Wsrtjtyk

I am working on a project that requires the use of a large number of objects of classes extending a particular abstract class. Instantiating manually requires a call to each object's individual constructor, which is starting to pollute my client class.

The raw code from the project is too lengthy to post on this site, so I have a simplified example below:

Suppose there is an abstract class called Letter:

public abstract class Letter{

    String letter;

}

And 26 individual letter classes that extend the Letter abstract class:

public class A extends Letter{

    this.letter = "A";

}



public class B extends Letter{

    this.letter = "B";

}

... and so on

This is where I seek help - There is a client class called Alphabet:

public class Alphabet{

    public List<Letter> getLetters(){

        List<Letter> letters = new ArrayList<>();

        A a = new A();

        B b = new B();

        C c = new C();

        list.add(a);

        list.add(b);

        list.add(c);

        ...the list goes on



        return letters;

    }

}

As you can see, it is not ideal to instantiate each letter manually. I am looking for a way to create a list of class objects dynamically at runtime.

Perhaps something similar to this:

public class Alphabet{

    public List<Letter> getLetters(){

        List<Letter> letters = getAllClassesExtending(Letter.class);

        return letters;

        //list would then contain A,B,C,D,E,F,G, etc.

    }

}

I ultimately want to be able to create a new class and have it automatically added to a list of objects in the client class, without needing to modify the client class (e.g. I do not want to explicitly reference each letter class in the client class).

This sounds like a typical use case for an enum. The constants of an enum can have a class body with code, overriding methods of the base type, and even declare new fields.

The task of getting an instance of all subtypes implies a closed set of subtypes, which matches the behavior of enum types.

public enum Letter {

    A {

        @Override

        public boolean isVowel() {

            return true;

        }

    },

    B, C, D,

    E {

        @Override

        public boolean isVowel() {

            return true;

        }

    },

    F, G, H,

    I {

        @Override

        public boolean isVowel() {

            return true;

        }

    }, J, K, L, M, N,

    O {

        @Override

        public boolean isVowel() {

            return true;

        }

    }, P, Q, R, S, T,

    U {

        @Override

        public boolean isVowel() {

            return true;

        }

    }, V, W, X, Y, Z;

    ;

    String letter = name();



    public static List<Letter> getLetters() {

        return Arrays.asList(values());

    }



    public boolean isVowel() {

        return false;

    }

}

This demonstrates that overriding methods is possible and also a method for getting all instances (as an array) comes for free. Further, the names are specified intrinsically without redundancy.

If you want to have an open set of subclasses instead, to be potentially extended after compiling the code, you should look at the Service Provider concept and the ServiceLoader class. This mechanism will not automatically find subclasses but only those declared as service providers either within a file in META-INF or through the module declararation, on the other hand, it provides you with a robust, established solution that will be maintained by the Java developers or even evolve with the platform, like the integration into the new modules concept.

You can use Reflections library to find the classes in runtime, e.g. if all of them are located under com.letters package and have a no-arg constructor:

public List<? extends Letter> getLetters(){

  Reflections reflections = new Reflections("com.letters");

  Set<Class<? extends Letter>> classes = reflections.getSubTypesOf(Letter.class);

  return classes.stream()

        .map(clazz -> {

            try {

                return clazz.newInstance();

            } catch (InstantiationException | IllegalAccessException ex) {

                throw new RuntimeException(ex);

            }

        })

        .collect(Collectors.toList());

}

It's not the most efficient way to lookup classes, you might want to cache the results if this code is on your critical path.