Representation Invariants
CpSc 372:
Introduction to
Software Engineering
Jason O. Hallstrom
[email protected]
Authorship Disclaimer. These slides are intended to serve as teaching instruments for an undergraduate course in Software
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CpSc 372
Implementation Rules
When implementing an interface, we sometimes
impose rules to allow our method implementations
to work together.
modeled by:
Set of Object
represents
modeled by:
String of Object
Consider implementing the Set interface using an
implementation of Sequence.
CpSc 372
Interface Review
Recall the interface specifications that we’ve discussed.
public interface Set {
void clear();
void add(Object x);
void remove(Object x);
Object removeAny();
boolean isIn(Object x);
int getSize(); }
public interface Sequence {
void clear();
void add(int pos, Object x);
Object remove(int pos);
Object getElement(int pos);
int getLength(); }
modeled by:
Set of Object
modeled by:
String of Object
CpSc 372
Correct Implementation?
public class SetImpl1 implements Set {
/* representation: self = elements(self.items) */
private Sequence items;
public SetImpl1() {
items = new SequenceImpl1();
}
…
public void remove(Object x) {
for(int i = 0; i < items.getLength(); i++) {
if(items.getElement(i).equals(x)) {
items.remove(i);
return;
}
}
…
}
CpSc 372
Correct Implementation?
public class SetImpl1 implements Set {
/* representation: self = elements(self.items) */
private Sequence items;
public SetImpl1() {
items = new SequenceImpl1();
…
public void add(Object x) {
items.add(x);
items.add(x);
}
…
}
}
Considered in isolation, is the body of add() correct?
CpSc 372
What is a Representation Invariant?
A representation invariant characterizes a set
of rules that must be respected by every method
implementation.
A representation invariant is expressed as a
property that must be satisfied at the start and
end of every method invocation.
 Expressed
over concrete state space
 Satisfied at object creation
 Satisfied at method termination
CpSc 372
The Convention Clause
public class SetImpl1 implements Set {
/* representation:
self = elements(self.items)
convention:
*/
private Sequence items;
public SetImpl1() {
items = new SequenceImpl1();
…
}
}
CpSc 372
Another Example
What if we wanted to use binary search within the
body of isIn()?
public class SetImpl1 implements Set {
/* representation:
self = elements(self.items)
convention:
*/
…
}