1. Java Coding 6_part3 David Davenport Computer Eng. Dept.,
Clone wars…
David Davenport
Computer Eng. Dept.,
Bilkent University Ankara - Turkey.
Last update: 19/12/2016 ~original

2. IMPORTANT… Students… Instructors…
This presentation is designed to be used in class as part of a guided discovery sequence. It is not self-explanatory! Please use it only for revision purposes after having taken the class. Simply flicking through the slides will teach you nothing. You must be actively thinking, doing and questioning to learn!
Instructors…
You are free to use this presentation in your classes and to make any modifications to it that you wish. All I ask is an saying where and when it is/was used. I would also appreciate any suggestions you may have for improving it.
thank you, David.

3. To clone or not to clone…
Caution needed
To clone or not to clone…

4. no problem if IMMUTABLE
Encapsulation lost?
name
“David”
dob
name
“Gunes”
dob
Create date d, then use it to create Persons p & p2…
If constructor simply copies reference, i.e. this.dob = dob;
result is that d holds a reference to one of p’s properties
& can tweak it, thus breaking encapsulation!
Note: there is no problem with primitive types or immutable classes
IMMUTABLE objects ~ones whose state cannot change after creation
Java Strings are immutable, so the name property is safe! p
{Person} p2
{Person} d
{Date}
no problem if IMMUTABLE

5. Regained…? 1-7-2001 1-7-2001 1-7-1800 1-7-2001 name “David” dob name
“Gunes”
dob
tmp
{Date} p
{Person} p2
{Person}
Cloning d in the Person constructor is safer,
i.e. this.dob = d.clone(); or this.dob = new Date( dob);
But accessor methods can again cause “problems”…
e.g. if Person’s getDob() method simply returns the reference
to the Date property, i.e. return p.getDob(); d
{Date}
tmp = p.getDob();

6. Clones everywhere…? 1-7-2001 1-7-2001 1-7-2001 1-7-1800 1-7-2001 name
“David”
dob p
{Person}
name
“Gunes”
dob p2
{Person}
Solved if it getDob() returns a clone of p’s dob Date object.
tmp
{Date} d
{Date}
tmp = p.getDob();

7. Shallow vs. Deep Clone 1-7-2001 1-7-2001 name “David” dob p name
{Person}
name
“David”
dob
pShallowClone
{Person}
Similar problems can occur when cloning objects with object-type properties.
Shallow clone ~ object-type properties copied by reference only (clone shares property object!)
Deep clone ~ object-type properties also cloned ( clones completely separate! )
Java objects all have: toString, equals, clone
but may not do what you expect or want
~ clone is usually shallow clone!
name
“David”
dob
pDeepClone
{Person}

8. but may not do what you want/expect!
Shallow vs. Deep clone x
Java objects have
toString
equals
clone y
Similar problems can occur when cloning objects with object-type properties.
Shallow clone ~ object-type properties copied by reference only (clone shares property object!)
Deep clone ~ object-type properties also cloned ( clones completely separate! )
Java objects all have: toString, equals, clone
but may not do what you expect or want
~ clone is usually shallow clone! y
but may not do what you want/expect!