web application where I would like to pull user settings from a database and store them for Global access. Would it make more sense to store the data in a Singleton, or a Session object? What's the difference between the two? Is it better to store the data as an object reference or break it up into value type objects (ints and strings)? Thanks!

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  • Database design

    - by Hadad
    Hello, I've a system, that have two types of users (Companies and individuals).all types have a shared set of properties but they differ in another. What is the best design merge all in one table that allows null for unmatched properties, or separate them in two tables related to a basic table with a one to one relationship. Thanks.

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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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  • What pattern is this? php

    - by user151841
    I have several classes that are basically interfaces to database rows. Since the class assumes that a row already exists ( __construct expects a field value ), there is a public static function that allows creation of the row and returns an instance of the class. Here's an example ( without the actual database inserts ): class selfStarter { public $type; public function __construct( $type ) { $this->type = $type; } public static function create( $type ) { if ( ! empty($type) ) { $starter = & new selfStarter($type); return $starter; } } } $obj1 = selfStarter::create( "apple" ); $obj2 = & new selfStarter( "banana" ); What is this pattern called?

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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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  • Sequential coupling in code

    - by dotnetdev
    Hi, Is sequential coupling (http://en.wikipedia.org/wiki/Sequential_coupling) really a bad thing in code? Although it's an anti-pattern, the only risk I see is calling methods in the wrong order but documentation of an API/class library with this anti-pattern should take care of that. What other problems are there from code which is sequential? Also, this pattern could easily be fixed by using a facade it seems. Thanks

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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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  • How to handle request/response propagation up and down a widget hierarchy in a GUI app?

    - by fig-gnuton
    Given a GUI application where widgets can be composed of other widgets: If the user triggers an event resulting in a lower level widget needing data from a model, what's the cleanest way to be able to send that request to a controller (or the datastore itself)? And subsequently get the response back to that widget? Presumably one wouldn't want the controller or datastore to be a singleton directly available to all levels of widgets, or is this an acceptable use of singleton? Or should a top level controller be injected as a dependency through a widget hierarchy, as far down as the lowest level widget that might need that controller? Or a different approach entirely?

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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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  • What's wrong (or right) with this JS Object Pattern?

    - by unsane1
    Here's an example of the pattern I'm using in my javascript objects these days (this example relies on jQuery). http://pastie.org/private/ryn0m1gnjsxdos9onsyxg It works for me reasonably well, but I'm guessing there's something wrong, or at least sub-optimal about it, I'm just curious to get people's opinions. Here's a smaller, inline example of it: sample = function(attach) { // set internal reference to self var self = this; // public variable(s) self.iAmPublic = true; // private variable(s) var debug = false; var host = attach; var pane = { element: false, display: false } // public function(s) self.show = function() { if (!pane.display) { position(); $(pane.element).show('fast'); pane.display = true; } } self.hide = function() { if (pane.display) { $(pane.element).hide('fast'); pane.display = false; } } // private function(s) function init () { // do whatever stuff is needed on instantiation of this object // like perhaps positioning a hidden div pane.element = document.createElement('div'); return self; } function position() { var h = { 'h': $(host).outerHeight(), 'w': $(host).outerWidth(), 'pos': $(host).offset() }; var p = { 'w': $(pane.element).outerWidth() }; $(pane.element).css({ top: h.pos.top + (h.h-1), left: h.pos.left + ((h.w - p.w) / 2) }); } function log () { if (debug) { console.log(arguments); } } // on-instantiation let's set ourselves up return init(); } I'm really curious to get people's thoughts on this.

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  • Abstract away a compound identity value for use in business logic?

    - by John K
    While separating business logic and data access logic into two different assemblies, I want to abstract away the concept of identity so that the business logic deals with one consistent identity without having to understand its actual representation in the data source. I've been calling this a compound identity abstraction. Data sources in this project are swappable and various and the business logic shouldn't care which data source is currently in use. The identity is the toughest part because its implementation can change per kind of data source, whereas other fields like name, address, etc are consistently scalar values. What I'm searching for is a good way to abstract the concept of identity, whether it be an existing library, a software pattern or just a solid good idea of some kind is provided. The proposed compound identity value would have to be comparable and usable in the business logic and passed back to the data source to specify records, entities and/or documents to affect, so the data source must be able to parse back out the details of its own compound ids. Data Source Examples: This serves to provide an idea of what I mean by various data sources having different identity implementations. A relational data source might express a piece of content with an integer identifier plus a language specific code. For example. content_id language Other Columns expressing details of content 1 en_us 1 fr_ca The identity of the first record in the above example is: 1 + en_us However when a NoSQL data source is substituted, it might somehow represent each piece of content with a GUID string 936DA01F-9ABD-4d9d-80C7-02AF85C822A8 plus language code of a different standardization, And a third kind of data source might use just a simple scalar value. So on and so forth, you get the idea.

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  • Exception handling pattern

    - by treefrog
    It is a common pattern I see where the error codes associated with an exception are stored as Static final ints. when the exception is created to be thrown, it is constructed with one of these codes along with an error message. This results in the method that is going to catch it having to look at the code and then decide on a course of action. The alternative seems to be- declare a class for EVERY exception error case Is there a middle ground ? what is the recommended method ?

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