Adding sub classes to array lists then advanced for

Question

private ArrayList<Doctor> doctors = new ArrayList<Doctor>();

doctors.add(new Doctor());
        doctors.add(new Doctor());
        doctors.add(new Doctor());

        doctors.add(new Surgeon());
        doctors.add(new Surgeon());
        doctors.add(new Surgeon());

for (Doctor doctor: doctors) {
            if (doctor.getAssignedPatient() != null) {
                if (doctor.aDayPasses()) {
                    System.out.println("A " + convertSpecialism(doctor.getSpecialism()) + " treated their patient.");
                    shortBreak();
                }
            } else {
                break;
            }
        }

Works fine however when I try to do this:

        for (Surgeon doctor: doctors) {
            if (doctor.getAssignedPatient() != null) {
                if (doctor.aDayPasses()) {
                    System.out.println("A " + convertSpecialism(doctor.getSpecialism()) + " treated their patient.");
                    shortBreak();
                }
            } else {
                break;
            }
        }

There is a syntax error, how do I for loop through the surgeons I have added to the ArrayList of type doctor.

Assume that Surgeon extends doctor.

No it's the surgeon that extends doctor: public class Surgeon extends Doctor — Thahleel al-Aleem
– Thahleel al-Aleem, Commented Dec 7, 2014 at 19:36
Type mismatch: cannot convert from element type Doctor to Surgeon, at for (Surgeon doctor: doctors) { — Thahleel al-Aleem
– Thahleel al-Aleem, Commented Dec 7, 2014 at 19:38
Yes, user2177940, you can't do what you're trying to do in the second snippet. What's wrong with the code in the first snippet? — Dawood ibn Kareem
– Dawood ibn Kareem, Commented Dec 7, 2014 at 19:39
Ah, misread the problem statement, I thought the last code was the one that worked. — Keppil
– Keppil, Commented Dec 7, 2014 at 19:39

manzur · Accepted Answer · 2014-12-07 19:48:03Z

2

In two words: you can't. Because ArrayList contains Doctor, and you can't iterate that list as a list of Surgeons as Java doesn't support implicit downcasting. It is the same as assigning Doctor to Surgeon without explicitly downcasting.

So if you want to get Surgeon, you should explicitly convert it to Surgeon like this:

   for(Doctor d :doctors){
        if (d instanceof Surgeon){
            Surgeon s = (Surgeon) d;
            ...
        }
   }

But that's very bad practice and you shouldn't be doing it.

answered Dec 7, 2014 at 19:48

manzur

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1 Comment

Dawood ibn Kareem Over a year ago

This won't make any difference to the result of any of the method calls that look like doctor.something().

DavidGSola · Accepted Answer · 2014-12-07 19:40:09Z

0

You can't create a for-each loop (for (SubClass subClass : superClasses)) because the compiler cannot ensure that there are only objects of type SubClass (Surgeon) in there.

answered Dec 7, 2014 at 19:40

DavidGSola

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Comments

Tkachuk_Evgen · Accepted Answer · 2014-12-07 19:45:29Z

0

You can do like this:

for (Doctor doctor : doctors) {
    if (doctor instanceof Surgeon) {
        if (doctor.getAssignedPatient() != null) {
            if (doctor.aDayPasses()) {
                System.out.println("A "
                    + convertSpecialism(doctor.getSpecialism())
                    + " treated their patient.");
                shortBreak();
            }
        } else {
            break;
        }
    }
}

answered Dec 7, 2014 at 19:45

Tkachuk_Evgen

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2 Comments

Thahleel al-Aleem Over a year ago

Is there no better way to do this rather than using instanceof?

Dawood ibn Kareem Over a year ago

No, you don't need to do this at all.

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