Cracking the Array Interview: 50 Must-Know Questions and Solutions

Use our engineer-created questions to interview and hire the most qualified Objective-C developers for your organization.

Objective-C is still popular even though Swift has become the language of choice for macOS and iOS apps. This is because it works with both C and C, has a dynamic runtime, and has a mature ecosystem.

Our team has made functional programming tasks and discussion questions that are specifically designed to test developers’ Objective-C skills during coding tests. We have also put together a list of the best ways to make sure that your interview questions are accurate when testing candidates’ Objective-C knowledge.

Objective-C skills to assess

Question:Explain the concept of properties in Objective-C. What are the advantages of using properties over instance variables?

In Objective-C, properties make it easy to hide an object’s state and define the getter and setter methods that can be used to access and change that state. They are declared using the @property syntax and can have attributes like nonatomic, strong, weak, etc.

Advantages of using properties over instance variables include:

• Data encapsulation is possible with properties because they limit who can change an object’s state.
• Automatic Getter and Setter Methods: Properties generate getter and setter methods automatically, which cuts down on repetitive code and makes code easier to read.
• Memory Management: Properties with appropriate memory management attributes (e. g. , strong, weak, copy) automatically manage memory, making tasks related to memory management easier.
• Key-Value Observing (KVO): KVO lets you watch properties and let other objects know when the value of a property changes.
• Dot Notation Syntax: The dot notation syntax lets you access properties, which makes the code shorter and easier to read.

Which of the following is not the same as retain, strong, weak, or copy in Objective-C? When would you use each one?

Answer:

• The words “retain” and “strong” both mean to keep the object and make a strong connection to it. The difference lies in the context:
• In manual reference counting (MRC), retain is used to keep track of who owns an object by hand.
• In automatic reference counting (ARC), “strong” means a strong reference that doesn’t need to be managed by hand. In ARC, this is the default attribute that is often used for instance variables and properties.
• weak means that the reference is weak and doesn’t keep the object alive. It is often used to stop strong reference cycles between objects and break retain cycles.
• When you assign an object to a property, copy makes a copy of it. It’s useful when you want to make sure the value of the object doesn’t change.

The choice between retain/strong and weak depends on the ownership and lifetime requirements of the object. copy is used when you want to ensure the immutability of the object.

Question:Explain the concept of protocols in Objective-C and their usage. Provide an example of creating a protocol and implementing it in a class.

Answer:Protocols in Objective-C define a set of methods that a class can implement. They let you set a contract or interface that many classes can follow, which allows for polymorphism and code reuse.

Here’s an example of creating a protocol and implementing it in a class:

In this example, the Printable protocol is defined using the @protocol directive. It declares a single method, print. The MyClass class adopts the Printable protocol using the syntax. It must implement the required method print defined in the protocol.

Protocols allow polymorphism, which means that different classes can follow the same protocol and be used in place of each other based on the protocol contract.

What are Objective-C blocks, and how are they different from regular methods or functions? Give an example of how blocks can be used.

Answer:Blocks in Objective-C are self-contained, anonymous segments of code that can be passed around and executed later. They are similar to closures in other programming languages. Blocks hold variables and constants from the scope around them, enclosing both the code and the environment in which it was written.

Blocks are different from regular functions or methods because they can be treated as first-class objects. This means they can be stored in variables, passed as arguments, and returned from functions. They provide a way to create and pass behavior as data.

Here’s an example demonstrating the usage of blocks:

In this example, a block type CompletionBlock is defined using the typedef keyword. The performTaskWithCompletion: method takes a block as a parameter and executes it after performing a task. The block takes a BOOL parameter indicating the success status of the task.

Blocks are commonly used for asynchronous operations, callback mechanisms, and implementing higher-order functions.

What’s the point of using the @synthesize directive in Objective-C? When do you need it and when do you not?

Answer:The @synthesize directive in Objective-C is used to automatically generate the getter and setter methods for properties. Prior to Objective-C 2. 0, properties had to be explicitly synthesized in the implementation file using @synthesize propertyName = _propertyName;. However, since Objective-C 2. 0, the @synthesize directive is no longer required in most cases.

When a property is declared, the compiler automatically creates the right accessor methods and an instance variable with the same name as the property, but with an underscore (_) in front of it.

However, there are cases when explicit @synthesize is still useful:

• If you want to change the name of the instance variable that supports the property, you can synthesize it with @synthesize propertyName = _customName;
• You can explicitly synthesize one method and implement the other method by hand if you want to provide custom getter or setter implementations.

With modern Objective-C and automatic synthesis, the @synthesize directive is not needed and can be left out most of the time.

Question:Explain the concept of exceptions in Objective-C. How are exceptions different from error handling using NSError?

Answer:Exceptions in Objective-C provide a mechanism to handle exceptional conditions and unexpected runtime errors. They are thrown with the @throw statement, and @try, @catch, and @finally blocks can catch them and do something with them.

Exceptions differ from error handling using NSError in the following ways:

• Exceptions are used for rare events and runtime errors that were not expected, while NSErrors are usually used for expected errors or conditions that can be fixed.
• Similar to how exceptions are handled in other programming languages, @throw, @try, @catch, and @finally are used to throw and catch exceptions. NSError is usually sent as an error message through return values or as an argument.
• Exceptions can be caught at different levels of the call stack, which lets them be handled centrally. Usually, NSError is handled at the caller level, which gives clear error handling and recovery options.

It is important to remember that Objective-C exceptions should only be used very rarely and only for rare errors that can’t be fixed, like programming mistakes or invalid conditions. For expected errors and recoverable conditions, error handling using NSError is the preferred approach.

Question:What are the differences between NSMutableArray and NSArray in Objective-C? When would you use one over the other?

Answer:

• In Objective-C, NSArray is an immutable array, which means that once it is created, its contents cannot be changed. You can’t add, remove, or change objects in an NSArray after it has been initialized.
• On the other hand, NSMutableArray is a mutable array that lets you add, remove, and change objects on the fly.

You would use NSArray when you have a collection of objects that does not need to be modified. It provides better performance and memory efficiency since it does not need to support mutation operations.

NSMutableArray is used when you need to dynamically modify the array’s contents. It provides methods to add, remove, and replace objects at specific indexes.

To make sure that NSArray is immutable by default and to avoid unintended side effects, it is best to use it whenever possible and only switch to NSMutableArray when mutability is needed.

Question:Explain the concept of key-value coding (KVC) in Objective-C. How does it simplify accessing and manipulating object properties?

Answer: Key-Value Coding (KVC) is a feature in Objective-C that lets you access and change an object’s properties by using strings of keys. Instead of calling the getter and setter methods for properties directly, KVC gives you a single way to read and change property values on the fly.

KVC simplifies accessing and manipulating object properties by providing the following benefits:

• Dynamic Access: KVC lets you read and write properties even if you don’t know their names at compile time. This enables generic code and reduces coupling between classes.
• Collection Operators: KVC has collection operators like @sum, @avg, @max, @min, and @distinctUnionOfObjects that make working with groups of objects easier.
• Key-Value Observing (KVO): KVO is built on KVC, which lets objects see when certain properties of other objects change.
• Introspection and reflection: KVC allows introspection and reflection, which lets you look at an object’s properties as they change over time.

To use KVC, you can use the methods provided by NSObject such as valueForKey: and setValue:forKey: Additionally, KVC-compliant properties can be accessed using dot notation or subscripting.

Question:What is the role of the @autoreleasepool block in Objective-C memory management? When should you use it?

Answer:In Objective-C, the @autoreleasepool block is used to control the autorelease pool, which is in charge of controlling objects that have been automatically released. Things that are autoreleased will have their memory freed automatically when the current run loop iteration ends.

The @autoreleasepool block sets up a local autorelease pool that lets you manage how long autoreleased objects in the block last. When the block is exited, the autorelease pool releases the autoreleased objects, freeing up memory.

When you need to make a lot of temporary objects, like in a loop or an operation that takes a long time, you should use the @autoreleasepool block. By making a local autorelease pool, you can make sure that the memory used by autoreleased objects is freed up more often, which lowers the amount of memory that is used at peak times.

It’s important to remember that after Automatic Reference Counting (ARC) was introduced, managing autorelease pools by hand became a lot easier. In most cases, ARC handles autorelease pools automatically, and explicit use of @autoreleasepool blocks is not necessary. However, using @autoreleasepool blocks can still be helpful in situations where you have clear control over memory management, like in non-ARC code or performance-critical parts.

What is the difference between using dot notation and square bracket syntax to get to object properties and methods in Objective-C?

Answer: In Objective-C, you can access object properties and call methods using either dot notation or square bracket syntax. However, they are not the same and should not be used in the same situations.

Dot Notation:

• Dot notation is a shorthand syntax introduced in Objective-C 2. 0 that lets you get to properties directly by using the dot ( ) operator.
• It makes it easier to read and understand how to get or set property values, which makes the code more expressive.
• You can only use dot notation for properties that are set or retrieved with getter and setter methods.
• It is a feature that is added at compile time and doesn’t affect how the program runs.

Example:

Square Bracket Syntax:

• In Objective-C, square bracket syntax is the standard way to call methods and get to properties.
• It lets you call any method, even ones that don’t have getter or setter semantics, and send messages to objects.
• It’s more adaptable and can be used in more situations, like when passing multiple arguments, calling methods with selectors, or getting to elements in arrays.
• Using square brackets has effects on runtime, since the message dispatch takes place at runtime.

Example:

In general, dot notation is preferred for accessing properties when available since it enhances code readability. When you call a general method, pass arguments, or interact with objects dynamically at runtime, you use square bracket syntax.

Help us design a parking lot

Hey candidate! Welcome to your interview. Boilerplate is provided. Feel free to change the code as you see fit. To run the code at any time, please hit the run button located in the top left corner.

Goals: Design a parking lot using object-oriented principles

Here are a few methods that you should be able to run:

• Tell us how many spots are remaining
• As a whole, how many parking spots are there?
• Tell us when the parking lot is full
• Tell us when the parking lot is empty
• Tell us when certain spots are full e. g. when all motorcycle spots are taken.
• Tell us how many spots vans are taking up

Assumptions:

• The parking lot can hold motorcycles, cars and vans
• Some spots are for motorcycles, some are for cars, and some are big.
• A motorcycle can park in any spot
• A car can park in either a regular spot or a single tight spot.
• There is room for a van, but it will take up three space
• These are just a few assumptions. If you need to, you can ask your interviewer about more assumptions.

NSArray vs NSMutableArray

FAQ

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