Question

a child class extending a mother class which, in its constructor call a template method instianted in the child class because it need a value obtained in the child constructor.

How Can I do something like this, without changing my Parent constructor (Foo, here. I cannot do any changes in the real Foo class):

public abstract class Foo{
    public Foo(){
        register();
    }

    public abstract void register();

    public void aMethod(int aValue){
        // body
    }

}

public class Bar extends Foo{

    public Foo(int aValue){
        // body
    }

    register(){
        aMethod(aValue);
    }
}

Here, even if I put aValue in a field, aValue is not even created at

aMethod(aValue);.

How can I solve my problem?

I am looking for any pattern, ULM, solution.

Answer 1

a child class extending a mother class which, in its constructor call a template method instianted in the child class because it need a value obtained in the child constructor.

Why doesn’t the subclass constructor simply pass the value up to the superclass constructor?

public abstract class Foo {
    protected Foo(int value) {
       ... use value ...
   }
}

public class Bar extends Foo {
    public Foo(int value) {
        super(value);
    }
    ...
}

Note that calling virtual methods in a constructor is a dangerous practice, as the subclass won’t have had the opportunity to initialize itself yet.