What is the use of creating a constructor for an abstract class in Java?

Question

I would like to know what purpose a constructor for an abstract class serves; as we do not instantiate abstract classes, why would we ever need such a constructor?

Answer 1

You don't instantiate abstract classes but the constructor is invoked when a subclass is instantiated.

The use could be to initialize common attributes ie.

import java.util.List;
import java.util.ArrayList;
abstract class BaseClass {
    protected List list; // visible from subclasses

    public BaseClass() {
        System.out.println("to abstract...");
        // common initialization to all subclasses
        list = new ArrayList();
        list.add("a");
        list.add("a");
        list.add("a");
    }
}

class ConcreteClass extends BaseClass {
    public ConcreteClass(){
        // The list is initialized already
        System.out.println("now it is concrete and the list is: = "+ this.list );


    }
}

class TestAbstractClass {
    public static void main( String [] args ) {
        BaseClass instance = new ConcreteClass();
    }

}

Output

$ java TestAbstractClass
to abstract...
now it is concrete and the list is: = [a, a, a]