Objective-C Interview Questions and Answers

1. What is Objective-C?

   • Answer: Objective-C is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. It's the primary language used for developing macOS and iOS applications.

2. Explain the difference between `@interface` and `@implementation`

   • Answer: `@interface` declares the class's public interface – its properties and methods. `@implementation` provides the actual code for the methods declared in the interface.

3. What is a protocol in Objective-C?

   • Answer: A protocol defines a set of methods that a class may choose to implement. It's similar to an interface in Java but doesn't require inheritance. It allows for polymorphism and loose coupling.

4. What is the difference between `strong`, `weak`, `assign`, and `copy` properties?

   • Answer:
     • `strong`: Retains a strong reference to the object, preventing it from being deallocated until the property is set to nil.
     • `weak`: Retains a weak reference to the object. The object is not retained; when the last strong reference is released, the object is deallocated.
     • `assign`: Assigns the object directly, without retaining it. Use only for non-object types or when you explicitly manage memory.
     • `copy`: Creates a copy of the object and retains that copy. Useful for immutable objects like NSString.

5. Explain the concept of memory management in Objective-C.

   • Answer: Objective-C traditionally used manual reference counting (MRC), where developers manually managed the retain and release of objects. ARC (Automatic Reference Counting) automates this process, reducing memory leaks and crashes. ARC tracks object references and automatically deallocates objects when they're no longer needed.

6. What is a category in Objective-C?

   • Answer: A category adds methods to an existing class without subclassing. It's useful for extending functionality without modifying the original class's implementation.

7. What is a class extension in Objective-C?

   • Answer: A class extension is a nameless category that is declared in the same file as the class implementation. It's used to add private methods and properties to a class.

8. What is the difference between a category and a class extension?

   • Answer: A category adds methods publicly, while a class extension adds methods privately, only accessible within the same implementation file. Both extend the functionality of a class without subclassing.

9. What is polymorphism in Objective-C?

   • Answer: Polymorphism allows objects of different classes to respond to the same method call in their own specific way. This is achieved through inheritance and protocols.

10. What is inheritance in Objective-C?

    • Answer: Inheritance is a mechanism where a class (subclass) inherits properties and methods from another class (superclass). It promotes code reusability and establishes an "is-a" relationship.

11. What is encapsulation in Objective-C?

    • Answer: Encapsulation bundles data (instance variables) and methods that operate on that data within a class. It protects data integrity and hides internal implementation details.

12. What is delegation in Objective-C?

    • Answer: Delegation is a design pattern where an object (the delegate) handles events or tasks on behalf of another object (the delegator). It decouples objects and improves code organization.

13. Explain the use of `id` in Objective-C.

    • Answer: `id` is a generic object pointer. It can point to any Objective-C object. It's often used when the exact type of an object isn't known or is not important.

14. What is a singleton in Objective-C?

    • Answer: A singleton is a design pattern that ensures only one instance of a class is created. It's often used for managing global resources or configurations.

15. How do you create a singleton in Objective-C?

    • Answer: Typically, you create a shared instance method that checks if an instance exists. If not, it creates one and returns it; otherwise, it returns the existing instance.

16. What is KVO (Key-Value Observing)?

    • Answer: KVO is a mechanism that allows an object to observe changes to a property of another object. When the observed property changes, the observer is notified.

17. What is KVC (Key-Value Coding)?

    • Answer: KVC is a mechanism that allows access to an object's properties using strings representing the property names. This is useful for data binding and introspection.

18. Explain the difference between KVO and KVC.

    • Answer: KVC provides access to properties, while KVO provides a mechanism to observe changes in properties. They often work together.

19. What are blocks in Objective-C?

    • Answer: Blocks are anonymous functions that can be passed as arguments to methods or used as variables. They provide a way to create closures and encapsulate code.

20. What is Grand Central Dispatch (GCD)?

    • Answer: GCD is a low-level concurrency API that provides a way to manage tasks and execute them concurrently on multiple cores. It simplifies multithreading and improves performance.

21. How do you perform asynchronous operations using GCD?

    • Answer: You use functions like `dispatch_async` to submit tasks to a dispatch queue. The task is executed asynchronously without blocking the main thread.

22. What are the different types of dispatch queues?

    • Answer: There are serial queues (execute tasks one at a time), concurrent queues (execute tasks concurrently), and the main queue (associated with the main thread).

23. What is a `nonatomic` property?

    • Answer: A `nonatomic` property means that access to the property is not atomic; multiple threads can access it simultaneously, potentially leading to data races. `atomic` (the default) provides thread safety but with a performance overhead.

24. What is the difference between `self` and `super`?

    • Answer: `self` refers to the current instance of the class, while `super` refers to the superclass. `super` is used to call methods in the superclass.

25. Explain the role of the `dealloc` method.

    • Answer: The `dealloc` method is called by the runtime just before an object is deallocated. It's used to release any resources held by the object (like memory or file handles) and perform any necessary cleanup.

26. What is the purpose of the `init` method?

    • Answer: The `init` method is the designated initializer of a class. It's responsible for initializing the object's instance variables and setting its initial state.

27. What is a property synthesized by the compiler?

    • Answer: When you declare a property with `@synthesize`, the compiler automatically generates getter and setter methods for that property.

28. What is a property declared with `@dynamic`?

    • Answer: A property declared `@dynamic` tells the compiler that the getter and setter methods will be implemented elsewhere (perhaps manually or at runtime).

29. How do you handle exceptions in Objective-C?

    • Answer: Objective-C uses `@try`, `@catch`, and `@finally` blocks to handle exceptions. This is less common than error handling with return codes in many Objective-C projects.

30. What are mutable and immutable objects?

    • Answer: Mutable objects can be changed after creation (e.g., NSMutableArray), while immutable objects cannot be changed after creation (e.g., NSArray).

31. Explain the difference between `isEqual:` and `==` operators.

    • Answer: `==` compares memory addresses, while `isEqual:` compares the content of the objects (you should implement this method in your custom classes).

32. What is NSCoding protocol?

    • Answer: The `NSCoding` protocol allows objects to be archived and unarchived. This is useful for persisting object data.

33. How do you implement NSCoding protocol?

    • Answer: You implement the `initWithCoder:` and `encodeWithCoder:` methods to encode and decode object data using an `NSCoder` object.

34. What is NSNotificationCenter?

    • Answer: `NSNotificationCenter` is a mechanism for broadcasting notifications to registered observers. It facilitates communication between different parts of an application.

35. How to post a notification?

    • Answer: You use `[[NSNotificationCenter defaultCenter] postNotificationName:object:userInfo:]` to post a notification with a name, an optional object, and a dictionary of user info.

36. How to observe a notification?

    • Answer: You use `[[NSNotificationCenter defaultCenter] addObserver:selector:name:object:]` to register an observer that will receive notifications matching the specified name and object.

37. What is `performSelector:`?

    • Answer: `performSelector:` is a method that allows you to call a method on an object at a later time or on a different thread.

38. What are the advantages and disadvantages of using `performSelector:`?

    • Answer: Advantages: Flexible for scheduling and delayed execution. Disadvantages: Can lead to runtime errors if the selector is invalid and lacks type safety.

39. What is the difference between `copy` and `mutableCopy`?

    • Answer: `copy` returns an immutable copy, while `mutableCopy` returns a mutable copy of the object.

40. What is the difference between shallow copy and deep copy?

    • Answer: A shallow copy creates a new object but copies only the references to the contained objects. A deep copy creates a new object and recursively copies all contained objects.

41. What are some common design patterns used in Objective-C?

    • Answer: Singleton, Delegate, Observer, Model-View-Controller (MVC), Factory, and more.

42. Explain the Model-View-Controller (MVC) design pattern.

    • Answer: MVC separates the application into three interconnected parts: the model (data and business logic), the view (user interface), and the controller (handles user input and updates the model and view).

43. What is a runtime in Objective-C?

    • Answer: The Objective-C runtime is a layer of code that manages the execution of Objective-C programs. It handles tasks like message passing, memory management, and object creation.

44. What is method swizzling?

    • Answer: Method swizzling is a technique that allows you to exchange the implementations of two methods at runtime. It's powerful but should be used cautiously.

45. What are some common ways to handle asynchronous network requests in Objective-C?

    • Answer: NSURLSession, NSURLConnection (deprecated), third-party libraries like AFNetworking.

46. How do you handle errors in network requests?

    • Answer: Check the response status code, handle potential exceptions, use error callbacks provided by the networking API.

47. What is the difference between synchronous and asynchronous operations?

    • Answer: Synchronous operations block execution until they complete. Asynchronous operations don't block; they return immediately, allowing the program to continue executing other tasks.

48. What are some ways to improve the performance of an Objective-C application?

    • Answer: Use ARC, optimize algorithms, avoid unnecessary object creation, use efficient data structures, profile the application.

49. What are some tools for debugging Objective-C code?

    • Answer: Xcode debugger, Instruments, logging, breakpoints.

50. How do you handle memory leaks in Objective-C?

    • Answer: Use ARC, use Instruments' Leaks tool, carefully manage object lifetimes, and use weak references where appropriate.

51. What is the difference between a pointer and a reference?

    • Answer: In Objective-C, pointers hold memory addresses while references are syntactic sugar that works in a similar way, but more easily used.

52. Explain the concept of a "zombie object".

    • Answer: A zombie object is an object that has been deallocated but its memory hasn't been overwritten yet. Accessing a zombie object leads to undefined behavior.

53. How can you prevent zombie objects?

    • Answer: Proper memory management (ARC or careful manual reference counting) and avoiding accessing objects after they've been deallocated.

54. What is the purpose of the `retainCount` method (and why is it generally avoided in ARC)?

    • Answer: `retainCount` returns the number of strong references to an object. It's generally avoided in ARC because relying on it can make code fragile and less readable; ARC handles reference counting automatically.

55. What is a selector in Objective-C?

    • Answer: A selector is a type representing a method name; it's used to invoke methods dynamically at runtime.

56. What is the role of the `respondsToSelector:` method?

    • Answer: `respondsToSelector:` checks whether an object can respond to a given selector (meaning it has a method with that name).

57. What is the difference between a class method and an instance method?

    • Answer: Class methods operate on the class itself (`+`), while instance methods operate on instances of the class (`-`).

58. How do you declare a class method?

    • Answer: Using a `+` sign before the method declaration.

59. How do you declare an instance method?

    • Answer: Using a `-` sign before the method declaration.

60. Explain the concept of "message passing" in Objective-C.

    • Answer: In Objective-C, method calls are actually messages sent to objects. The runtime determines which method to execute based on the message and the object's class.

61. What are some common ways to serialize and deserialize data in Objective-C?

    • Answer: `NSCoding` (for archiving), JSON serialization (using libraries like SBJson or built-in frameworks in later iOS versions), property lists.

62. What is a deadlock? How can you avoid deadlocks?

    • Answer: A deadlock occurs when two or more threads are blocked indefinitely, waiting for each other to release resources. Avoid deadlocks by careful resource ordering and avoiding circular dependencies.

63. What are some common Objective-C frameworks?

    • Answer: Foundation, UIKit (for iOS), AppKit (for macOS), Core Data, Core Graphics, etc.

64. What is the difference between a strong reference and a retain?

    • Answer: In ARC, `strong` is a property attribute that indicates a strong reference. `retain` was a method used in manual reference counting to increase the retain count; it is now mostly handled implicitly by ARC.

65. What are some best practices for writing clean and maintainable Objective-C code?

    • Answer: Use descriptive names, follow coding conventions, use proper indentation, write modular code, use version control.

66. How do you perform unit testing in Objective-C?

    • Answer: Use XCTest framework included with Xcode.

67. Explain the concept of a "first responder" in iOS.

    • Answer: The first responder is the UI element that is currently receiving user input events (touches, keyboard input, etc.).

68. What is the purpose of the `@property` directive?

    • Answer: `@property` declares properties for a class, which automatically create accessors (getters and setters) to manage instance variables.

69. What is the difference between `typedef` and `#define`?

    • Answer: `typedef` creates an alias for a data type; `#define` is a preprocessor directive that performs textual substitution.

70. Explain the use of `id` in method signatures.

    • Answer: When `id` appears in a method signature, it indicates that the method can accept or return any Objective-C object type.

71. What are some common uses of NSOperationQueue?

    • Answer: Managing concurrent operations, handling dependencies between operations, adding operations with priorities, cancelling operations.

72. What is the difference between `dispatch_async` and `dispatch_sync`?

    • Answer: `dispatch_async` performs the block asynchronously (non-blocking); `dispatch_sync` performs the block synchronously (blocking).

73. What are some strategies for handling long-running tasks on the main thread?

    • Answer: Move long-running tasks to background threads using GCD or NSOperationQueue; use progress indicators to provide feedback to the user.

74. How can you prevent race conditions in multithreaded applications?

    • Answer: Use synchronization primitives like locks, mutexes, semaphores, or atomic operations; design thread-safe data structures; minimize shared mutable state.

75. What is the purpose of the `NSRunLoop`?

    • Answer: `NSRunLoop` manages the execution of tasks on a thread; it handles events, timers, and other sources.

76. What are the benefits of using blocks over delegates for asynchronous operations?

    • Answer: Blocks can be more concise and easier to read for simple asynchronous tasks; they avoid the need to create separate delegate classes.

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