Objective-C Interview Questions and Answers (2024)

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 is the primary programming language used to develop software for Apple's macOS and iOS operating systems.

2. What are the key features of Objective-C?

   • Answer: Key features include:
     • Object-oriented programming (OOP): Encapsulation, inheritance, and polymorphism.
     • Dynamic runtime: Allows for dynamic method dispatch and late binding.
     • Message passing: Objects communicate by sending messages to each other.
     • Memory management: Uses reference counting or automatic reference counting (ARC).
     • Foundation framework: Provides a rich set of classes for common tasks like data structures, file handling, and networking.

3. Explain the concept of "objects" and "classes" in Objective-C.

   • Answer:
     • Class: A blueprint or template that defines the structure and behavior of objects. It specifies the data (instance variables) and methods (functions) that objects of that class will have.
     • Object: An instance of a class. It is a real-world entity that represents the data and behavior defined by its class.

4. What is a method in Objective-C?

   • Answer: A method is a function associated with a class that defines the behavior of objects of that class. Methods are invoked using the "dot" (.) operator. For example, [object methodName].

5. What are instance variables in Objective-C?

   • Answer: Instance variables are variables that belong to individual objects of a class. They store the data specific to each object. They are declared within the @interface section of a class definition.

6. Explain the difference between "instance methods" and "class methods" in Objective-C.

   • Answer:
     • Instance methods: Operate on individual objects of a class. They are invoked using the "dot" (.) operator on an object instance.
     • Class methods: Operate on the class itself. They are invoked using the "plus" (+) operator on the class name. They are often used for utility functions or to create objects of the class.

7. What is a protocol in Objective-C?

   • Answer: A protocol is a blueprint or interface that defines a set of methods that a class can implement. It allows for loose coupling between objects and provides a way to specify expected behavior without dictating implementation details.

8. What is the purpose of the "id" data type in Objective-C?

   • Answer: The id data type is a generic pointer that can hold a reference to any object. It is often used when the exact type of object is unknown or when a method can accept different types of objects as arguments.

9. Explain the difference between "strong" and "weak" references in Objective-C (ARC).

   • Answer:
     • Strong reference: The object is kept alive as long as the strong reference exists. If the last strong reference is removed, the object will be deallocated.
     • Weak reference: The object is not kept alive by the weak reference. The weak reference will become nil when the last strong reference to the object is removed.

10. What is the purpose of the "dealloc" method in Objective-C?

    • Answer: The dealloc method is called by the runtime when an object is about to be deallocated. It is used to release any resources held by the object, such as memory, file handles, or network connections.

11. What is the difference between "mutable" and "immutable" objects in Objective-C?

    • Answer:
      • Mutable objects: Can be modified after they are created. They have methods for adding, removing, or changing their contents.
      • Immutable objects: Cannot be modified after they are created. They provide a safe and predictable way to work with data.

12. Explain the concept of "delegation" in Objective-C.

    • Answer: Delegation is a design pattern where one object (the delegate) handles the messages or events that are sent to another object (the delegator). It allows for separating concerns and providing customization points.

13. What is a "category" in Objective-C?

    • Answer: A category is a mechanism for adding methods to an existing class without modifying its original source code. It allows you to extend the functionality of a class without subclassing.

14. What is a "block" in Objective-C?

    • Answer: A block is a lightweight, anonymous function that can be passed around like a value. They are used to create closures and handle asynchronous operations.

15. What is the difference between "NSInteger" and "int" in Objective-C?

    • Answer:
      • NSInteger: An integer data type that is compatible with the platform's native integer size (32-bit or 64-bit). It ensures that your code behaves correctly on different architectures.
      • int: A standard C integer data type, which may have a different size depending on the architecture.

16. What is the purpose of the "Foundation" framework in Objective-C?

    • Answer: The Foundation framework provides a wide range of classes that are essential for developing applications in Objective-C. It includes classes for data structures, strings, collections, files, networking, and more.

17. What is the difference between "NSArray" and "NSMutableArray" in Objective-C?

    • Answer:
      • NSArray: An immutable array that cannot be modified after creation.
      • NSMutableArray: A mutable array that can be modified by adding, removing, or changing its elements.

18. What is the difference between "NSDictionary" and "NSMutableDictionary" in Objective-C?

    • Answer:
      • NSDictionary: An immutable dictionary that cannot be modified after creation.
      • NSMutableDictionary: A mutable dictionary that can be modified by adding, removing, or changing key-value pairs.

19. Explain the concept of "KVO (Key-Value Observing)" in Objective-C.

    • Answer: KVO is a mechanism for observing changes in the values of an object's properties. An observer can register to be notified when a specific property of an object changes.

20. What are the different ways to handle errors in Objective-C?

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    • Answer:
      • Exceptions: Throwing and catching exceptions for unexpected errors.
      • Error objects: Returning an error object to indicate failure.
      • Boolean return values: Returning a Boolean value to indicate success or failure.

21. What is the difference between "static" and "global" variables in Objective-C?

    • Answer:
      • Static variables: Have file scope, meaning they are only accessible within the file they are defined. They retain their values between function calls.
      • Global variables: Have global scope, meaning they are accessible from anywhere in the program. They are also retained between function calls.

22. What is the difference between "NSLog()" and "printf()" in Objective-C?

    • Answer:
      • NSLog(): An Objective-C function for logging messages to the console. It formats the output automatically and includes timestamps.
      • printf(): A standard C function for formatted output. It requires manual formatting and does not include timestamps.

23. What is a "GCD (Grand Central Dispatch)" in Objective-C?

    • Answer: GCD is a framework for managing concurrent tasks in Objective-C. It provides a way to execute tasks asynchronously on multiple threads and manage dependencies between tasks.

24. What is a "dispatch queue" in GCD?

    • Answer: A dispatch queue is a data structure that holds a collection of tasks to be executed. Tasks are executed in a FIFO (First-In, First-Out) order.

25. What is the difference between a "serial dispatch queue" and a "concurrent dispatch queue" in GCD?

    • Answer:
      • Serial dispatch queue: Executes tasks one at a time, ensuring that only one task is running at any given moment.
      • Concurrent dispatch queue: Can execute multiple tasks concurrently, allowing for parallel processing.

26. What is a "dispatch group" in GCD?

    • Answer: A dispatch group is a mechanism for coordinating the execution of multiple tasks and notifying when all tasks in the group have completed.

27. Explain the difference between "nonatomic" and "atomic" properties in Objective-C.

    • Answer:
      • Atomic: Provides thread safety by ensuring that access to the property is synchronized. It guarantees that reads and writes to the property will be atomic.
      • nonatomic: Does not provide thread safety. Access to the property is not synchronized, which can lead to data corruption in multithreaded environments.

28. What is the purpose of the "property" keyword in Objective-C?

    • Answer: The property keyword is used to declare properties for a class. Properties are accessors (getter and setter methods) that provide a controlled interface for accessing instance variables. They provide a mechanism for encapsulation and allow you to manage memory management and thread safety.

29. What are the different ways to create an object in Objective-C?

    • Answer:
      • alloc/init: Use the alloc method to allocate memory for the object and then the init method to initialize its instance variables.
      • new: This method is a convenience method that combines allocation and initialization.
      • [ClassName alloc] initWith... : This method allows you to provide specific initialization parameters to the object.

30. Explain the concept of "inheritance" in Objective-C.

    • Answer: Inheritance is a mechanism for creating new classes (subclasses) based on existing classes (superclasses). Subclasses inherit the properties and methods of their superclasses and can add their own unique features. It allows for code reuse and promotes a hierarchical organization of classes.

31. What is the difference between "super" and "self" in Objective-C?

    • Answer:
      • super: Refers to the superclass of the current class. It is used to invoke methods of the superclass when overriding them in a subclass.
      • self: Refers to the current object instance. It is used to access properties and methods of the current object.

32. What is the purpose of the "NSObject" class in Objective-C?

    • Answer: NSObject is the root class of all objects in Objective-C. It provides fundamental methods that are common to all objects, such as memory management, object comparison, and string representation.

33. What is a "selector" in Objective-C?

    • Answer: A selector is a unique identifier that represents a method. It is used by the runtime to find the correct implementation of a method at runtime. Selectors are declared using the @selector directive.

34. Explain the concept of "polymorphism" in Objective-C.

    • Answer: Polymorphism refers to the ability of objects of different classes to respond to the same message (method call) in different ways. It is achieved through method overriding, where a subclass provides its own implementation of a method inherited from its superclass.

35. What is a "convenience initializer" in Objective-C?

    • Answer: A convenience initializer is a special type of initializer that calls another initializer of the same class or its superclass. It is used to provide alternative ways to initialize an object with different parameters or to simplify the initialization process.

36. What is a "singleton" in Objective-C?

    • Answer: A singleton is a design pattern that ensures that a class has only one instance and provides a global point of access to that instance. It is often used for managing shared resources or centralizing application logic.

37. How do you implement a singleton in Objective-C?

    • Answer:
      • Create a static instance variable of the class within the class definition.
      • Create a class method to provide access to the shared instance.
      • In the class method, check if the shared instance is nil. If it is, create a new instance of the class and assign it to the static instance variable.
      • Return the static instance variable.

38. What is the difference between "mutableCopy" and "copy" in Objective-C?

    • Answer:
      • mutableCopy: Creates a mutable copy of an object. The copy can be modified after creation.
      • copy: Creates an immutable copy of an object. The copy cannot be modified after creation.

39. What is the difference between "retain" and "assign" in Objective-C (ARC)?

    • Answer:
      • retain: Increments the retain count of the object, keeping it alive.
      • assign: Simply assigns the pointer to the object without affecting its retain count.

40. What is the difference between "autorelease" and "release" in Objective-C?

    • Answer:
      • autorelease: Decrements the retain count of the object, but does not immediately deallocate it. The object is placed in an autorelease pool, which will release it later when the pool is drained.
      • release: Decrements the retain count of the object, potentially deallocating it if the retain count reaches zero.

41. What is an "autorelease pool" in Objective-C?

    • Answer: An autorelease pool is a mechanism for delaying the deallocation of objects. Objects that are autoreleased are added to an autorelease pool, and the pool will release them all at once when it is drained. Autorelease pools are used to ensure that objects are not deallocated prematurely and to simplify memory management.

42. What is the difference between "getter" and "setter" methods in Objective-C?

    • Answer:
      • Getter: A method that provides access to the value of a property. It is invoked using the dot (.) operator.
      • Setter: A method that allows you to change the value of a property. It is invoked using the dot (.) operator and assignment.

43. What is the purpose of the "synthesize" keyword in Objective-C?

    • Answer: The synthesize keyword tells the compiler to automatically generate the getter and setter methods for a property. It simplifies the implementation of properties and reduces boilerplate code.

44. What is the difference between "IBOutlet" and "IBAction" in Objective-C?

    • Answer:
      • IBOutlet: An attribute that connects an object in a storyboard or NIB file to an instance variable in your code. It allows you to access and manipulate UI elements programmatically.
      • IBAction: An attribute that connects an event in a storyboard or NIB file to a method in your code. It allows you to handle user interactions, such as button clicks or text field changes.

45. What is a "delegate" in Objective-C?

    • Answer: A delegate is an object that is responsible for handling events or messages for another object. It allows for separating concerns and providing customization points. For example, a table view delegate handles events related to the table view, such as row selection or cell configuration.

46. What is the difference between "performSelector" and "performSelector:withObject:" in Objective-C?

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    • Answer:
      • performSelector: Invokes a method on an object without passing any arguments.
      • performSelector:withObject: Invokes a method on an object and passes a single argument to the method.

47. What is a "notification" in Objective-C?

    • Answer: A notification is a mechanism for broadcasting events to interested objects. It allows for loose coupling between objects and provides a way for objects to communicate without knowing each other directly. Notifications are often used to respond to system events, such as application launch or low memory warnings.

48. What is the difference between "NSNotificationCenter" and "NSUserDefaults" in Objective-C?

    • Answer:
      • NSNotificationCenter: Used for broadcasting events and notifying interested objects.
      • NSUserDefaults: Used for storing small amounts of user preferences or application data that needs to persist across application launches.

49. What is the difference between "Core Data" and "SQLite" in Objective-C?

    • Answer:
      • Core Data: An object-oriented framework for managing persistent data. It provides an abstraction layer over SQLite and offers features such as object-relational mapping, data validation, and undo/redo.
      • SQLite: A lightweight, embedded relational database engine that is used by Core Data. It provides a lower-level interface for working with data directly.

50. What is the difference between "NSURLSession" and "NSURLConnection" in Objective-C?

    • Answer:
      • NSURLSession: The modern, recommended API for performing network requests in Objective-C. It provides a more efficient and flexible approach to network communication.
      • NSURLConnection: A legacy API for network requests. While still functional, NSURLSession is generally preferred.

51. What is a "NSOperationQueue" in Objective-C?

    • Answer: An NSOperationQueue is an object that manages the execution of a group of NSOperation objects. It allows you to schedule and prioritize operations and handle dependencies between them. It is a powerful tool for managing asynchronous tasks and performing concurrent operations.

52. What is the difference between "NSOperation" and "NSThread" in Objective-C?

    • Answer:
      • NSOperation: An object-oriented wrapper for a block of code to be executed asynchronously. It provides a more structured approach to managing concurrent tasks than threads.
      • NSThread: A lower-level abstraction for creating and managing threads. It provides more control over thread execution but requires more manual management.

53. What is a "view controller" in Objective-C?

    • Answer: A view controller is an object that manages a view or a set of views in a user interface. It is responsible for handling user input, updating the view, and managing the lifecycle of the view.

54. What is the difference between "UIViewController" and "UIView" in Objective-C?

    • Answer:
      • UIViewController: A class that manages a view or a set of views and handles user interactions.
      • UIView: A class that represents a rectangular area on the screen that can be drawn on and interacted with.

55. What is the "view lifecycle" in Objective-C?

    • Answer: The view lifecycle refers to the sequence of methods that are called on a view controller when it is created, presented, and dismissed. These methods allow you to initialize the view, handle transitions, and respond to user interactions.

56. What are some common view controller transitions in Objective-C?

    • Answer: Common transitions include:
      • Push: Pushing a new view controller onto the navigation stack.
      • Pop: Removing the top view controller from the navigation stack.
      • Modal: Presenting a view controller over the current view controller.
      • Custom: Implementing your own custom transitions using the UIViewControllerAnimatedTransitioning protocol.

57. What is the purpose of the "UITableView" class in Objective-C?

    • Answer: UITableView is a class that displays a list of items in a scrolling view. It is highly customizable and provides features for handling row selection, cell configuration, and data management.

58. What is the purpose of the "UITableViewCell" class in Objective-C?

    • Answer: UITableViewCell is a class that represents a single row in a UITableView. It contains the UI elements that are displayed for each item in the list. You can customize the appearance and behavior of table view cells to display different types of data.

59. What is the difference between "UITableViewDataSource" and "UITableViewDelegate" in Objective-C?

    • Answer:
      • UITableViewDataSource: A protocol that defines methods for providing data to the table view. It is responsible for determining the number of rows, the content of each row, and the configuration of each cell.
      • UITableViewDelegate: A protocol that defines methods for handling user interactions with the table view. It is responsible for responding to row selection, editing, and other user events.

60. What is the purpose of the "UICollectionView" class in Objective-C?

    • Answer: UICollectionView is a class that displays a collection of items in a scrolling view. It is similar to UITableView but provides more flexibility for customizing the layout and appearance of the items. It is ideal for creating complex layouts, such as grids or staggered layouts.

61. What is the purpose of the "UICollectionViewCell" class in Objective-C?

    • Answer: UICollectionViewCell is a class that represents a single item in a UICollectionView. It contains the UI elements that are displayed for each item in the collection. You can customize the appearance and behavior of collection view cells to display different types of data.

62. What is the difference between "UICollectionViewDataSource" and "UICollectionViewDelegate" in Objective-C?

    • Answer:
      • UICollectionViewDataSource: A protocol that defines methods for providing data to the collection view. It is responsible for determining the number of items, the content of each item, and the configuration of each cell.
      • UICollectionViewDelegate: A protocol that defines methods for handling user interactions with the collection view. It is responsible for responding to item selection, editing, and other user events.

63. What is the purpose of the "UIScrollView" class in Objective-C?

    • Answer: UIScrollView is a class that enables scrolling content that is larger than the screen. It provides features for bouncing, zooming, and paging. UITableView and UICollectionView are subclasses of UIScrollView.

64. What is the purpose of the "UITextField" class in Objective-C?

    • Answer: UITextField is a class that represents a single-line text input field. It allows users to enter and edit text and provides features for handling text editing, validation, and formatting.

65. What is the purpose of the "UITextView" class in Objective-C?

    • Answer: UITextView is a class that represents a multi-line text view. It allows users to enter and edit large amounts of text and provides features for handling text editing, formatting, and scrolling.

66. What is the purpose of the "UIButton" class in Objective-C?

    • Answer: UIButton is a class that represents a button that users can tap to trigger an action. It provides various styles and customization options and allows you to handle button taps using target-action mechanisms.

67. What is the purpose of the "UIImageView" class in Objective-C?

    • Answer: UIImageView is a class that displays an image in a view. It provides options for scaling, resizing, and animating images.

68. What is the purpose of the "UILabel" class in Objective-C?

    • Answer: UILabel is a class that displays a text string in a view. It provides options for text alignment, font, and color.

69. What is the purpose of the "UIAlertController" class in Objective-C?

    • Answer: UIAlertController is a class that displays an alert dialog to the user. It provides options for adding titles, messages, actions, and text fields. Alerts are used to display important information or to prompt the user for input.

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