NSDictionary中的键可以复制吗？ [英] Keys in NSDictionary can be duplicated?

问题描述

根据我所阅读的内容，字典中的键是唯一的。

From what I have read, keys in dictionaries are unique.

请考虑以下代码：

NSMutableDictionary *mydic = [NSMutableDictionary dictionary];

[mydic setObject:@"value1" forKey:@"key1"]; 
[mydic setObject:@"value1" forKey:@"key1"];
[mydic setObject:@"value1" forKey:@"key1"];

为什么我可以运行这个没有任何错误？

Why can I run this without any error? What should I do to avoid duplicate keys?

推荐答案

是的键是唯一的。调用 -setObject：forKey：使用现有键覆盖旧值 - 设置值，而不是添加值。您可以检查：

Yes keys are unique. Calling -setObject:forKey: with an existing key overrides the old value — it sets values, not adds values. You can check that:

[mydict setObject:@"1" forKey:@"key1"];
[mydict setObject:@"2" forKey:@"key1"];
NSLog(@"%@", mydict);

如果不希望覆盖现有项目，请检查其是否存在 -objectForKey：：

If you don't want existing items to be overridden, check if it exists with -objectForKey::

@implementation NSMutableDictionary (AddItem)
-(void)addObjectWithoutReplacing:(id)obj forKey:(id)key {
   if ([self objectForKey:key] == nil)
      [self setObject:obj forKey:key];
}
@end

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