如何在Java中将对象从超类转换为子类？

Question

A class Person has several subclasses including Employee .

There is a method which is supposed to return another List which consists of just the Employee objects from the original list of Person .

How do I accomplish this in Java?

Answer 1

Simple example for you:

public class Test {
public static void main(String[] args) {
    List<Person> lst = new ArrayList<Person>() {
        {
            add(new Person());
            add(new Person());
            add(new Employee());
            add(new Employee());
            add(new AnotherPerson());
            add(new AnotherPerson());
        }
    };

    List<Person> employes = lst.stream().filter(item -> item instanceof Employee).collect(Collectors.toList());
    System.out.println(employes.size());
}

class Person {

}

class Employee extends Person {

}

class AnotherPerson extends Person {

}
}

Answer 2

Since

Person has several subclasses

and

return another List which consists of just the Employee objects

You shall use instanceOf to accomplish looking for specifically looking for Employee class from a List<Person> . Just for eg:

List<Person> personList // assigned to something
List<Employee> employeeList = new ArrayList<>();
for(Person person : personList)  {
    if(person !=null && person instanceOf Employee) { 
        employeeList.add((Employee) person);
    }
}

The sample from java-nutsandbolts might help you.

When using the instanceof operator, keep in mind that null is not an instance of anything.

Answer 3

Other answers and comments should provide a solution, but if you want a generic solution, then you could do something like this:

public static <T extends Person> List<T> getPersonTypes(Class<T> cls, Collection<Person> c)
{
  return c.stream()
      .filter(Objects::nonNull)
      .filter(p -> cls.isAssignableFrom(p.getClass()))
      .map(cls::cast)
      .collect(Collectors.toList());
}

And then call something like this:

List<Employee> employees = getPersonTypes(Employee.class, listOfPersons);