The document discusses two approaches to encapsulation in object-oriented programming. The usual approach is to prevent access to private methods and fields. However, it also considers an alternative where encapsulation is voluntary and violations result in warnings rather than errors. This second approach aims to educate programmers about risks while still allowing access when needed. It also notes issues around representation exposure and difficulties avoiding violations of invariants.

2. The Usual Approach
Correct functioning of classes depend on a lot of invariants.
Programmers may not be aware of all of them.
So let’s prevent programmers from accessing private metods
and fields.
Michal P´se (CTU in Prague)
ıˇ Object Programming Lect. 10: Encapsulation December 1, 2010 2/7

3. But. . .
What if you really need to?
What if you are willing to risk failure?
After all, you may only hurt yourself. . .
Michal P´se (CTU in Prague)
ıˇ Object Programming Lect. 10: Encapsulation December 1, 2010 3/7

4. The Other Option
Encapsulation is voluntary
Violations are reported as a warning, not an error
Programmers are educated about possible risks
Michal P´se (CTU in Prague)
ıˇ Object Programming Lect. 10: Encapsulation December 1, 2010 4/7

5. The Other Option
Encapsulation is voluntary
Violations are reported as a warning, not an error
Programmers are educated about possible risks
Encapsulation = ”not having to care what is inside” instead of
”not having access to what is inside”.
Michal P´se (CTU in Prague)
ıˇ Object Programming Lect. 10: Encapsulation December 1, 2010 4/7

6. Representation Exposure
Pointers pointing to subobject may manipulate them without parent’s
object knowledge
This may violate important invariants
However, it is very difficult to avoid
Michal P´se (CTU in Prague)
ıˇ Object Programming Lect. 10: Encapsulation December 1, 2010 5/7

7. Possible Solutions
Immutable objects
Various sorts of const modifiers
Copying
Michal P´se (CTU in Prague)
ıˇ Object Programming Lect. 10: Encapsulation December 1, 2010 6/7

8. Possible Solutions
Immutable objects
Various sorts of const modifiers
Copying
None of these solutions is satisfactory enough
Michal P´se (CTU in Prague)
ıˇ Object Programming Lect. 10: Encapsulation December 1, 2010 6/7

9. See
Boyland. Why we should not add readonly to Java (yet). Journal of
Object Technology (2006)
Michal P´se (CTU in Prague)
ıˇ Object Programming Lect. 10: Encapsulation December 1, 2010 7/7