Expanding Objects

Question

I am a relatively experienced Java coder, but I am having trouble with this one for some reason...
Let's say I have 4 Objects. Form, IntField, TextField, DropField
All the *Field Objects have some similar attributes (let's say idNumber & name)
And a Form can contain any amount or combinations of these Objects.
So I started with an Abstract Object GeneralField it contained the name, idNumber variables.
Then I had
public class IntField extends GeneralField{...}etc.
My Form Object had a List<GeneralField> and that works well.
But it became more complex. The *Field types all have different methods. For instance an IntField can have a getMaxInt(); and a DropField has a getIndex();
But if I have a loop: for(GeneralField gf : list)
gf can never have access to getMaxInt(); (Obviously).
So I then add into the mix IField (an Interface which contains all the possible methods)
and now I have:
public class IntField extends GeneralField implements IField{}and my List<IField>
But what I now get is IField always has access to getMaxInt() even for a TextField which that makes no sense for. Now I know I could easily just throw an NonsensicalAttributeException or return null, but that doesn't seem appropriate.
So I started reading more about the Decorator pattern, which I would build an Interface called IIntField and call
IIntField myInt = new IIntField(new IntField()); and that easily goes into my List<IField>
but then if I do my loop for(IField f : list) I need some way of knowing that this instance of f is an IIntField. Without resorting toinstanceof.
I read up on various other patterns and nothing seems to do what I want. The only thing I can think of is to have a boolean on IField that says isIIntField() and when I create a new IIntfield set it to true then I can cast it:
for(IField f : list){
     if(f.isIIntField()){
           IIntField i = (IIntField)f;
}But that is horribly messy and not that extensible.
I am sure I am missing something super basic, but I just cannot see it yet.
Can anyone point me in the correct direction?
I basically need a way of going through List<*Field> and having access to all the various attributes that that particular instance may have.
Thanks,
Craig

Answer 1

cstoss wrote:
@Horst_M I used the Visitor pattern in a previous project and had a lot of success with it, I peaked at it for this but couldn't visualize it. I will have another look for sure.Looking at your code example the visitor pattern cries out loud "visit me".
Original code:
for(IField f : list){
     if(f.isIIntField()){
           IIntField i = (IIntField)f;
}will be changed to:
ConcreteVisitorA implements IVisitor
for(IField f : list){
   f.accept(this);
public void visitIntField(IIntField i) {
}And IIntField will be extended with:
public void accept(IVisitor v) {
  v.visitIntField(this);
}