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Array sort not working, no instance(s) of variable(s) T exist so that Employee conforms to Comparable

By : Jwd
Date : September 15 2020, 11:00 AM
I hope this helps you . You need to implement Comparable interface and override its compareTo method like following:
code :
public class Employee implements Comparable<Employee> {
    String name;

    public Employee(String name) {
        this.name = name;

    public String toString() {
        return this.name;

    public int compareTo(Employee employee) {
        return this.name.compareTo(employee.name);
Comparator<Employee> employeeNameComarator = new Comparator<Employee>() {
    public int compare(Employee employee1, Employee employee2) {
        return employee1.name.compareTo(employee2.name);
Collections.sort(listOfEmployees, employeeNameComarator);

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No instance of type variable V,K exist so that HashMultiMap<V,K> conforms to MultiMap<..,..>

By : IllusionalQuestions
Date : March 29 2020, 07:55 AM
it should still fix some issue check your imports and class/interface names: MultiMap and HashMultimap should be imported from the same framework.
For instance Apache Collections has MultiMap and Guava has Multimap - typo and wrong import is likely to happen.

No instance(s) of type variable(s) U exist so that Optional<U> conforms to Response

By : Ojsmith
Date : March 29 2020, 07:55 AM
it fixes the issue Based on the error, the return type of your method is Response. However, update(resourceID, data).map(updatedResource -> Response.status(Response.Status.OK).entity(updatedResource).build()) returns an Optional, so you have to change the return type to Optional.
So the method would look like this:
code :
public Optional<Response> yourMethod (...) {
    return update(resourceID, data).map(updatedResource -> Response.status(Response.Status.OK).entity(updatedResource).build());
public Response yourMethod (...) {
    return update(resourceID, data).map(updatedResource -> Response.status(Response.Status.OK).entity(updatedResource).build()).orElse(defaultValue);

When using Collections.sort - no instance of Variable T exist so that Collection conforms etc

By : Stephen
Date : March 29 2020, 07:55 AM
I hope this helps you . As per request, here's a compilation of my comments to answer the question:
The immediate problem is that Collections.sort(List) takes a List parameter and not just a Collection because collections in general don't have to be sortable (e.g. hash sets aren't). Additionally the method returns void and sorts the passed list in place, i.e. the way you call it won't compile.
code :
public class GenreManager{
   private List<Genre> genres;

   public GenreManager(Collection<Genre> genres){

     //create a list out of the passed collection
     this.genres = new ArrayList<Genre>( genres );

     //sort the list    
public class GenreManager{
   private SortedSet<Genre> genres;

   public GenreManager(Collection<Genre> genres){
     this.genres = new TreeSet<>( genres );
public GenreManager(Collection<Genre> genres){
  this.genres = new TreeSet<>(); //the generic type is inferred from this.genres

  for( Genre element : genres ) {
    //If the element didn't exist in the set add() will return true, false if it existed  
    boolean nonDuplicate = this.genres.add( element );

    //handle the duplicate element here

no instance(s) of type variable(s) U exist so that Foo conforms to CompletionStage<U>

By : Shawn Zhang
Date : March 29 2020, 07:55 AM
seems to work fine The last thenCompose in the chain should be thenApply. The function passed to it is returning a GetUsersForAdAccountResponse which is not CompletionStage.
thenCompose is the flatMap of CompletableFuture. It takes a function that returns a CompletionStage and flattens the result.

No instance of type variable(s) T exist so that ID conforms to Comparable<? super T>

By : nielsAD
Date : March 29 2020, 07:55 AM
This might help you This error means the elements in the parameter Collection of method Collections.max should implement interface Comparable. So it can use the compareTo to find the max elements in the Collection.
You can make it compile by declaring it this way:
code :
public abstract class AbstractMapService<T, ID extends Comparable<ID>> 

private ID getNextId() {
    return Collections.max(map.keySet());
public abstract class AbstractMapService<T> {

    protected Map<Long, T> map = new HashMap<>();

    Set<T> findAll(){
        return new HashSet<>(map.values());

    T findById(Long aLong) {
        return map.get(aLong);

    T save(Long aLong, T object){
       map.put(aLong, object);
       return object;

    void deleteById(Long aLong){

    void delete(T object){
        map.entrySet().removeIf(entry -> entry.getValue().equals(object));

    private Long getNextId() {
        return Collections.max(map.keySet()) + 1L;
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