Question

If I have two entities, Parent and Child, Parent needs to know about all of its Child children, and every Child instance needs to know about its parent Parent instance, how do I do this properly (in terms of DDD etc)?

The easy way would be to do parent.addChild(new Child(parent)), but this seems ugly – as well as:

parent.addChild(new Child()); // Then call some setParent method on child, which needs to be public

Do I need to use a factory here? And if so, how?

Thanks

Answer 1

One option would be to have a relevant instance method on the Parent class. A static Factory is probably not necessary here since you are operating on existing objects and simply need to connect object with another.

public class Parent {

    // ...

    public Child createChild() {
        Child c = new Child(this);
        this.addChild(c);
        return c;
    }

    protected void addChild(c) {
        // ...
    }

    // ...

}

public class Child {

    public Child(Parent p) {
        // ...
        this.addParent(p);
    }

    protected addParent(Parent p) {
        // ...
    }
}

If the Child constructor needs arguments, you can pass those to the createChild method.