DCI - Data, Context and Interaction @ Jug Lugano May 2011

DCI
Data, Context and
Interaction
Fabrizio Giudici, Senior Java Architect
Tidalwave s.a.s - fabrizio.giudici@tidalwave.it

Wednesday, May 18, 2011

About the speaker

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About the speaker

• Senior Software Architect, Mentor, Technical Writer
• Fourteen years of Java experience (JSE, JEE, JME, etc...)
• Sun partner since 1998, Oracle consultant since 2010
• Author of a number of open source projects
• Speaker at JavaOne, Devoxx, Jazoon, JAX and other events
• Member of the NetBeans Dream Team
• Co-leader of JUG Milano
• Java.Net blogger at http://www.java.net/blogs/fabriziogiudici

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Agenda

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Agenda

• A real world example

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Agenda

• DCI - Basic concepts

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Agenda

• DCI - How to implement?

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Agenda

• DCI - How to implement?
• Some simple examples

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A real world example


Shameless Plug

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Shameless Plug

• Exercises of Design - my design book...

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Shameless Plug

• Just started!

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Shameless Plug

• Just started!

• http://exercisesofdesign.java.net

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The Observation API

• A core component of blueBill Mobile
• An application about recording bird observations
• Designed with a high degree of abstraction

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The Taxonomy API

• Models the taxonomy of a philogenetic tree
• The standard way of biology for representing (bird) species

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So, what about a Bird?

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• Both a Taxon and an Observable

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• Multiple inheritance?

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• Multiple interface implementation?

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• Composition

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• Composition
• Taxon and Observable are roles

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• Composition
• Taxon and Observable are roles
• A Bird, in some contexts, could be a Taxon and/or an
Observable

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DCI - Basic Concepts


Data, Context and Interaction

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• OOD best practice formalized by Trygve Reenskaug

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• The formalizer of MVC

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• Seen by somebody as an evolution of MVC

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• But it doesn’t replace it

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• It rather broadens the analysis scope

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• It rather broadens the analysis scope

• http://www.artima.com/articles/dci_vision.html

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DCI - aims

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DCI - aims

• Improve readability of a OO system

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DCI - aims

• Give system behaviour ﬁrst-class status

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DCI - aims

• Recover readability of system properties on the whole

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DCI - aims


• Separate responsibilities for behaviour and domain

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DCI - aims


• Behaviour: what the system does

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DCI - aims



• Domain: what the system is

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DCI - aims



• Domain: what the system is

• Be close to people’s mental model

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DCI - Data

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DCI - Data

• What the system is

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DCI - Data

• Relatively static with relations

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DCI - Data

• Domain structure implemented with “conventional” classes

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DCI - Data

• Hmm... perhaps anemic classes?

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DCI - Data


• Typically it includes persistence

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DCI - Data



• Comes from the mental model of system stakeholders

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DCI - Data



• Close to the “model” in MVC

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DCI - Data



• Close to the “model” in MVC
• E.g.: BankAccount with increase(), decrease(); no deposit()

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DCI - Context (and Roles)

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• Associated to a use case, user story, scenario, or
algorithm

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algorithm
• Identiﬁes objects participating in a scenario

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algorithm
• Assign to each object one or more stateless roles

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algorithm
• Objects can have multiple roles at the same time

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algorithm

• Decompose the scenario into roles, not objects

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algorithm

• Contrast this with polymorphism and classic OO decomposition

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algorithm

• Contrast this with polymorphism and classic OO decomposition

• E.g.: MoneyTransfer.{.withDraw(),.deposit()} with srcAccount
and destAccount

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Quick terminology note

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• Methodless role: abstract role

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• Interface, abstract class

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• Methodful role: concrete role

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• Methodful role: concrete role
• Concrete class

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DCI - Interaction

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DCI - Interaction

• What the system does

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DCI - Interaction

• Occurs among roles, which act as adapters among objects

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DCI - Interaction

• Roles are bound in different ways for each context

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DCI - Interaction

• Roles life cycle is likely to be bound to a given interaction

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DCI - Interaction


• Roles should be generic

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DCI - Interaction


• Interaction should be explicit given roles nature

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DCI - Interaction


• Interaction should be explicit given roles nature
• Hmm.... rather than emergent as in agile design?

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DCI - Execution Model

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• The Context ﬁnds object participants

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• Then, it assigns (injects?) roles to them

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• Roles are discovered by type (methodless roles)

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• Then, it triggers a method on the ﬁrst role

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• Then, it triggers a method on the ﬁrst role
• Interaction goes on among other roles until the job is done

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Some issues

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Some issues

• What about anemic objects?

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Some issues

• Perhaps not a problem considering clusters object + roles

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Some issues


• Object schizophrenia

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Some issues


• Which is the identity of an object in a cluster?

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Some issues


• What about equals() / hashcode()? Are roles parts of identity?

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Some issues



• Possible solution is an Identifiable role

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Some issues



• Injected by context

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Some issues



• Injected by context

• Objects with the same identity are “equals”

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DCI - How to
implement?


Implementation issues

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• Static vs dynamic

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• Java

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• Java
• AOP, annotation-driven code generation (e.g. Qi4J)

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• Java
• Other languages

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• Java
• Other languages
• Traits, mix-ins

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• Java
• Other languages
• Traits, mix-ins

• First, acknowledge that DCI is a best practice

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• Java
• Other languages
• Traits, mix-ins

• Some languages can ﬁt better than others

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• Java
• Other languages
• Traits, mix-ins

• Frameworks might help... but they force a new nature

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• Java
• Other languages
• Traits, mix-ins

• Frameworks might help... but they force a new nature

• My point: DCI must be addressed in design
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Abstracting and sweetening

• Would be nice to be technology independent
• Would be nice to have some syntactic sugar

• Note: not central in this presentation, but needed for code
examples

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NetBeans Platform Lookup; as()

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• Role role = object.getLookup().lookup(Role.class) ...

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• NetBeans Platform’s Lookup, as a bag of roles

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• Allows both static implementation and dynamic injection

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• Role role = object.as(Role);

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• Role role = object.as(Role);
• My syntactic sugar around Lookup

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Some examples


Some example reusable roles

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• Displayable:

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• Displayable:
• String s = myObject.as(Displayable).getDisplayName();

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• Displayable:

• IconProvider:

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• Displayable:

• IconProvider:
• Icon i = myObject.as(IconProvider).getIcon(16);

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• Displayable:

• IconProvider: “ask” approach

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• Displayable:


“tell” approach

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• Displayable:


• Renderable:
“tell” approach

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• Displayable:


• Renderable:
• PrintWriter pw = ... // “tell” approach
myObject.as(TextRenderable).render(pw);

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• Displayable:


• Renderable:
• JLabel label = ... // from Swing
myObject.as(JLabelRenderable).render(label);

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• Displayable:


• Renderable:
• TextView textView = ... // from Swing
myObject.as(TextViewRenderable).render(textView);
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Roles can be dynamically injected,
• Displayable: so myObject does not depend on them


• Renderable:
• TextView textView = ... // from Swing
myObject.as(TextViewRenderable).render(textView);
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Examples from blueBill

• ObservationSet set = ... ;
set.as(ObservationSetTraverser)
.visit(new KMLReportGenerator()); // visitor pattern
• ObservationItem item = ...;
TextView textView = ...;
item.as(Taxon)
.as(TextViewRenderable)
.render(textView);

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Example: decorating roles

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Q &A

• Question Time

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DCI - Data, Context and Interaction @ Jug Lugano May 2011

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