How to iterate over ArrayLists of different objects with a single method?

Question

I am trying to iterate over two ArrayList fields of different types. While I could write methods for each type, is there a way to iterate over them with only a single method? I can't quite seem to figure out if it is possible to pass on a field as a method argument. In the below code, the method iteratePhrase() is supposed to print all elements in either of the ArrayLists. In it, [...] signifies either numbers or letters.

import java.util.ArrayList;
import java.util.List;

public class Phrase {
    private List<Number> numbers;
    private List<Letter> letters;

    public Phrase() {
        numbers;
        letters;
    }

    public void iteratePhrase() {
        for (int i=0; i<[...].size(); i++)
            System.out.println([...].get(i));
    }
}

Elliott Frisch · Accepted Answer · 2019-04-27 19:46:35Z

3

You could make iteratePhrase generic, that is make it take a List<T>; like

public <T> void iteratePhrase(List<T> al) {
    for (int i = 0; i < al.size(); i++) {
        System.out.println(al.get(i));
    }
}

You could also use a for-each loop like,

for (T t : al) {
    System.out.println(t);
}

In either case, you would call

iteratePhrase(numbers);

and

iteratePhrase(letters);

answered Apr 27, 2019 at 19:46

Elliott Frisch

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2 Comments

theejazz Over a year ago

Thank you! It seems like I cannot call a method on the result of al.get(i) (presumably because its type is unknown?). Do you know how I would do (al.get(i)).call(), where call() is a method both numbers and letters have?

Elliott Frisch Over a year ago

Program to a common interface, change T in <T> to T extends Foo.

Idan Str · Accepted Answer · 2019-04-27 21:35:56Z

2

You can use java 8 stream:

public <T> void iteratePhrase(List<T> al) {
   al.stream.forEach(System.out::println);
}

answered Apr 27, 2019 at 21:35

Idan Str

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Comments

anon · Accepted Answer · 2019-04-28 10:03:30Z

1

You could make both numbers and letters extend a common class, for example IterableObject.

Then, iterate like this

public void iteratePhrase(List<IterableObject> l) {
    for (int i = 0; i < l.size(); i++) {
        System.out.println(l.get(i));
    }
}

or this

public void iteratePhrase(List<IterableObject> objects) {
    for (IterableObject object : objects) {
        System.out.println(object);
    }
}

However, iteratePhrase function will only accept objects, that extend IterableObject, unlike answer above with generics.

edited Apr 28, 2019 at 10:03

answered Apr 27, 2019 at 20:05

anon

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