Design Scenario Question

Question

Hello, I have a question about how to architect a certain part of my code and I will try to formulate an example here:
Say I have a class Parent that should only be constructed with variables a and b. A no-arg constructor does not make logical sense for Parent, as it should not exist without the a and b instance variables having a value.
Parent has two subclasses, ChildOne and ChildTwo which are also constructed using the a and b variables in Parent. The a and b attributes in Parent should be used across the board for all Parent�s children to do class-specific processing. For instance, ChildOne will use Parent�s a and b attribute to derive ChildOne-specific information.
Parent also has a method, getDataFromChildren(int id) which does something similar to:
if (id == 1) {
     ChildOne one = new ChildOne(this.a, this.b);
     one.getChildData();
} else if (id==2) {
     ChildTwo two = new ChildTwo(this.a, this.b);
     two.getChildData();
}     I understand this may violate OO polymorphism principals, which will be one of my two questions listed below:
1. My first problem is that I understand to be correct, this getDataFromChildren method should accept a Parent parameter and then simply invoke a method polymorphically, such as:
getDataFromChildren(Parent p) {
     p.getChildData();
}Then from other parts of my application, one of the child classes will be passed to this method. But, my other parts of the application know nothing about the Parent, ChildOne and ChildTwo classes. All they know is an ID. So, how can they possibly pass a Parent object to this method? Furthermore, this getDataFromChildren method exists within Parent itself. That doesn�t sound correct either.
2.     My second problem involves constructors. Within my children, I have constructors to match the Parent constructor. I also have explicit calls to the super(a, b) constructor in Parent. So, when a child class is instantiated as say ChildOne(a, b), it makes an explicit call to super(a, b). That creates a new instance of parent with a and b, when I already had one in the first place. To make matters worse, my original Parent object contained some other information specific to that instance that the child classes need. When they are instantiated and make the call to super(a, b), they create new instances of Parent completely separate from my original, relevant one.
I suspect that what I need to do here is have some type of intermediate class that takes the ID from another part of my system and interprets it there. I considered that my child classes should accept a Parent object in their constructor, but that doesn�t seem to make sense.
Please help, sorry for being so verbose. Hopefully this made sense.

Answer 1

Learn about object-oriented design patterns a bit. It will help you design applications a bit better and come up with working solutions for isues such as the one you describe.
if (id == 1) {
     ChildOne one = new ChildOne(this.a, this.b);
     one.getChildData();
} else if (id==2) {
     ChildTwo two = new ChildTwo(this.a, this.b);
     two.getChildData();
}Considering the fragment above, you should check out the Abstract Factory design pattern. You could have a factory return the appropriate object based on the logic in the conditional structure. Also, coding to an interface is much more flexible that what you have right now. Below is a example:
Child child = null;
if (id == 1) {
     child = ChildFactory.getChild("1", this.a, this.b);
} else if (id==2) {
     child = ChildFactory.getChild("2", this.a, this.b);
child.getChildData();
...The goal is to reduce the number of hard-coded class names in your code. The only place where there are hard-coded class names is inside the factory. When you want to introduce a new behavior and a new class type, all you need is to add it to the factory.