Java abstract class fields override

Regarding

....and i IMAGINE that the field "child" in SpecialContainer is automatically overloading the field 'child' of the GenericContainer...

No. Fields don't get overridden, only methods do.

This is one reason why use of (overridable) getter and setter methods are preferred to direct access to fields. Your fields should almost all be private.

As for your design, there's no need for your SpecialContainer class to have a SpecialChild field, but instead the SpecialChild object should be placed in the GenericChild field.

In Java, data members/attributes are not polymorphic. Overloading means that a field will have a different value depending from which class it's accessed. The field in the subclass will hide the field in the super-class, but both exists. The fields are invoked based on reference types, while methods are used of actual object. You can try it yourself.

It's called, variable hiding/shadowing, for more details look on here

It isn't overriding anything, you're just hiding the original field at the current class scope. If you use a variable with the subtype you will still be able to access the original property. Example:

abstract class GenericContainer {
    public GenericChild child;       
}

abstract class GenericChild {
    public int prop1=1 ;
}

class SpecialChild extends GenericChild {
    public int prop1=2;
}

class SpecialContainer extends GenericContainer {
    public SpecialChild child;
}

public class Main {

    public static void main( String ... args ) {

        GenericContainer container = new SpecialContainer();
        container.child = new SpecialChild();

        System.out.println( container.child.prop1 );

        SpecialChild child = (SpecialChild) container.child;        
        System.out.println( child.prop1 );
    }

}

This prints 1 and then 2.

From SpecialChild you would also be able to go up one level using super:

class SpecialChild extends GenericChild {
    public int prop1=2;

    public int getOriginalProp1() {
        return super.prop1;
    }

}