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Constraints in Java

Constraints in Java. Markus Völter, MATHEMA AG. 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

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Constraints in Java

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  1. Constraints in Java Markus Völter, MATHEMA AG

  2. 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)

  3. 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

  4. Examples for the problem II • Semantic consistency in subclasses: • The following sublass-overriding is legal:

  5. 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.

  6. 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!

  7. 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

  8. Constraints in Design (UML) • OCL (Object Constraint Language) can be used to do it!

  9. 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

  10. Constraints in Eiffel II • Eiffel also allows correct constraints in subclasses: • Require else • Ensure then

  11. 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

  12. 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...

  13. 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

  14. Using the API for Constraints

  15. Handle the invocations

  16. Handle the invocations

  17. 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()

  18. An Example (Interface) • The following is a simple Interface for vehicles:

  19. An Example (Implementation) • The following is a trivial implementation of the interface:

  20. An Example (Constraints) • Precondition: In accelerate(), delta must be > 0 • Invariant: Truck must never drive faster than 80 km/h

  21. An Example (Constraints II) • Postcondition: After decelerate, speed must be less than before:

  22. 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:

  23. A small disadvantage II • Client application needs to use this factory: • Constraint checking can turned on or off easily

  24. Constraint failure • If constraint fails, an Exception is thrown:

  25. Want to know more? Check out the current issue of Or ask questions! Thank you!

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