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今天讲Objective-C的两个小知识点，分别是NSDictionary和NSNull，NSDictionary接上节，我们讲NSDictionary的检索，可变的NSDictionary，NSMutilateDictionary。

Querying Dictionaries

Once you’ve created a dictionary, you can ask it for the object stored against a given key, like this:

NSNumber *storedNumber = [dictionary objectForKey:@"magicNumber"];

If the object isn’t found, the objectForKey: method will return nil.

There’s also a subscript syntax alternative to using objectForKey:, which looks like this:

NSNumber *storedNumber = dictionary[@"magicNumber"];

Mutability

If you need to add or remove objects from a dictionary after creation, you need to use the NSMutableDictionary subclass, like this:

[dictionary setObject:@"another string" forKey:@"secondString"];
[dictionary removeObjectForKey:@"anObject"];

Represent nil with NSNull

It’s not possible to add nil to the collection classes described in this section because nil in Objective-C means “no object.” If you need to represent “no object” in a collection, you can use the NSNull class:

NSArray *array = @[ @"string", @42, [NSNull null] ];

NSNull is a singleton class, which means that the null method will always return the same instance. This means that you can check whether an object in an array is equal to the shared NSNull instance:

for (id object in array) {
	if (object == [NSNull null]) {
		NSLog(@"Found a null object");
	}
}