hashmap

问题 I have a json file with same key but different values as follows, { "domains" : { "A" : { "name" : "a", "type" : "a1" }, "B" :{ "name" : "r", "type" : "g1" }, "A" : { "name" : "b", "type" : "b1" } } } which is coming from external system . How to convert the json to java map object and access the different values of the key: A I am using something like below, map = mapper.readValue(json, new TypeReference<HashMap<String,String>>(){}); which returns a map with unique keys. But I need a map

问题 I am java class and I really concern about performance. There are some methods which has Property as their input parameter and those methods are invoked several time in my execution. Will Replacing Property with Hashmap help to increase performance ? Hashmap is sufficient in my scenario. 回答1: java.util.Properties extends java.util.Hashtable , so it is a kind of Hashtable. Below, there are couple differences relevant for your case: java.util.Hashtable is synchronized, but java.util.HashMap is

问题 I have the main class Library where I create: HashMap<String, HashSet<String>> students_books = new HashMap<String, HashSet<String>>(); Then I'll have class Student where I'll make a constructor who will take the HashMap as a parameter like this: public class Student { private Student(HashMap students_books){ then, back in my main class (Library) I create a student object where I want to put the HashMap as parameter: Student student = new Student(*HashMap as parameter*); What I'm not finding

问题 I already know that we use HashMap when we have Key-Value pair and we use HashSet when we do not need repetitive data. But I always get confused where Hashtable comes into play. Is it a totally different data structure? Or both HashMap and HashSet use hash function to store data in Hashtable ? Thorough explanation will be really helpful . 回答1: More specific, HashSet uses internally a HashMap , look at the implementation of HashSet.java private transient HashMap<E,Object> map; As stated in a

问题 I'm documenting some code, and I have a private HashMap. I'd like to specify information about what is expected from the key and value. Right now I have: /** * HashMap where key=word, value=part of speech */ private HashMap<String, String> dictionary; However, this seems hard to read, and also like it won't work well when I have something more complex like HashMap<String, HashMap<String, String>> What are best/common practices for documenting maps? 回答1: If you need a small javadoc, I suggest

问题 I want to know how to extract a List<D> from a HashMap<E,R> considering these constraints: E is a custom class; R is a custom class containing a Set<D> of custom objects; What I have tried: I tried addressing the issue in this question. In that previous case, I had a simple Map<E,List<R>> , but in this case I have to access the R class which has the targeted Set<D> . What I want to do in the following portion of the code is to get the Elements of the Set<D> whose country names are equal to

问题 We are working with an in memory data grid (IMDG) and we have a migration tool. In order to verify that all the objects are migrated successfully, we calculate the chucksum of the objects from its serialized version. We are seeing some problems with HashMap, where we serialize it, but when we deserialize it the checksum changes. Here is a simple test case: @Test public void testMapSerialization() throws IOException, ClassNotFoundException { TestClass tc1 = new TestClass(); tc1.init(); String

问题 In a hashmap, when we have the same hashcodes, we insert objects as a linked list which are later converted to TreeNode. Every New object with the same hashcode is added to the last of the linked list attached. So, my question here is why don't we add the new element as first element of the internal linked list attached to the bucket? Why do we traverse till the last element, and then add the new element. Time taken by Linked list to: Insert New element at start = O(1) Insert New element at

问题 I have a static HashMap that is initialized on server startup. Clients initialize their data from this map when they login. Now I need to refresh this map, but clients can login and get data from this map at the same time. Can I change reference of map like below while they read? I cant use synchronized because they can read in at the same time and only one thread is writing. public void refresh() { Map<String, Object> newMap = prepareData(); map = newMap; } 回答1: Lets assume that "refresh"

问题 I have a static HashMap that is initialized on server startup. Clients initialize their data from this map when they login. Now I need to refresh this map, but clients can login and get data from this map at the same time. Can I change reference of map like below while they read? I cant use synchronized because they can read in at the same time and only one thread is writing. public void refresh() { Map<String, Object> newMap = prepareData(); map = newMap; } 回答1: Lets assume that "refresh"