Advertisements The signature of HashMap::entry is: pub fn entry(&mut self, key: K) -> Entry<‘_, K, V> Ok, looks good. I want to take an entry from the HashMap whose key is K, so of course I need to give it a K. However, the signature of HashMap:get is: pub fn get<Q>(&self, k: &Q) -> Option<&V>… Read More Why does HashMap::get and HashMap::entry take different types of key?
Advertisements I have been reading https://www.alibabacloud.com/blog/the-clever-design-of-java-map_597979 for the internal implementation of the HashMap and CHM. There is an interesting line for which I could not find more explanation for : Can someone point me to the bug this refers to? I could only find the bug related to HashMap improvement with RB-Trees, but nothing about… Read More What is the dead loop bug in HashMap in earlier versions?
Advertisements I’ve a list of payloads and a payload looks like Payload { public int id; public String name; public String foo; public List<String> list; } now I want this to a HashMap where the key is a value of the list property, like payloads = [ {id:45, name:"Alfred", foo:"bar", list:["AAA", "BBB"]}, {id:2, name:"John", foo:"various",… Read More Convert list of payloads with a property List<String> to HashMap<String, Payload)
Advertisements import java.util.*; public class Hash_Map_Demo { public static void main(String[] args) { Map<String, String> toEmails = Collections .singletonMap("key", "Value"); Map<String, String> userEmails = new HashMap<>(); userEmails.put("abc@gmail.com", "abc"); toEmails.putAll(userEmails); } } when i’m trying to run the above code, I’m encountering the following exception Exception in thread "main" java.lang.UnsupportedOperationException at java.util.AbstractMap.put(AbstractMap.java:209) at java.util.AbstractMap.putAll(AbstractMap.java:281) at Hash_Map_Demo.main(Hash_Map_Demo.java:13)… Read More Hashmap is unable to be added to a collecitons.singletonmap
Advertisements The purpose and goal of the method is to print the hash map in reverse from <Key, Value> to <Value, Key>. But I’m having a problem on line 7(revrse();) where where I get the error "The method reverse(Map<Integer,String>) in the type lab8_Part3 is not applicable for the arguments ()". All the solutions eclipse suggest… Read More Having trouble running public HashMap method
Advertisements I define a Triple class with equals and hashcode method. According the docs,HashSet.contains() method of Java HashSet must returns the same value of Objects.equals(). But contains() method of Java HashSet method return true, when it suppose to be false. class Triple{ private List<Integer> triple; public Triple(int one, int two, int three){ this.triple = new… Read More Java HashSet contains method return true when it supposes to be false
Advertisements In python, I’d use a list of dictionaries to collect data (like name, age, height, etc.) of several people that a user enters, but now I want to do the same thing in Java. Is an array of HashMaps the best way to do that? static List<Map<String, Object>> driverList = new ArrayList<>(); >Solution :… Read More Array of hashmaps
Advertisements I have a Map<String, Object> which I call myMap, and I want to iterate through each key and ensure that its value is either of type String, int & Boolean. I’ve tried something along these lines: myMap.entrySet().forEach(key -> myMap.get(key)); I don’t know how to do the actual check… >Solution : You can check all… Read More How do I type validate values in a map?
Advertisements I’m trying to check whether 2 HashMaps are equal using a custom function match. I expect the output to be true, but the program outputs false. Here is the code snippet. import java.util.HashMap; public class HashMapEquals{ private static boolean matches1(HashMap<Character, Integer> one, HashMap<Character, Integer> two){ for(char c: one.keySet()){ if(one.get(c) != two.get(c)) return false; }… Read More Custom function to check whether 2 HashMaps are equal is not working in Java
Advertisements I have the following Java 8 code: final Person[] personEntities = personRepository.getPersons(groupIds); Map<String, List<Person>> personMapByDepartmentId = new HashMap<>(); for (Person person: personEntities ) { // create hashmap:departmentId as the key, and person entities as the value } Person object is a standard POJO with fields Id, Name and departmentId What is the best way… Read More Most efficient way to create a hash map from a list of objects – with the object field as key?