hashmap

问题 I know that in Java 8 HashMap was optimized for poorly distributed hashCode . And in cases when threshold was exceeded it rebuilds nodes in bucket from linked list into tree. Also it is stated that this optimization doesn't work for not comparable keys (at leas performance is not improved). In the example below I put not Comparable keys into HashMap import java.util.HashMap; import java.util.Map; import java.util.concurrent.TimeUnit; import java.util.stream.IntStream; class Main { public

问题 I have a data structure which has, <Book title>, <Author>, and <rate> Since Book title or Author can be duplicated, I'd like to build a composite key. (let's say I cannot make extra unique key, such as ID) Since data is pretty huge, I'm using GCC unordered_map for the sake of speed, and I built my structure like this: typedef pair<string, string> keys_t typedef unordered_map<keys_t, double> map_t; Everything works okay in general, But the problem happens when I want to refer one specific key.

问题 I have added the functionality to the app I am designing to get the List of all "Job_Class" objects from my Firebase database, this is working fine, I can get all the jobs. The issue is I need to use the collection of jobs I have retrieved however I am getting the error: java.util.HashMap cannot be cast to (packages removed).Job_Class This error occurs at the for loop you will see in the code below. If anyone could help me in being able to actually use the jobs retrieved I would be very

问题 When we add values to hashmap<Key, Value> variable using put() , are they always ordering? Because when I tried with simple codes they are ordering. Example: Map<Integer, Integer> ctrArMap = new HashMap<Integer, Integer>(); ctrArMap.put( 1, 11); ctrArMap.put( 2, 12); ctrArMap.put( 3, 13); ctrArMap.put( 4, 14); ctrArMap.put( 5, 15); System.out.println(ctrArMap); But in my case they aren't ordering. 回答1: HashMap :- HashMap never preserves your Insertion Order. It Internally Use a hashing

问题 I'm practicing java by building a simple directory. I have 4 Classes. These are: Person Address Contact testClass I have already finished creating this system and it works the way I want it. I did this by making 3 different arrays for the Person, Address and Contact. To link the Person, Address and Contact together I place them on there respective array with the same index number. (Not literally linking them together, just a way to know which address or contact to access when editing a person

问题 I have the following code to count the instances of different strings in an array; String words[] = {"the","cat","in","the","hat"}; HashMap<String,Integer> wordCounts = new HashMap<String,Integer>(50,10); for(String w : words) { Integer i = wordCounts.get(w); if(i == null) wordCounts.put(w, 1); else wordCounts.put(w, i + 1); } Is this a correct way of doing it? It seems a bit long-winded for a simple task. The HashMap result is useful to me because I will be indexing it by the string. I am

问题 I was studying about internal structure of Hash Map where i came across following details: Implementation is an array of HashMap$Entry objects: Each HashMap$Entry contains: – int KeyHash – Object next – Object key – Object Value Default capacity is 16 entries Empty size is 128 bytes Overhead is 48 bytes for HashMap, plus (16 + (entries * 4bytes)) for array – Plus overhead of HashMap$Entry objects Additional 32bytes per key ↔ value entry Overhead of HashMap is therefore: – 48 bytes, plus 36

问题 I am currently using 2 for loops to compare all entries but I am getting duplicate comparisons. Because HashMaps aren't ordered, I can't figure out how to eliminate comparisons that have already been made. For example, I have something like: for(Entry<String, String> e1: map.entrySet()) { for(Entry<String, String> e2: map.entrySet()) { if (e1.getKey() != e2.getKey()) { //compare e1.getValue() to e2.getValue() } } } The problem with this is that the first entry will be compared to the second

问题 The best look-up structure is a HashTable . It provides constant access on average (linear in worst case). This depends on the hash function. Ok. My question is the following. Assuming a good implementation of a HashTable e.g. HashMap is there a best practice concerning the keys passed in the map?I mean it is recommended that the key must be an immutable object but I was wondering if there are other recommendations. Example the size of the key? For example in a good hashmap (in the way

问题 I want that the string of keys must act as header and the list must be inflated under that map key in the RecyclerView . thanks for any help public class Adapter extends RecyclerView.Adapter<RecyclerView.ViewHolder>{ private WeakHashMap<String, List<VideoItem>> mData = new WeakHashMap<>(); private ArrayList<String> mKeys; ArrayList<WeakHashMap<String,List<VideoItem>>> hashMapArrayList; public Adapter(WeakHashMap<String, List<VideoItem>> mData, ArrayList<WeakHashMap<String,List<VideoItem>>>