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• Conflict resolution for two-way sync

  - by K.Steff

  How do you manage two-way synchronization between a 'main' database server and many 'secondary' servers, in particular conflict resolution, assuming a connection is not always available? For example, I have an mobile app that uses CoreData as the 'database' on the iOS and I'd like to allow users to edit the contents without Internet connection. In the same time, this information is available on a website the devices will connect to. What do I do if/when the data on the two DB servers is in conflict? (I refer to CoreData as a DB server, though I am aware it is something slightly different.) Are there any general strategies for dealing with this sort of issue? These are the options I can think of: 1. Always use the client-side data as higher-priority 2. Same for server-side 3. Try to resolve conflicts by marking each field's edit timestamp and taking the latest edit Though I'm certain the 3rd option will open room for some devastating data corruption. I'm aware that the CAP theorem concerns this, but I only want eventual consistency, so it doesn't rule it out completely, right? Related question: Best practice patterns for two-way data synchronization. The second answer to this question says it probably can't be done.

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• When is it better to offload work to the RDBMS rather than to do it in code?

  - by GeminiDomino

  Okay, I'll cop to it: I'm a better coder than I am at databases, and I'm wondering where thoughts on "best practices" lie on the subject of doing "simple" calculations in the SQL query vs. in the code, such as this MySQL example (I didn't write it, I just have to maintain it!) -- This returns the username, and the users age as of the last event. SELECT u.username as user, IF ((DAY(max(e.date)) - DAY(u.DOB)) &lt; 0 , TRUNCATE(((((YEAR(max(e.date))*12)+MONTH(max(e.date))) -((YEAR(u.DOB)*12)+MONTH(u.DOB)))-1)/12, 0), TRUNCATE((((YEAR(max(e.date))*12)+MONTH(max(e.date))) - ((YEAR(u.DOB)*12)+MONTH(u.DOB)))/12, 0)) AS age FROM users as u JOIN events as e ON u.id = e.uid ... Compared to doing the "heavy" lifting in code: Query: SELECT u.username, u.DOB as dob, e.event_date as edate FROM users as u JOIN events as e ON u.id = e.uid code: function ageAsOfDate($birth, $aod) { //expects dates in mysql Y-m-d format... list($by,$bm,$bd) = explode('-',$birth); list($ay,$am,$ad) = explode('-',$aod); //Insert Calculations here ... return $Dy; //Difference in years } echo "Hey! ". $row['user'] ." was ". ageAsOfDate($row['dob'], $row['edate']) . " when we last saw him."; I'm pretty sure in a simple case like this it wouldn't make much difference (other than the creeping feeling of horror when I have to make changes to queries like the first one), but I think it makes it clearer what I'm looking for. Thanks!

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• What is Rainbow (not the CMS)

  - by Jeremy Thompson

  I was reading this excellent blog article regarding speeding up the badge page and in the last comment the author @waffles (a.k.a Sam Saffron) mentions these tools: dapper and a bunch of custom helpers like rainbow, sql builder etc Dapper and sql builder was easy to look up but rainbow keeps pointing me to a CMS, can someone please point me to the real source? Thanks. Obviously the architecture of these [SE] sites is uber cool and ultra fast so no comments on that thanks.

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• How to find out what Hackathons are coming up near me? [closed]

  - by codeulike

  I just went to my first hackathon (London GreenHackathon) and I want to go to lots more. Specifically, hackathons that have a theme of helping people or making the world a better place in some way. I know about Random Hacks of Kindness and a couple of others but is there some central aggregator that will help me find out about hackathons? Also, if anyone else can suggest specific hackathon initiatives I should check out that would be handy too. I'm in London but obviously for other people reading this question other locations will be of interest.

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• Any algorithm to dedicate a set of known resources to a set of known requirements (scheduling)

  - by Saeed Neamati

  I'm developing an application to help school principals in dedicating teachers to classes and courses over the hours of a week (scheduling). The scenario is roughly something like this: User enters the list of teachers and their free times into the system User enters the list of courses for this semester User enters the list of available classes into the system Well, up to here, there is no big deal. Just simple CRUD operations and nothing extraordinary. However, now what makes this system useful is that the application should automatically and based on an algorithm create the semester scheduling. I think you've got the main idea here. For example application should suggest that teacher A should go to class 1 for mathematics, and at the same time teacher B should go to class 2 for physics. This way all of the classes would be dedicated to lessons and teacher times won't overlap each other. Piece a cake for school principal. However, I can't find a good algorithm for this resource dedication. I mean it seems hard to me. Searching Google resulted in articles from different websites, but they are of no help and use to me. For example: http://en.wikipedia.org/wiki/Resource_allocation or http://en.wikipedia.org/wiki/Scheduling_(production_processes) Is there any algorithm out there, or any application or engine which can help me here? Does this requirements have a known name, like for example time scheduling engine? Any help would be appreciated.

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• What is the situation about OpenGL under Ubuntu Unity and Gnome3?

  - by user827992

  In a GNU/linux distribution is usually installed Xorg as main graphical server, it operates with a client-server logic, a special windows is designate as desktop environment and this special windows can handle all the eyecandy stuff like decorations, icons and effects. The problem is that the latest UI heavily relies on hardware acceleration, Unity is an overlay on Compiz and the Gnome-shell also require an active driver for the GPU to work well: the problem is: on the same OS I can find multiple implementations of OpenGL who is handling my OpenGL buffer? how the OpenGL buffer is managed compared to the other windows? how can I be sure that my OpenGL implementation is glued to the hardware and is not related to the client-server logic of Xorg? For example I have tried the clutter library and I have only experienced problems under both Unity and GTK/Gnome, no problem under other OS.

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• How to keep your eyes on the prize?

  - by Ziv

  I'm about to go in a very intensive programming course (8AM-8PM every day of the week for three months), at the end of which I will be given a developer job-the job's quality and the project I will work on will depend on my performance in the course. Getting a good position in the company could be very beneficial and I would very much want that, does anyone have specific techniques or ways that he keeps himself concentrated on a goal for a long period of time?

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• Algorithmic problem - quickly finding all #'s where value %x is some given value

  - by Steve B.

  Problem I'm trying to solve, apologies in advance for the length: Given a large number of stored records, each with a unique (String) field S. I'd like to be able to find through an indexed query all records where Hash(S) % N == K for any arbitrary N, K (e.g. given a million strings, find all strings where HashCode(s) % 17 = 5. Is there some way of memoizing this so that we can quickly answer any question of this form without doing the % on every value? The motivation for this is a system of N distributed nodes, where each record has to be assigned to at least one node. The nodes are numbered 0 - (K-1) , and each node has to load up all of the records that match it's number: If we have 3 nodes Node 0 loads all records where Hash % 3 ==0 Node 1 loads all records where Hash % 3 ==1 Node 2 loads all records where Hash % 3 ==2 adding a 4th node, obviously all the assignments have to be recomputed - Node 0 loads all records where Hash % 4 ==0 ... etc I'd like to easily find these records through an indexed query without having to compute the mod individually. The best I've been able to come up with so far: If we take the prime factors of N (p1 * p2 * ... ) if N % M == I then p % M == I % p for all of N's prime factors e.g. 10 nodes : N % 10 == 6 then N % 2 = 0 == 6 %2 N % 5 = 1 == 6 %5 so storing an array of the "%" of N for the first "reasonable" number of primes for my data set should be helpful. For example in the above example we store the hash and the primes HASH PRIMES (array of %2, %3, %5, %7, ... ]) 16 [0 1 1 2 .. ] so looking for N%10 == 6 is equivalent to looking for all values where array[1]==1 and array[2] == 1. However, this breaks at the first prime larger than the highest number I'm storing in the factor table. Is there a better way?

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• One codebase - lots of hosted services (similar to a basecamp style service) - planning structure

  - by RickM

  We have built a service (PHP Based) for a client, and are now looking to offer it to other clients as a hosted service. For this example, think of it like a hosted forum service, where a client signs up on our site, and is given a subdomain or can use their own domain, and the code picks up the domain, checks it against a 'master' users table, and then loads the content as needed. I'm trying to work out the best way of handling multiple clients. At the moment I can only think of two options that would work: Option 1 - Have 1 set of database tables, but on each table have a column called 'siteid' - this would mean every query has to check the siteid. This would effectively work with just 1 codebase, and 1 database. Option 2 - Have 1 'master' database with all the core stuff such as the client details and their domain. Then when the systen checks the domain, it pulls the clients database details (username/password/dbname) from a table, and loads a second database. The issue here is security of the mysql server details, however it does have the benefit that they are running their own database instead of sharing one. Which option would I be better taking here, and why? Ideally I want it to be fairly easy to convert the 'standalone' script to the 'multi-domain' script as we're on a tight deadline.

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• Why does working processors harder use more electrical power?

  - by GazTheDestroyer

  Back in the mists of time when I started coding, at least as far as I'm aware, processors all used a fixed amount of power. There was no such thing as a processor being "idle". These days there are all sorts of technologies for reducing power usage when the processor is not very busy, mostly by dynamically reducing the clock rate. My question is why does running at a lower clock rate use less power? My mental picture of a processor is of a reference voltage (say 5V) representing a binary 1, and 0V representing 0. Therefore I tend to think of of a constant 5V being applied across the entire chip, with the various logic gates disconnecting this voltage when "off", meaning a constant amount of power is being used. The rate at which these gates are turned on and off seems to have no relation to the power used. I have no doubt this is a hopelessly naive picture, but I am no electrical engineer. Can someone explain what's really going on with frequency scaling, and how it saves power. Are there any other ways that a processor uses more or less power depending on state? eg Does it use more power if more gates are open? How are mobile / low power processors different from their desktop cousins? Are they just simpler (less transistors?), or is there some other fundamental design difference?

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• In MVC , DAO should be called from Controller or Model

  - by tito

  I have seen various arguments against the DAO being called from the Controller class directly and also the DAO from the Model class.Infact I personally feel that if we are following the MVC pattern , the controller should not coupled with the DAO , but the Model class should invoke the DAO from within and controller should invoke the model class.Why because , we can decouple the model class apart from a webapplication and expose the functionalities for various ways like for a REST service to use our model class. If we write the DAO invocation in the controller , it would not be possible for a REST service to reuse the functionality right ? I have summarized both the approaches below. Approach #1 public class CustomerController extends HttpServlet { proctected void doPost(....) { Customer customer = new Customer("xxxxx","23",1); new CustomerDAO().save(customer); } } Approach #2 public class CustomerController extends HttpServlet { proctected void doPost(....) { Customer customer = new Customer("xxxxx","23",1); customer.save(customer); } } public class Customer { ........... private void save(Customer customer){ new CustomerDAO().save(customer); } } Note- Here is what a definition of Model is : Model: The model manages the behavior and data of the application domain, responds to requests for information about its state (usually from the view), and responds to instructions to change state (usually from the controller). In event-driven systems, the model notifies observers (usually views) when the information changes so that they can react. I would need an expert opinion on this because I find many using #1 or #2 , So which one is it ?

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• How can I test database access methods in Java?

  - by javaStudent

  I want to write a test for a method that accesses a database such as following. public class MyClass{ public String getAddress(Int id){ String query = "Select * from Address where id="+id; //some additional statements resultSet = statement.executeQuery(); return result.getString(ADDRESS); } } How can I test this method? I am using Java.

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• What do you code first to learn a new language, library, or framework?

  - by Griffin

  Every language, framework, and library has its own syntax, quirks, and pitfalls. What Program, Game, etc. do you code in order to learn these unique characteristics? How do you decide on what previous programming experience is applicable? I'd imagine that the task would have to be complicated enough to force you to use applicable programming techniques and idioms, but simple enough that it wouldn't take a ton of time.

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• Nested Entities and calculation on leaf entity property - SQL or NoSQL approach

  - by Chandu

  I am working on a hobby project called Menu/Recipe Management. This is how my entities and their relations look like. A Nutrient has properties Code and Value An Ingredient has a collection of Nutrients A Recipe has a Collection of Ingredients and occasionally can have a collection of other recipes A Meal has a Collection of Recipes and Ingredients A Menu has a Collection of Meals The relations can be depicted as In one of the pages, for a selected menu I need to display the effective nutrients information calculated based on its constituents (Meals, Recipes, Ingredients and the corresponding nutrients). As of now am using SQL Server to store the data and I am navigating the chain from my C# code, starting from each meal of the menu and then aggregating the nutrient values. I think this is not an efficient way as this calculation is being done every time the page is requested and the constituents change occasionally. I was thinking about a having a background service that maintains a table called MenuNutrients ({MenuId, NutrientId, Value}) and will populate/update this table with the effective nutrients when any of the component (Meal, Recipe, Ingredient) changes. I feel that a GraphDB would be a good fit for this requirement, but my exposure to NoSQL is limited. I want to know what are the alternative solutions/approaches to this requirement of displaying the nutrients of a given menu. Hope my description of the scenario is clear.

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• How to use data mining principles in this project?

  - by Simon

  I'm getting a Data Mining class this semester and we are free for the final project. For a few months I'm working on procedural planets rendering (something like this http://www.youtube.com/watch?v=rL8zDgTlXso). Do you have any idea of which data mining principles I could use to keep working this project ? Maybe I could try to generate interesting terrains from a set of real maps ? Any publications on that subject ? Any other ideas ?

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• Should I blog in english or in my native language?

  - by Jérémy

  I had a blog which was written in my native language, but now I'm wondering if I should switch to english because of a wider audience. For sure, I want to share my knowledge, but at the meantime I'd like to get hired or be recognized from my peers. Reputation can be important and it can help in making my professional network larger. Do you have any feedback? Btw, my native language is french if that matters.

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• What is the next promotion for a scrum master

  - by gnebar

  I'm currently a scrum master. I have been offered a promotion to a role that will allow me to have a wider impact. (more involved in company wide architectural decisions, possible secondment to kick start major projects, etc). The role and title of the job has yet to be decided but my company are open to guidance from me. I'm happy I can mould the role to suit me and the company but I'm unsure about the job title that fits this role. Technical Evangelist has been suggested but i'm not sure that is the correct title. I'm keen to proceed down the technical route. What would you suggest? What other roles do people take after scrum master/technical lead? EDIT: (I am aware that my current role is a mix of a technical lead and scrum master role, but that's how we do it in my company :) )

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• What kind of language will replace C++ as C++ replaced C ? [closed]

  - by jokoon

  I think I'm not totally wrong when thinking that C++0x (or C++1x) is still C++, just better, with functionnalities coming from boost. I can't stop thinking that computer sciences, even with all that has been made so far, have to evolve again. I don't really like D since it just try to be some sort of "what C++ should have been", and Go seems to be too sophisticated when I dig a little into it, especially after watching some presentation video like this one http://www.youtube.com/watch?v=rKnDgT73v8s The first thing that come into my mind is a new kind of syntax to directly handle specific datatypes and containers such as map, vectors, queues... What kind of things are researchers thinking about ? What are the real features that could make C++ better or a new C-like language could invent ? Does Go features such things ? Would there be a new kind of syntax that would "unbloat" C++ while keeping its advantages ? Could C++ have some of the interesting stuff of languages such as C# and ObjC ? EDIT: Please consider that I'm talking about a system language, not a VM/CLI/bytecode thing.

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• Are the only types of data "sources" static and dynamic?

  - by blunders

  Thinking that there might be others, but not sure -- but before getting into that, let me explain what I mean by static and dynamic data sources. Static (or datastore) - Meaning that the data's state is non-changing, and if was changed, that would be a new state, and the old data would be considered stateless; meaning it no longer is known to exist, or not exist. Another way of possibly looking at a static data source might be that if read and written back without modification, the checksum for before and after should be exactly the same regardless of the duration of time between the reading and rewriting of the data. Examples: Photos, Files, Database Record, Dynamic (or datastream) - Meaning that the data's state is known to be in flux, and never expected to be the same per input. Example: Live video/audio feed, Stock Market feed, First let me say, the above is a very loose mapping of the concepts, and I'd welcome any feedback. Next, onto the core of the question, that being are these the only two types of data sources. My guess, is that yes, they are -- but that there are hybrid versions of the two. That being, streaming data that has a fixed state. For example, the data being streamed has a checksum given and each unique checksum is known to be a single instance of static data. On the flip side, static data could be chained via say a version control system; when played back, each version might be viewed as a segment of a stream; thing is, the very fact that it can be played back makes the data source static. Another type might be that the data source is being organically discovered, and it's simply unknown what the state is. Questions, feedback, requests -- just comment, thanks!!

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• Setting coding priorities

  - by dotnetdev

  Hi, In the dev shops I've worked in, nobody has ever mentioned "coding priorities". I read this in a book or site somewhere, and sets the expectation of what priority should be first in the code. In places where this is not specified, what should the first priority be? It may sound simple to say "do what the business need requires", but that could be at the expense of performance/maintainability. Many people say maintainability first, regardless, some say fulfill the need regardless. I am a young developer, so I am probably missing the point somewhere. Of course, programming is engineering and tough because you can never have the perfect solution. Thanks

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• How can I implement an escrow payment system in my website?

  - by BeachRunnerJoe

  Hello. I'd like to build a web service similar to kickstarter that allows users to pledge money to an idea, tho I'm unsure how I can implement this kind of payment system. If the the idea receives a specified amount of money, then the donors are charged. If it doesn't, the donors are not charged. I've done some preliminary research and have found Amazon Payments to be a possible solution provider for this, but I'm still unsure where to start with this and was hoping someone could point me in some right directions for how I can go about implementing this kind of payment structure in my web site. I should also note that this is primarily a prototype I'm building, so it's ok if the solution is limited to U.S. customers only. Also, I plan to build the site using Ruby on Rails. Thanks so much for your wisdom!

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• How to deal with data on the model specific to the technology being used?

  - by user1620696

  There are some cases where some of the data on a class of the domain model of an application seems to be dependent on the technology being used. One example of this is the following: suppose we are building one application in .NET such that there's the need of an Employee class. Suppose further that we are going to implement relational database, then the Employee has a primary key right? So that the classe would be something like public class Employee { public int EmployeeID { get; set; } public string Name { get; set; } ... } Now, that EmployeeID is dependent on the technology right? That's something that has to do with the way we've choose to persist our data. Should we write down a class independent of such things? If we do it this way, how should we work? I think I would need to map all the time between domain model and persistence specific types, but I'm not sure.

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• Canonical representation of a class object containing a list element in XML

  - by dendini

  I see that most implementations of JAX-RS represent a class object containing a list of elements as follows (assume a class House containing a list of People) <houses> <house> <person> <name>Adam</name> </person> <person> <name>Blake</name> </person> </house> <house> </house> </houses> The result above is obtained for instance from Jersey 2 JAX-RS implementation, notice Jersey creates a wrapper class "houses" around each house, however strangely it doesn't create a wrapper class around each person! I don't feel this is a correct mapping of a list, in other words I'd feel more confortable with something like this: <houses> <house> <persons> <person> <name>Adam</name> </person> <person> <name>Blake</name> </person> </persons> </house> <house> </house> </houses> Is there any document explaining how an object should be correctly mapped apart from any opninion?

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• How do I organize a GUI application for passing around events and for setting up reads from a shared resource

  - by Savanni D'Gerinel

  My tools involved here are GTK and Haskell. My questions are probably pretty trivial for anyone who has done significant GUI work, but I've been off in the equivalent of CGI applications for my whole career. I'm building an application that displays tabular data, displays the same data in a graph form, and has an edit field for both entering new data and for editing existing data. After asking about sharing resources, I decided that all of the data involved will be stored in an MVar so that every component can just read the current state from the MVar. All of that works, but now it is time for me to rearrange the application so that it can be interactive. With that in mind, I have three widgets: a TextView (for editing), a TreeView (for displaying the data), and a DrawingArea (for displaying the data as a graph). I THINK I need to do two things, and the core of my question is, are these the right things, or is there a better way. Thing the first: All event handlers, those functions that will be called any time a redisplay is needed, need to be written at a high level and then passed into the function that actually constructs the widget to begin with. For instance: drawStatData :: DrawingArea -> MVar Core.ST -> (Core.ST -> SetRepWorkout.WorkoutStore) -> IO () createStatView :: (DrawingArea -> IO ()) -> IO VBox createUI :: MVar Core.ST -> (Core.ST -> SetRepWorkout.WorkoutStore) -> IO HBox createUI storeMVar field = do graphs <- createStatView (\area -> drawStatData area storeMVar field) hbox <- hBoxNew False 10 boxPackStart hbox graphs PackNatural 0 return hbox In this case, createStatView builds up a VBox that contains a DrawingArea to graph the data and potentially other widgets. It attaches drawStatData to the realize and exposeEvent events for the DrawingArea. I would do something similar for the TreeView, but I am not completely sure what since I have not yet done it and what I am thinking of would involve replacing the TreeModel every time the TreeView needs to be updated. My alternative to the above would be... drawStatData :: DrawingArea -> MVar Core.ST -> (Core.ST -> SetRepWorkout.WorkoutStore) -> IO () createStatView :: IO (VBox, DrawingArea) ... but in this case, I would arrange createUI like so: createUI :: MVar Core.ST -> (Core.ST -> SetRepWorkout.WorkoutStore) -> IO HBox createUI storeMVar field = do (graphbox, graph) <- createStatView (\area -> drawStatData area storeMVar field) hbox <- hBoxNew False 10 boxPackStart hbox graphs PackNatural 0 on graph realize (drawStatData graph storeMVar field) on graph exposeEvent (do liftIO $ drawStatData graph storeMVar field return ()) return hbox I'm not sure which is better, but that does lead me to... Thing the second: it will be necessary for me to rig up an event system so that various events can send signals all the way to my widgets. I'm going to need a mediator of some kind to pass events around and to translate application-semantic events to the actual events that my widgets respond to. Is it better for me to pass my addressable widgets up the call stack to the level where the mediator lives, or to pass the mediator down the call stack and have the widgets register directly with it? So, in summary, my two questions: 1) pass widgets up the call stack to a global mediator, or pass the global mediator down and have the widgets register themselves to it? 2) pass my redraw functions to the builders and have the builders attach the redraw functions to the constructed widgets, or pass the constructed widgets back and have a higher level attach the redraw functions (and potentially link some widgets together)? Okay, and... 3) Books or wikis about GUI application architecture, preferably coherent architectures where people aren't arguing about minute details? The application in its current form (displays data but does not write data or allow for much interaction) is available at https://bitbucket.org/savannidgerinel/fitness . You can run the application by going to the root directory and typing runhaskell -isrc src/Main.hs data/ or... cabal build dist/build/fitness/fitness data/ You may need to install libraries, but cabal should tell you which ones.

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• Jump into Android or learn Java basics first?

  - by daniels

  I am quite proficient in C and know some C++, but never touched Java. Recently I got an idea for an app which I think has potential, and I want to develop it. I am planning to go Android first, cause it doesn't require a mac/iPhone. So my question is: can I go straight to learning Android development, picking up the Java syntax as I go along and need it (after all I don't think it's much different from C/C++), or should I take a couple of weeks to learn the basics of Java first and then start with Android development?

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