comparison difference object in one arraylist

Question

need help.Have create arraylist and then add multiple object(in my case,add 2 type object inside arraylist).Then sort each object inside arraylist .Able to sort but I think my coding is too long.Is their any way or method to make it simple and effective.Hope you guys can help.That all,hope you can help.Thank you.

public class Sample {


    static ArrayList<Object> object = new ArrayList<>();

    public static void main(String[] args){


        //initialize object
        User user1 = new User("user1","1");
        User user2 = new User("user1","3");
        Staff staf1 = new Staff("staf1","2");
        Staff staf2 = new Staff("staf2","4");

        //add object to arraylist
        object.add(user1);
        object.add(user2);
        object.add(staf1);
        object.add(staf2);


        //sort
        Collections.sort(object,new Comparator<Object>() {
            Object one,two;

            @Override
            public int compare(Object o1, Object o2) {

                User user1,user2;
                Staff staf1,staf2;

                ///identify ol class
                if(o1 instanceof User){
                   user1 = (User) o1;
                   one = (Object) user1;
                }else if(o1 instanceof Staff){
                    staf1 = (Staff) o1;
                    one = (Object) staf1;
                }

                //identify o2 class
                if(o2 instanceof User){
                     user2 = (User) o2;
                     two = (Object) user2;

                }else if(o2 instanceof Staff){
                    staf2 = (Staff) o2;
                    two = (Object) staf2;
                }

                //identify each object class
                //then compare value
                if(one instanceof User && two instanceof User){
                    //execute process
                    User userOne = (User) one;
                    User userTwo = (User) two;
                    return userOne.getRegister().compareTo(userTwo.getRegister());

                }else if(one instanceof Staff && two instanceof Staff){
                     //execute process
                    Staff stafOne = (Staff) one;
                    Staff stafTwo = (Staff) two;
                    return stafOne.getRegister().compareTo(stafTwo.getRegister());

                }else if(one instanceof User && two instanceof Staff){
                     //execute process 
                    User userOne = (User) one;
                    Staff stafTwo = (Staff) two;
                     return userOne.getRegister().compareTo(stafTwo.getRegister());
                }else if(one instanceof Staff && two instanceof User){
                     //execute process
                    Staff stafOne = (Staff) one;
                    User userTwo = (User) two;
                    return  stafOne.getRegister().compareTo(userTwo.getRegister());
                }else{
                    return 0;
                }
            }

        });

        //display item
        for(Object object : object){   
           if(object instanceof User){
               User user = (User)object;
               //display data
               System.out.println(user.getRegister());
           }else if(object instanceof Staff){
               Staff staf = (Staff) object;
               //display data
               System.out.println(staf.getRegister());
           }
        }


    }
}

Answer 1

As your objects have the same fields and methods I am recomend you to use inheritance int this situation. First you have to exclude one abstract class which be a parent of yours two classes:

public abstract class Parent implements Comparable<Parent> { 

    abstract Integer getRegister();

    @Override
    public int compareTo(final Parent parent) {
        return this.getRegister().compareTo(parent.getRegister());
    }

}

Then you have to make, that your classes User and Staff will extends Person class. Then, you can create list: List<Parent> list and sort it by its own comparator. So assuming there are matchinbg constructors available, User extends Parent and Staff extends Parent:

final List<Parent> list = new ArrayList<>();
list.add(new User("user1","1"));
list.add(new User("user1","3"));
list.add(new Staff("staf1","2"));
list.add(new Staff("staf2","4"));
Collections.sort(list);

EDIT (as OP don't want to use inheritance):

Instead of checking few times, do it only once and get register field in moment you know about type.

@Override
public int compare(Object o1, Object o2) {
    Integer register1 = null;
    Integer register2 = null;

    if(o1 instanceof User) {
        register1 = ((User)o1).getRegister();
    } else if(o1 instanceof Staff) {
        register1 = ((Staff)o1).getRegister();
    }

    if(o2 instanceof User) {
        register2 = ((User)o2).getRegister();
    } else if(o1 instanceof Staff) {
        register2 = ((Staff)o2).getRegister();
    }

    if(register1 != null && register2 != null) {
        return register1.compareTo(register2);
    }
    return 0;
}