Java - List, array type thing of different type objects?

Question

So I have a bunch of java classes that are structured like this

class Something {
    public static String name;
    public static String description;

    public void run(String[] args);
}

The problem is, they are all different types. So if I try to make a HashSet of them and iterate over them

HashSet<Object> things = new HashSet<Object>();
things.add(new Something());
things.add(new Otherthing());

things.forEach(thing -> {
    thing.run();
})

it gives this error

  symbol:   variable run
  location: variable thing of type java.lang.Object

So how can I make an iterable Set of these objects and use their properties?

Make an interface interface MyRunnable { void run(String[] args); } and have each of your classes implement it. Then use HashSet<MyRunnable>. — tgdavies
– tgdavies, Commented Dec 1, 2022 at 22:26
Have a read of docs.oracle.com/javase/tutorial/java/concepts/interface.html — tgdavies
– tgdavies, Commented Dec 1, 2022 at 22:28

hayly · Accepted Answer · 2022-12-02 00:03:12Z

1

They need to be of the same type to be used like that. Luckily, we can fix this by implementing a fitting functional interface.

public class Something implements Runnable {

    public String name;
    public String description;

    @Override
    public void run() {

        System.out.println("something");

    }

}

Runnable has the method run() that takes no arguments and returns nothing (aka it has to do something).

So this works only if you're not actually going to be using the String array as an argument, as you don't include it in your calling of run(). However, if you do need it, you must change the implemented interface to Consumer instead of Runnable, with accept() instead, as this method actually takes an argument as opposed to run().

public class Main {
    public static void main(String[] args) {

        HashSet<Runnable> things = new HashSet<>();
        things.add(new Something());
        things.add(new Something2());

        things.forEach(thing -> {
            thing.run();
        });

    }
}

Now, we set the HashSet to the type of the implemented interface, and it works!

Here's the other class:

public class Something2 implements Runnable {

    public String name;
    public String description;

    @Override
    public void run() { 

        System.out.println("something2");

    }

}

Now you can create as many classes as you want and have their methods do different things, while still being able to gather them in the same set(as long as they implement the same type)!

Side note: if you didn't need the attributes, you could've used lambda expressions to implement different versions of run() (or accept(), or any other functional interface-method).

edited Dec 2, 2022 at 0:03

answered Dec 1, 2022 at 23:53

hayly

262 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

shmosel Over a year ago

you must change the implemented interface to Consumer instead of Runnable - or create a custom interface. But if Runnable is adequate, note that the lambda can be simplified to forEach(Runnable::run).

Collectives™ on Stack Overflow

Java - List, array type thing of different type objects?

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related