hashmap | 易学教程

“Cannot create generic array of ..” - how to create an Array of Map<String, Object>?

阅读更多关于 “Cannot create generic array of ..” - how to create an Array of Map?

问题 I would like to use simpleJdbcInsert class and executeBatch method public int[] executeBatch(Map<String,Object>[] batch) http://static.springsource.org/spring/docs/2.5.x/api/org/springframework/jdbc/core/simple/SimpleJdbcInsert.html So I need to pass an array of Map<String,Object> as parameter. How to create such an array? What I tried is Map<String, Object>[] myArray = new HashMap<String, Object>[10] It is error: Cannot create generic array of Map<String, Object> A List<Map<String, Object>>

Iterating through a LinkedHashMap in reverse order

阅读更多关于 Iterating through a LinkedHashMap in reverse order

问题 I have a LinkedHashMap: LinkedHashMap<String, RecordItemElement> that I need to iterate through from a given key's position, backwards. So if I was given the 10th item's key, I'd need iterate backwards through the hashmap 9, 8, 7 etc. 回答1: You don't have to iterate through it. But it would be handy to pull the keys off and store it in a list. Thats the only way you can do indexOf() type operations. List<String> keyList = new ArrayList<String>(map.keySet()); // Given 10th element's key String

Iterating through a LinkedHashMap in reverse order

阅读更多关于 Iterating through a LinkedHashMap in reverse order

How to create a HashMap with two keys (Key-Pair, Value)?

阅读更多关于 How to create a HashMap with two keys (Key-Pair, Value)?

问题 I have a 2D array of Integers. I want them to be put into a HashMap. But I want to access the elements from the HashMap based on Array Index. Something like: For A[2][5], map.get(2,5) which returns a value associated with that key. But how do I create a hashMap with a pair of keys? Or in general, multiple keys: Map<((key1, key2,..,keyN), Value) in a way that I can access the element with using get(key1,key2,...keyN). EDIT : 3 years after posting the question, I want to add a bit more to it I

Is there a clean way to avoid calling a method on nil in a nested params hash? [duplicate]

阅读更多关于 Is there a clean way to avoid calling a method on nil in a nested params hash? [duplicate]

问题 This question already has answers here : How to avoid NoMethodError for missing elements in nested hashes, without repeated nil checks? (17 answers) Closed 3 years ago . I'm interested in getting the nested 'name' parameter of a params hash. Calling something like params[:subject][:name] throws an error when params[:subject] is empty. To avoid this error I usually write something like this: if params[:subject] && params[:subject][:name] Is there a cleaner way to implement this? 回答1: Check Ick

Finding Key associated with max Value in a Java Map

阅读更多关于 Finding Key associated with max Value in a Java Map

问题 What is the easiest way to get key associated with the max value in a map? I believe that Collections.max(someMap) will return the max Key, when you want the key that corresponds to the max value. 回答1: Basically you'd need to iterate over the map's entry set, remembering both the "currently known maximum" and the key associated with it. (Or just the entry containing both, of course.) For example: Map.Entry<Foo, Bar> maxEntry = null; for (Map.Entry<Foo, Bar> entry : map.entrySet()) { if

What happens when a duplicate key is put into a HashMap?

阅读更多关于 What happens when a duplicate key is put into a HashMap?

问题 If I pass the same key multiple times to HashMap ’s put method, what happens to the original value? And what if even the value repeats? I didn’t find any documentation on this. Case 1: Overwritten values for a key Map mymap = new HashMap(); mymap.put("1","one"); mymap.put("1","not one"); mymap.put("1","surely not one"); System.out.println(mymap.get("1")); We get surely not one . Case 2: Duplicate value Map mymap = new HashMap(); mymap.put("1","one"); mymap.put("1","not one"); mymap.put("1",

Does Java have a HashMap with reverse lookup?

阅读更多关于 Does Java have a HashMap with reverse lookup?

问题 I have data that is organized in kind of a "key-key" format, rather than "key-value". It's like a HashMap, but I will need O(1) lookup in both directions. Is there a name for this type of data structure, and is anything like this included in Java's standard libraries? (or maybe Apache Commons?) I could write my own class that basically uses two mirrored Maps, but I'd rather not reinvent the wheel (if this already exists but I'm just not searching for the right term). 回答1: There is no such

How to preserve insertion order in HashMap? [duplicate]

阅读更多关于 How to preserve insertion order in HashMap? [duplicate]

问题 This question already has answers here : Java Class that implements Map and keeps insertion order? (9 answers) Closed 4 years ago . I'm using a HashMap . When I iterate over the map, the data is returned in (often the same) random order. But the data was inserted in a specific order, and I need to preserve the insertion order. How can I do this? 回答1: LinkedHashMap is precisely what you're looking for. It is exactly like HashMap , except that when you iterate over it, it presents the items in

HashMap 的底层原理

阅读更多关于 HashMap 的底层原理

HashMap 的底层原理 1. HashMap的数据结构数组数组存储区间是连续的，占用内存严重，故空间复杂的很大。但数组的二分查找时间复杂度小，为O(1)；数组的特点是：寻址容易，插入和删除困难；链表链表存储区间离散，占用内存比较宽松，故空间复杂度很小，但时间复杂度很大，达O（N）。链表的特点是：寻址困难，插入和删除容易。哈希表哈希表（(Hash table）既满足了数据的查找方便，同时不占用太多的内容空间，使用也十分方便。哈希表是由数组+链表组成的。例如：一个长度为16的数组中，每个元素存储的是一个链表的头结点。那么这些元素是按照什么样的规则存储到数组中呢。一般情况是通过hash(key)%len获得，也就是元素的key的哈希值对数组长度取模得到。比如哈希表中，12%16=12,28%16=12,108%16=12,140%16=12。所以12、28、108以及140都存储在数组下标为12的位置。 HashMap其实也是一个线性的数组实现的,所以可以理解为其存储数据的容器就是一个线性数组。这可能让我们很不解，一个线性的数组怎么实现按键值对来存取数据呢？这里HashMap有做一些处理。首先HashMap里面实现一个静态内部类Entry，其重要的属性有 key , value, next，从属性key