iPhone interview questions and Answers

When I wrote following post of interview questions


I did not provide answers because my intention was that; you should know all these before showing up for interview. A lot of people suggested that I should also write answers for these. So, I am trying to answer these questions as I get time and I will be updating this post on regular basis.So, Keep in touch.

Difference between shallow copy and deep copy?

Ans: Shallow copy is also known as address copy. In this process you only copy address not actual data while in deep copy you copy data.

Suppose there are two objects A and B. A is pointing to a different array while B is pointing to different array. Now what I will do is following to do shallow copy.
Char *A = {‘a’,’b’,’c’};
Char *B = {‘x’,’y’,’z’};
B = A;
Now B is pointing is at same location where A pointer is pointing.Both A and B in this case sharing same data. if change is made both will get altered value of data.Advantage is that coping process is very fast and is independent of size of array.

while in deep copy data is also copied. This process is slow but Both A and B have their own copies and changes made to any copy, other will copy will not be affected.

What is advantage of categories? What is difference between implementing a category and inheritance? 

Ans: You can add method to existing class even to that class whose source is not available to you. You can extend functionality of a class without subclassing. You can split implementation in multiple classes. While in Inheritance you subclass from parent class and extend its functionality.

Difference between categories and extensions?

Ans:Class extensions are similar to categories. The main difference is that with an extension, the compiler will expect you to implement the methods within your main @implementation, whereas with a category you have a separate @implementation block. So you should pretty much only use an extension at the top of your main .m file (the only place you should care about ivars, incidentally) — it’s meant to be just that, an extension.

Difference between protocol in objective c and interfaces in java?

Ans:Protocol is also way to relate classes that are not related in inheritance hierarchy. Protocols and interfaces both are used to achieve multiple inheritance.

There is minor difference between these two. In Objective-C, protocols also implement NSObject protocol to access all the mehthods in NSObject

@protocol WebProtocol <NSObject>


If I don’t implement NSObject explicitly, I will not be able to access NSObject methods like retain, release etc. when I access through WebProtocol instance.

While in Java you don’t need to implement Object interface explicitly. It is implicitly implemented.

This link will help you. Thanks to Tom Jefferys.


What are KVO and KVC?

KVC: Normally instance variables are accessed through properties or accessors but KVC gives another way to access variables in form of strings. In this way your class acts like a dictionary and your property name for example “age” becomes key and value that property holds becomes value for that key. For example, you have employee class with name property.

You access property like

NSString age = emp.age;

setting property value.

emp.age = @”20″;

Now how KVC works is like this

[emp valueForKey:@”age”];

[emp setValue:@”25″ forKey:@”age”];

KVO : The mechanism through which objects are notified when there is change in any of property is called KVO.

For example, person object is interested in getting notification when accountBalance property is changed in BankAccount object.To achieve this, Person Object must register as an observer of the BankAccount’s accountBalance property by sending an addObserver:forKeyPath:options:context: message.

Can we use two tableview controllers on one view controller?

Ans: Yes, we can use two tableviews on the same view controllers and you can differentiate between two by assigning them tags…or you can also check them by comparing their memory addresses.

What is keyword atomic in Objective C?

Ans: When you place keyword atomic with a property, it means at one time only one thread can access it.

What are mutable and immutable types in Objective C?

Ans: Mutable means you can change its contents later but when you mark any object immutable, it means once they are initialized, their values cannot be changed. For example, NSArray, NSString values cannot be changed after initialized.

When we call objective c is runtime language what does it mean?

Ans: Objective-C runtime is runtime library that is open source that you can download and understand how it works. This library is written in C and adds object-oriented capabilities to C and makes it objective-c. It is only because of objective c runtime that it is legal to send messages to objects to which they don’t know how to respond to. Methods are not bound to implementation until runtime. Objective-C defers its decisions from compile time to run time as much as it can. For example, at runtime, it can decide to which object it will send message or function.

What is difference between NSNotification and delegate?

Ans:Delegate is passing message from one object to other object. It is like one to one communication while nsnotification is like passing message to multiple objects at the same time. All other objects that have subscribed to that notification or acting observers to that notification can or can’t respond to that event. Notifications are easier but you can get into trouble by using those like bad architecture. Delegates are more frequently used and are used with help of protocols.

Swap the two variable values without taking third variable?


int x=10;
int y=5;
NSLog(@”x==> %d”,x);

NSLog(@”Y Value==> %d”,y);

NSLog(@”x Value==> %d”,x);

Thanks to Subrat for answering this question.

What is push notification?

Imagine, you are looking for a job. You go to software company daily and ask sir “is there any job for me” and they keep on saying no.  Your time and money is wasted on each trip.(Pull Request mechanism)

So, one day owner says, if there is any suitable job for you, I will let you know. In this mechanism, your time and money is not wasted. (Push Mechanism)

How it works?

This service is provided by Apple in which rather than pinging server after specific interval for data which is also called pull mechanism, server will send notification to your device that there is new piece of information for you. Request is initiated by server not the device or client.

Flow of push notification

Your web server sends message (device token + payload) to Apple push notification service (APNS) , then APNS routes this message to device whose device token specified in notification.

What is polymorphism?

This is very famous question and every interviewer asks this. Few people say polymorphism means multiple forms and they start giving example of draw function which is right to some extent but interviewer is looking for more detailed answer.

Ability of base class pointer to call function from derived class at runtime is called polymorphism.

For example, there is super class human and there are two subclasses software engineer and hardware engineer. Now super class human can hold reference to any of subclass because software engineer is kind of human. Suppose there is speak function in super class and every subclass has also speak function. So at runtime, super class reference is pointing to whatever subclass, speak function will be called of that class. I hope I am able to make you understand.

Acknowledgement: Thanks to Adarsh Koppula,Nitu Chandra for answering on my previous post.

If anyone wants to adds something he/she is welcomed to do that.

Thanks to Waqar Iqabl for pointing out few omissions in previous post.