Call constructor in an abstract class

You can't call an abstract class constructor with a class instance creation expression, i.e.

// Invalid
AbstractClass x = new AbstractClass(...);

However, in constructing an object you always go through the constructors of the whole inheritance hierarchy. So a constructor from a subclass can call the constructor of its abstract superclass using super(...). For example:

public class Abstract {
    protected Abstract(int x) {
    }
}

public class Concrete {
    public Concrete(int x, int y) {
        super(x); // Call the superclass constructor
    }
}

As constructors of abstract classes can only be called within subclass constructors (and by chaining one to another within the same class), I typically make them protected... making them public would serve no purpose.

The normal rules apply if you don't specify a super(...) or this(...) call in a concrete subclass constructor - it's equivalent to a super(); statement at the start of a constructor, calling a parameterless constructor in the superclass... so there'd have to be such a constructor.