Class type as a parameter in Java

user_14554276 0
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I have a method:

private List<ApplicationForVacancy> createListOfApplicationsForVacancy(List<Document> documents) {
        return documents.stream()
                .filter(d -> d instanceof ApplicationForVacancy)
                .map(d -> (ApplicationForVacancy) d)
                .collect(Collectors.toList());
}

but I'd like to pass the name of the class/the type of the class in parameter and return type of the method like this:

private List<TypeOfSomeClass> createList (List<Document> documents, String or Type typeOfSomeClass) {
            return documents.stream()
                    .filter(d -> d instanceof typeOfSomeClass)
                    .map(d -> (typeOfSomeClass) d)
                    .collect(Collectors.toList());
}

Is it possible? How can I do that?

methods object oop java types
2 Answers

You have to provide a Class object to declare the type and add a type parameter to the method, like this:

private static <T> List<T> filterType(List<?> input, Class<T> clazz) {
    return input.stream()
            .filter(clazz::isInstance)
            .map(clazz::cast)
            .collect(Collectors.toList());
}

You then call the method like this:

List<Object> stuff = List.of("String", Integer.valueOf(3), Double.valueOf(42), new Object());
List<String> strings = filterType(stuff, String.class);
List<Number> numbers  = filterType(stuff, Number.class);

strings will only contain String and numbers will contain the 2 numeric values.

user_403420

To my knowledge you can't perform a type conversion like this. But you could create a new List instance at the start of your method, containing the specified type. Then loop through your existing list, filtering out all instances that should be contained in the result. Add those, then return the new List. Here some pseudo code.

public <T> List<T> filterInstances(List<?> input, Class<T> type){
  List<T> result = new ArrayList<>();
  for(Object o : input){
    if(type.isAssignableFrom(o.getClass()){
      result.add((T)o);
    }
  }
  return result;
}
user_34973380
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