1. Formal Specification of Java Interfaces
Jason Hallstrom and Murali Sitaraman
Clemson University

2. Basics An interface Describes what classes or components do
Does not describe how they should do it
Is a contract between component users (clients) and developers (implementers)
If the users satisfy the requirements for using the component, the component will provide guarantees

3. Principles of Interface Design
Information hiding
Hide details unnecessary to use the component
Abstraction
Provide a “cover story” or explanation in user-oriented terms so they can understand the interface

4. Contract specifications
Requirements and guarantees
Requires clauses are preconditions
Ensures clauses are postconditions
Who is responsible for requires clauses?
Client (i.e., caller)
Implementer
Neither
Both
Discussion of consequences

5. Contract specifications
Requirements and guarantees
Requires clauses are preconditions
Ensures clauses are postconditions
Who is responsible for requires clauses?
Client (i.e., caller)
Implementer
Neither
Both
Consequences

6. Specification of Stacks
Mathematical modeling
What can we think of stacks as “mathematically”?

7. Mathematical Strings Unlike sets, strings have order Notations
Example: Str(Z) for String of integers
Notations
Empty string (written empty_string or L)
Concatenation ( alpha o beta )
Length ( |alpha| )
String containing one entry ( <5> )

8. Specification of IntStack Interface
Suppose IntStack is an interface
uses Integer_Theory, String_Theory;
Think of stacks of Integers as “math strings” of integers
this: Str(Z);
Specification of Constructor
Initialization ensures this = empty_string;
Exercises: Specification of other Stack operations

9. Specification of IntStack Interface
Operation push (int x);
updates this; restores x;
ensures this = <x> o #this;
int Operation pop ();
updates this;
requires this /= empty_string;
ensures #this = <result of pop()> o this;
bool Operation is_empty();
preserves this;
ensures result of is_empty =
(this = empty_string);

10. Specification of IntStack Interface
Operation push (int x);
updates this; restores x;
ensures this = <x> o #this;
int Operation pop ();
updates this;
requires this /= empty_string;
ensures #this = <pop()> o this;
bool Operation is_empty();
preserves this;
ensures is_empty = (this = empty_string);

11. Java Specification Questions
What is the specification of “=“ to assign one IntStack object to another?
If you defined a “clone” method, what is its specification?
What are the advantages of using “=“ over “clone”?
What are the advantages of using “clone” over equal?