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Objective-C interview questions

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Interview questions for your next interview

What is difference between NSArray and NSMutableArray in Objective C?
Mutable means you can change its contents later, Immutable means once they are initialized, their values cannot be changed. So we cannot change NSArray's value after its initialization.
Whats the difference between frame and bounds?
The frame is the rectangle that defines the UIView with respect to its superview. The bounds of an UIView is the rectangle that is relative to its own coordinate system.

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By writing code show how can we achieve singleton pattern in Objective C?
+ (id)sharedObject {
static id shared;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
shared = [[self alloc] init];
return shared;
What is atomic and nonatomic? Which one is default?
You can use this attribute to specify that accessor methods are not atomic. It will ensure the present process is completed by the CPU, before another process accesses the variable. nonatomic specifies that accessors are nonatomic. It is faster and not thread-safe.

By default, accessors are atomic.
What does the following line says about my Object?id myObject;
It says that myObject conforms to the Painting protocol.
How can you declare a method, that can be set as the action to be performed on various events?
— (IBAction) action:(id) sender;
Write a function that will take date as NSString and converts it to NSDate object.
NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
[dateFormat setDateFormat:@"yyyy-MM-dd"];
NSDate *date = [dateFormat dateFromString:dateString];
return date;
Calculate total number of words and character in a given NSString.
NSString *text = @“Hello World“;

NSArray *words = [text componentsSeparatedByCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
NSInteger wordCount = [words count];

NSInteger characterCount = 0;
for (NSString *word in words) {
characterCount += [word length];
Sort a NSMutableArray with custom objects in it. (Assume custom array have type Person objects, sort these according to their date of birth)
NSArray *sortedArray;
sortedArray = [originalArray sortedArrayUsingComparator:^NSComparisonResult(id a, id b) {
NSDate *first = [(Person*)a birthDate];
NSDate *second = [(Person*)b birthDate];
return [first compare:second];

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