Most efficient way to instantiate large number of subclasses in Java?

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).