Vaadin 7 Creational Pattern

General
1

Hello,
I’m currently puzzling over something that came up when i was trying to port some old Delphi source code to vaadin. Here’s the code to a hypothetical View class. The View will have 3 ‘parts’: a header, a footer, and a body.

public abstract class MyView extends VerticalLayout implements View () {
   public MyView() {
       //wiring code goes here
   }  
   public ComponentContainer createHeader() {
       return new DefaultHeader(); 
   } 
   public ComponentContainer createFooter() {
       return new DefaultFooter();       
   }
   public abstract ComponentContainer createBody(); //to be defined in descendant classes 
}

If you need a new view, you just have to inherit from MyView, and a proper IDE will generate the abstract method that you have to implement, e.g:

public class HomeView extends MyView { public ComponentContainer createBody() { //add contents here } } Now here’s the problematic part, the wiring code in MyView constructor:

//MyView constructor public MyView() { ComponentContainer ccHeader = createHeader(); ComponentContainer ccBody = createBody(); ComponentContainer ccFooter = createFooter(); // if (ccHeader != null) this.addComponent(ccHeader); if (ccBody != null) this.addComponent(ccBody); if (ccFooter != null) this.addComponent(ccFooter); } The code sort of works, and i kinda like the structure, but Java and the IDE will complain about "
overridable call in method constructor"
 at line 3/4/5 above. I understand the problems underlying the java warnings and why it’s not advised, so i suppose my question now is, how do we do this “properly” in java?

Alternative 1: Do not use the constructor for the wiring code, use an init() method instead.

  • Pros: retain all the flexibility
  • Cons: need to remember calling init() after creating the object (might sound trivial, but i failed this a couple of times with interesting results :D, in general i would normally avoid extra-but-mandatory steps like this)

Alternative 2: Use something like a Factory Method pattern.

  • Pros: not sure
  • Cons: gets a bit complicated, messier code involving at least 2 new classes

Any opinions/pointers are welcome. Maybe share your favorite way to structure vaadin views/UI if you can?
Thanks in advance,
ts.

2

wow…man ! Great ! :slight_smile:

3

It’s more like a common problem. Another alternative is a construct like this (vaadin gives you the attach method):

public class MyView extends CustomComponent {
  private Component header;
  private VerticalLayout layout;

  public MyView() {
     layout = new VerticalLayout();
     setCompositionRoot(layout);
  }
 
  public void setHeader(Component header) { this.header = header; }
 
  @Override
  public void attach() {
    super.attach();
    layout.addComponent(header == null ? createDefaultHeader() : header);
    layout.addComponent(createBody());
  }

  protected abstract Component createBody();
  private Component createDefaultHeader() {
    return new MyDefaultHeader();
  }