How can I create an instance using a member in ArrayList<Class>

Question

I created an ArrayList to store Classes. Then I would like to use the members in the ArrayList to create an instance. How can I instantiate?

import java.util.ArrayList;

class Machine{
    public String toString(){
        return "I am a machine";
    }
}

class MyMachine extends Machine{
    public String toString(){
        return "This is a super Machine";
    }
}


class MyClass<T> {
    public static <T> void showString(T abc){
        System.out.println(abc);
    }
}

public class myfun {
    public static void main(String[] args) {
        ArrayList<Class> hahalist = new ArrayList<>();

        hahalist.add(MyClass.class);
        hahalist.add(Machine.class);
        hahalist.add(MyClass.class);

        // Machine abc = new Machine();
        // I can't do this
        // abc = hahalist.get(1);

        // I can't do this either
        // hahalist.get(1) abc = new hahalist.get(1)()
    }
}

Answer 1

If you have a no args constructor on all you classes you can just use the Class.newInstance(), eg

try {
    Object x = hahaList.get(0).newInstance(); 
} catch (InstantiationException | IllegalAccessException e) {
    throw new RuntimeException(e); 
}

(I haven't got my IDE open so haven't verified the exact syntax).

If you've got arguments to the constructor then its a bit more convoluted. You can read the full details here: https://docs.oracle.com/javase/tutorial/reflect/member/ctorInstance.html

Answer 2

Instead of storing classes in the list, you could store references to the constructors:

List<Supplier<Object>> hahalist = new ArrayList<>();

hahalist.add(MyClass::new);
hahalist.add(Machine::new);
hahalist.add(MyMachine::new);

Object instance = hahalist.get(1).get(); // Calling constructor

Because MyClass and Machine have a closest common ancestor of Object that is the return type of the Supplier I'm using here.