Constraints%20in%20Java

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Constraints in Java
Markus Völter, MATHEMA AG
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The Problem

• Goal Separation of Responsibilities
• Server class
• Client class
• Server class has well defined interface for use
  by clients
• Includes operations, each operations consists of
  name and parameters
• However, interfaces contain no semantic
  information (except for types, in some languages)

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Examples for the problem

• Assumptions about parameters
• Problem The assumption is not visible in the
  interface
• Use of Java Interfaces does not work !!!
• Implementation can forget to check it
• Therefore Client cannot rely on it

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Examples for the problem II

• Semantic consistency in subclasses
• The following sublass-overriding is legal

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Examples for the problem IIb

• But what about this one
• Problem A semantic assumption of the super class
  is broken.
• It can be argued that this should not be allowed.

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The problem conclusion

• Why does the client assume certain semantics
  although they are not formally ensured?
• It is ok to assume these semantics.
• Then we need a way to ensure them.
• This is the goal of this
  presentation!

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Solution (conceptual) Constraints

• Preconditions
• Scope Operation
• Must be true before operation is executed
• Postconditions
• Scope Operation
• Must be true after the operation has been
  executed
• Invariants
• Scope Class
• Must be true at any time

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Constraints in Design (UML)

• OCL (Object Constraint Language) can be used to
  do it!

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Constraints in Eiffel

• Eiffel provides Constraints Programming by
  Contract
• They can be specified on class interfaces
• They have different names
• Preconditions require clause
• Postconditions ensure clause
• Invariants are also possible, also on interfaces

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Constraints in Eiffel II

• Eiffel also allows correct constraints in
  subclasses
• Require else
• Ensure then

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Semantic of Constraints

• Preconditions
• Server requires them to be true
• Client must ensure this
• Runtime system checks it
• Server implementation expects them to be true
• Postconditions
• Server assures them to be true
• Server implementation must ensure that
• Runtime system checks it
• Client expects them to be true

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Constraints in Java

• Either do it manually (ifs at the beginning of
  each method)
• Use Precompiler and declarative statements
• Use Aspects (a kind of precompiler, in some way)
• Or, use the following approach...

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The Java Proxy API (JDK 1.3)

• GoF Proxy pattern
• Proxy API can dynamically create Proxies for any
  class in a system (at runtime!)
• Forwards any method invocation to an
  InvocationHandler

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Using the API for Constraints
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Handle the invocations
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Handle the invocations
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Specifying Constraints

• For an interface X, the constraints are specified
  in a class Xconstraints
• Naming conventions exist for methods
• For Operation visibility RetType op(params) 
  precondition is called public void pre_op(params
  )
• For Operation visibility RetType op(params) 
  postcondition is called public void
  post_op(Object retVal )
• The invariant is checked in invariant()

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An Example (Interface)

• The following is a simple Interface for vehicles

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An Example (Implementation)

• The following is a trivial implementation of the
  interface

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An Example (Constraints)

• Precondition In accelerate(), delta must be gt 0
• Invariant Truck must never drive faster than 80
  km/h

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An Example (Constraints II)

• Postcondition After decelerate, speed must be
  less than before

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A small disadvantage

• To allow Java to create the Proxies, a factory
  must be used to create instances of classes, of
  which the constraints should be checked

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A small disadvantage II

• Client application needs to use this factory
• Constraint checking can turned on or off easily

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Constraint failure

• If constraint fails, an Exception is thrown

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Want to know more?

• Check out the current issue of
• Or ask questions!
• Thank you!