Java从父类中检索子类对象

Question

Is it possible to write a method which allows me to take in a List of objects belonging to a Parent class Person .

Under Person class, there are several subclasses, which includes Employee class.

I want the method to return a separate List which consists of just the Employee objects from the original list.

Thank you

Answer 1

You need to do it by steps :

1. Iterate on the List<Person to check all of them
2. If the current element is en Employee you need to cast it as and keep it
3. Return the list of keeped Employee

---

1. Classic way with foreach-loop

public static List<Employee> getEmployeeListFromPersonList(List<Person> list) {
    List<Employee> res = new ArrayList<>();
    for (Person p : list) {                 // 1.Iterate
        if (p instanceof Employee) {        // 2.Check type
            res.add((Employee) p);          // 3.Cast and keep it
        }
    }
    return res;
}

---

2. Java-8 way with Streams

public static List<Employee> getEmployeeListFromPersonList(List<Person> list) {
    return list.stream()                            // 1.Iterate
               .filter(Employee.class::isInstance)  // 2.Check type
               .map(Employee.class::cast)           // 3.Cast
               .collect(Collectors.toList());       // 3.Keep them
}

Answer 2

Do you mean something like:

List<Employee> getEmployees(List<Person> personList){
    List<Employee> result = new ArrayList<Employee>();

    for(Person person : personList){
        if(person instanceof Employee) result.add((Employee)person);
    }

    return result;
}