The virtual field pattern

Jose jgetino at
Thu Aug 9 03:35:12 PDT 2012

Hi all

This post is an elaboration of a comment by Yuval Shavit

After that Brian called this pattern the "virtual field pattern"

I didn't find any refereces to this name, but neither has nothing better to

I suppose that programers used to work with Traits would find this post
trivial, but the pattern was a good surprisse to me because I immedially
could locate a bunch of places where my code would benefit form it.

I think this pattern shows a simple case where extension methods plays and
important role in the design of the type hierarchy, independently of API

Lets go:

Let P be an interface

interface P{
    T1 m1(...);
    T1 m2(...);

and CP a class implementing P:

CP implements P{
    T1 m1(...){...};
    T1 m2(...){...};

The VirtualField pattern allows you to replace the task of implementing P by
the task of implementing a Trait 


which extends P.

The advantage is that VirtualP has a only a single abstract method to
implement, one that returns an instance of P. 
Furthermore VirtualP must implement all methods of P by delegation:

interface VirtualP extends P{
    P delegate();
    T1 m1(...) default {
        return delegate().m1(...);
    T2 m2(...) default{
        return delegate().m2(...);

Let D be a class we want to implement P, but it can't extend CP because it
already extends another class.
To have D implementing P we only need to provide these lines of code:

Class D implements VirtualP{
    P delegate=new CP();
    P delegate(){
        return delegate;

If P has many methods and you have many candidates to implement P, like in
my case, this pattern saves code and makes these classes more readable.

Thanks to Yuval for the suggestion

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