//空的字典
    NSMutableDictionary *dic = [NSMutableDictionary dictionary];
    //使用一个初始化指定的size创建可变字典
    int size = 10 ;
    NSMutableDictionary *dic = [NSMutableDictionary dictionaryWithCapacity:size];
    NSMutableDictionary *dic = [[NSMutableDictionary alloc] initWithObjectsAndKeys: @"obj1", @"key1", @"obj2", @"key2", nil];

//空的字典
    NSMutableDictionary *dic = [NSMutableDictionary dictionary];
    //使用一个初始化指定的size创建可变字典
    int size = 10 ;
    NSMutableDictionary *dic = [NSMutableDictionary dictionaryWithCapacity:size];
    NSMutableDictionary *dic = [[NSMutableDictionary alloc] initWithObjectsAndKeys: @"obj1", @"key1", @"obj2", @"key2", nil];