design interview questions

Top 15 design interview questions

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Are there free iPhone navigation bar icon sets available?

I want to use up/down icons like the ones used by "mail" in the upper-right corner when you're viewing a specific message. Are these icons available for free anywhere?

Beyond that, are there any sites with free navigation bar, toolbar, or tab bar icons?

Thank you!

Source: (StackOverflow)

prototype based vs. class based inheritance

In JavaScript, every object is at the same time an instance and a class. To do inheritance, you can use any object instance as a prototype.

In Python, C++, etc.. there are classes, and instances, as separate concepts. In order to do inheritance, you have to use the base class to create a new class, which can then be used to produce derived instances.

Why did JavaScript go in this direction (prototype-based object orientation)? what are the advantages (and disadvantages) of prototype-based OO with respect to traditional, class-based OO?

Source: (StackOverflow)

Struct like objects in Java

Is it completely against the Java way to create struct like objects?

class SomeData1 {
    public int x;
    public int y;

I can see a class with accessors and mutators being more Java like.

class SomeData2 {
    int getX();
    void setX(int x);

    int getY();
    void setY(int y);

    private int x;
    private int y;

The class from the first example is notationally convenient.

// a function in a class
public int f(SomeData1 d) {
    return (3 * d.x) / d.y;

This is not as convenient.

// a function in a class
public int f(SomeData2 d) {
    return (3 * d.getX()) / d.getY();

Source: (StackOverflow)

What program should I use to mock up GUIs? [closed]

I want to mockup a GUI system so that I can figure out how best to represent what I need. Normally, I would design a GUI in something like glade and then write a quick mockup in Python which implements only the GUI logic (ie, making menus/sub windows etc appear) and not business logic.

Are there any tools which automate this?

I mean, I can use glade or Visual Studio or whatever to draw a GUI and then write some code to glue things together, but I wonder if there is some tool which would minimize (or eliminate) having to write code, so that I can get input for potential GUI designs from non-programmers too.

Ideally, it would let you do things like draw non-standard widgets and perhaps define simple behaviours to them (eg, draggable, or clickable or whatever).

Source: (StackOverflow)

How to inherit constructors?

Imagine a base class with many constructors and a virtual method

public class Foo
   public Foo() {...}
   public Foo(int i) {...}
   public virtual void SomethingElse() {...}

and now i want to create a descendant class that overrides the virtual method:

public class Bar : Foo 
   public override void SomethingElse() {...}

And another descendant that does some more stuff:

public class Bah : Bar
   public void DoMoreStuff() {...}

Do i really have to copy all constructors from Foo into Bar and Bah? And then if i change a constructor signature in Foo, do i have to update it in Bar and Bah?

Is there no way to inherit constructors? Is there no way to encourage code reuse?

Source: (StackOverflow)

How to design RESTful search/filtering?

I'm currently designing and implementing a RESTful API in PHP. However, I have been unsuccessful implementing my initial design.

GET /users # list of users
GET /user/1 # get user with id 1
POST /user # create new user
PUT /user/1 # modify user with id 1
DELETE /user/1 # delete user with id 1

So far pretty standard, right?

My problem is with the first one GET /users. I was considering sending parameters in the request body to filter the list. This is because I want to be able to specify complex filters without getting a super long url, like:

GET /users?parameter1=value1&parameter2=value2&parameter3=value3&parameter4=value4

Instead I wanted to have something like:

GET /users
# Request body:
    "parameter1": "value1",
    "parameter2": "value2",
    "parameter3": "value3",
    "parameter4": "value4"

which is much more readable and gives you great possibilities to set complex filters.

Anyway, file_get_contents('php://input') didn't return the request body for GET requests. I also tried http_get_request_body(), but the shared hosting that I'm using doesn't have pecl_http. Not sure it would have helped anyway.

I found this question and realized that GET probably isn't supposed to have a request body. It was a bit inconclusive, but they advised against it.

So now I'm not sure what to do. How do you design a RESTful search/filterng function?

I suppose I could use POST, but that doesn't seem very RESTful.

Source: (StackOverflow)

How do you design object oriented projects?

I'm working on a large project (for me) which will have many classes and will need to be extensible, but I'm not sure how to plan out my program and how the classes need to interact.

I took an OOD course a few semesters back and learned a lot from it; like writing UML, and translating requirements documents into objects and classes. We learned sequence diagrams too but somehow I missed the lecture or something, they didn't really stick with me.

With previous projects I've tried using methods I learned from the course but usually end up with code that as soon as I can say "yeah that looks something like what I had in mind" i have no desire to dig through the muck to add new features.

I've got a copy of Steve McConnell's Code Complete which I continually hear is amazing, here and elsewhere. I read the chapter on design and didn't seem to come out with the information I'm looking for. I know he says that it's not a cut and dried process, that it's mostly based on heuristics, but I can't seem to take all his information and apply it to my projects.

So what are things you do during the high level design phase (before you begin programming) to determine what are the classes you need (especially ones not based on any 'real world objects') and how will they interact with each other?

Specifically I'm interested in what are the methods you use? What is the process you follow that usually yeilds a good, clean design that will closely represent the final product?

Source: (StackOverflow)

What kind of solutions is Golang (Google Go) designed for? [closed]

Seems like Go is designed as a replacement for problems you previously would have solved with C++. Is this an accurate statement? What kind of solutions is Golang (Google Go) designed for?

Source: (StackOverflow)

Using "final" modifier whenever applicable in java [closed]

In Java, there is a practice of declaring every variable (local or class), parameter final if they really are. Though this makes the code a lot more verbose, this helps in easy reading/grasping of the code and also prevents mistakes as the intention is clearly marked.

What are your thoughts on this and what do you follow?

Source: (StackOverflow)

Good or bad practice? Initializing objects in getter

I have a strange habit it seems... according to my co-worker at least. We've been working on a small project together. The way I wrote the classes is (simplified example):

public class Foo
    public Foo()
    { }

    private Bar _bar;

    public Bar Bar
            if (_bar == null)
                _bar = new Bar();

            return _bar;
        set { _bar = value; }

So, basically, I only initialize any field when a getter is called and the field is still null. I figured this would reduce overload by not initializing any properties that aren't used anywhere.

ETA: The reason I did this is that my class has several properties that return an instance of another class, which in turn also have properties with yet more classes, and so on. Calling the constructor for the top class would subsequently call all constructors for all these classes, when they are not always all needed.

Are there any objections against this practice, other than personal preference?

UPDATE: I have considered the many differing opinions in regards to this question and I will stand by my accepted answer. However, I have now come to a much better understanding of the concept and I'm able to decide when to use it and when not.


  • Thread safety issues
  • Not obeying a "setter" request when the value passed is null
  • Micro-optimizations
  • Exception handling should take place in a constructor
  • Need to check for null in class' code


  • Micro-optimizations
  • Properties never return null
  • Delay or avoid loading "heavy" objects

Most of the cons are not applicable to my current library, however I would have to test to see if the "micro-optimizations" are actually optimizing anything at all.


Okay, I changed my answer. My original question was whether or not this is a good habit. And I'm now convinced that it's not. Maybe I will still use it in some parts of my current code, but not unconditionally and definitely not all the time. So I'm going to lose my habit and think about it before using it. Thanks everyone!

Source: (StackOverflow)

Websites like projecteuler.net [closed]

Sometimes I'm solving problems on projecteuler.net. Almost all problems are solvable with programs, but these tasks are more mathematical than programmatical.

Maybe someone knows similar sites with:

  • design tasks,
  • architecture tasks,
  • something like "find most elegant C++ solution"?

Source: (StackOverflow)

How many parameters are too many? [closed]

Routines can have parameters, that's no news. You can define as many parameters as you may need, but too many of them will make your routine difficult to understand and maintain.

Of course, you could use a structured variable as a workaround: putting all those variables in a single struct and passing it to the routine. In fact, using structures to simplify parameter lists is one of the techniques described by Steve McConnell in Code Complete. But as he says:

Careful programmers avoid bundling data any more than is logically necessary.

So if your routine has too many parameters or you use a struct to disguise a big parameter list, you're probably doing something wrong. That is, you're not keeping coupling loose.

My question is, when can I consider a parameter list too big? I think that more than 5 parameters, are too many. What do you think?

Source: (StackOverflow)

Missing return statement in a non-void method compiles

I encountered a situation where a non-void method is missing a return statement and the code still compiles. I know that the statements after the while loop are unreachable(dead code) and would never be executed. But why doesn't the compiler even warn about returning something? Or why would a language allow us to have a non-void method having an infinite loop and not returning anything?

public int doNotReturnAnything() {
    while(true) {
        //do something
    //no return statement

If I add a break statement(even a conditional one) in the while loop, the compiler complains of the infamous errors: 'Method does not return a value'(Eclipse) and 'Not all code paths return a value'(Visual Studio)

public int doNotReturnAnything() {
    while(true) {
        if(mustReturn) break;
        //do something
    //no return statement

This is true of both Java and C#

Source: (StackOverflow)

C state-machine design

I am crafting a small project in mixed C and C++. I am building one small-ish state-machine at the heart of one of my worker thread.

I was wondering if you gurus on SO would share your state-machine design techniques.

NOTE: I am primarily after tried & tested implementation techniques.

UPDATED: Based on all the great input gathered on SO, I've settled on this architecture:

alt text

Source: (StackOverflow)

How to make a vertical line in HTML

How Do you make a vertical line using HTML?

Source: (StackOverflow)