[objective-c] Difference between objectForKey and valueForKey?

I'll try to provide a comprehensive answer here. Much of the points appear in other answers, but I found each answer incomplete, and some incorrect.

First and foremost, objectForKey: is an NSDictionary method, while valueForKey: is a KVC protocol method required of any KVC complaint class - including NSDictionary.

Furthermore, as @dreamlax wrote, documentation hints that NSDictionary implements its valueForKey: method USING its objectForKey: implementation. In other words - [NSDictionary valueForKey:] calls on [NSDictionary objectForKey:].

This implies, that valueForKey: can never be faster than objectForKey: (on the same input key) although thorough testing I've done imply about 5% to 15% difference, over billions of random access to a huge NSDictionary. In normal situations - the difference is negligible.

Next: KVC protocol only works with NSString * keys, hence valueForKey: will only accept an NSString * (or subclass) as key, whilst NSDictionary can work with other kinds of objects as keys - so that the "lower level" objectForKey: accepts any copy-able (NSCopying protocol compliant) object as key.

Last, NSDictionary's implementation of valueForKey: deviates from the standard behavior defined in KVC's documentation, and will NOT emit a NSUnknownKeyException for a key it can't find - unless this is a "special" key - one that begins with '@' - which usually means an "aggregation" function key (e.g. @"@sum, @"@avg"). Instead, it will simply return a nil when a key is not found in the NSDictionary - behaving the same as objectForKey:

Following is some test code to demonstrate and prove my notes.

- (void) dictionaryAccess {
    NSLog(@"Value for Z:%@", [@{@"X":@(10), @"Y":@(20)} valueForKey:@"Z"]); // prints "Value for Z:(null)"

    uint32_t testItemsCount = 1000000;
    // create huge dictionary of numbers
    NSMutableDictionary *d = [NSMutableDictionary dictionaryWithCapacity:testItemsCount];
    for (long i=0; i<testItemsCount; ++i) {
        // make new random key value pair:
        NSString *key = [NSString stringWithFormat:@"K_%u",arc4random_uniform(testItemsCount)];
        NSNumber *value = @(arc4random_uniform(testItemsCount));
        [d setObject:value forKey:key];
    }
    // create huge set of random keys for testing.
    NSMutableArray *keys = [NSMutableArray arrayWithCapacity:testItemsCount];
    for (long i=0; i<testItemsCount; ++i) {
        NSString *key = [NSString stringWithFormat:@"K_%u",arc4random_uniform(testItemsCount)];
        [keys addObject:key];
    }

    NSDictionary *dict = [d copy];
    NSTimeInterval vtotal = 0.0, ototal = 0.0;

    NSDate *start;
    NSTimeInterval elapsed;

    for (int i = 0; i<10; i++) {

        start = [NSDate date];
        for (NSString *key in keys) {
            id value = [dict valueForKey:key];
        }
        elapsed = [[NSDate date] timeIntervalSinceDate:start];
        vtotal+=elapsed;
        NSLog (@"reading %lu values off dictionary via valueForKey took: %10.4f seconds", keys.count, elapsed);

        start = [NSDate date];
        for (NSString *key in keys) {
            id obj = [dict objectForKey:key];
        }
        elapsed = [[NSDate date] timeIntervalSinceDate:start];
        ototal+=elapsed;
        NSLog (@"reading %lu objects off dictionary via objectForKey took: %10.4f seconds", keys.count, elapsed);
    }

    NSString *slower = (vtotal > ototal) ? @"valueForKey" : @"objectForKey";
    NSString *faster = (vtotal > ototal) ? @"objectForKey" : @"valueForKey";
    NSLog (@"%@ takes %3.1f percent longer then %@", slower, 100.0 * ABS(vtotal-ototal) / MAX(ototal,vtotal), faster);
}