1. Objektorienterad programmering d2, förel. 12
12 Object copying
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2. Objektorienterad programmering d2, förel. 12
Overview
Object duplication – cloning
In following lectures:
Equality and identity relations
Algebraic properties
Comparison methods and objects for ordering relations
Object serialization (OOA)
Object streams (OOA)
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

3. Objektorienterad programmering d2, förel. 12
Well behaved classes
Objects that are handled by the JVM and many standard classes should have
No-arg constructor
String representation (toString())
Cloning (deep copying)
Equality and hashCode methods (lecture 14)
Serialization (for stream i/o) (OOA)
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

4. Objektorienterad programmering d2, förel. 12
Class Object
The class Object defines basic default implementations of
public String toString()
protected Object clone()
public boolean equals(Object other)
public int hashCode()
They can (should!) be overridden in subclasses
next!
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

5. Ex. Object model for GPS tracks
Objektorienterad programmering d2, förel. 12
Ex. Object model for GPS tracks
A track consists of a list of legs
... and a course angle
A leg has a start position … φ
N E
a length ...
Man brukar lagra tidpunkter, bentider och fart också
A position is made up of of a latitude ...
... and a longitude
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

6. Ex. Object model for GPS tracks
Objektorienterad programmering d2, förel. 12
Ex. Object model for GPS tracks
Suppose our GPS plotter collects a track during a boat travel at the starboard side in a fairway from A to B A
We want to compute a return route from B to A. In order to avoid collisions with other boats, the return route must be separated from the forward route
(Ow, we go on the port side) B
As a basis for creating the return route, we need a copy of the track
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

7. GPS track object configuration
Objektorienterad programmering d2, förel. 12
GPS track object configuration
:Track
totalLength
legs
Explore the gps_tracks project!
:LatitudeAngle
isNorth
degrees
minutes …
:Leg
course
length
startPos
:GeoPosition
latitude
longitude
:LongitudeAngle
isWest
degrees
minutes
- How can we copy such
complicated structures?
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

8. The Object.clone() method
Objektorienterad programmering d2, förel. 12
The Object.clone() method
The class Object defines a default implementation of
protected Object clone()
It returns a shallow field-by-field copy of the calling object
it does not copy referenced objects
clone should be overridden:
public MySubClass clone()
Eftersom Object.clone har skyddad access måste man överskugga den publikt. Sen kan ev. denna ärvas vidare, eller överskuggas, allt efter behov.
OBS! Vi utnyttjar kovarianta returtyper här.
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

9. Objektorienterad programmering d2, förel. 12
Object.clone()
LatitudeAngle la
:LatitudeAngle
true 57
42.349
equal
but not the same
LatitudeAngle laCopy
= la.clone();
:LatitudeAngle
true 57
42.349
Ok eftersom alla variabler har primitiv typ
Inga referenser
OBS! Kommentera hur det blir med strängar!
A field-by-field copy
of the object above
la.equals(laCopy)
la != laCopy
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

10. Object.clone() returns a shallow copy
Objektorienterad programmering d2, förel. 12
Object.clone() returns a shallow copy
GeoPosition gp
:LatitudeAngle
:GeoPosition
:LongitudeAngle
GeoPosition gpCopy
= gp.clone();
:GeoPosition
referenced parts
become shared
Om man kopierar referenser ”field-by-field” kopieras bara addressvärden
A field-by-field copy
of the object above
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

11. We probably want a DEEP copy!
Objektorienterad programmering d2, förel. 12
We probably want a DEEP copy!
GeoPosition gp
:LatitudeAngle
:GeoPosition
:LongitudeAngle
GeoPosition gpCopy
= gp.clone();
:LatitudeAngle
:GeoPosition
:LongitudeAngle
En djup kopiering måste göras i flera led
A DEEP copy
of the object above
referenced parts
are copied too
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

12. Recommended properties
Objektorienterad programmering d2, förel. 12
Recommended properties
It is recommended that overridings of clone satisfy the following properties:
For all x, each expression must evaluate to true:
Weak independence x.clone() != x
Equality (x.clone()).equals(x)
Type identity (x.clone()).getClass() == x.getClass()
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

13. The Cloneable interface
Objektorienterad programmering d2, förel. 12
The Cloneable interface
By implementing interface Cloneable a class declares that Object.clone may copy instances of the class field-by-field
CloneNotSupportedException is thrown if Object.clone is called for instances not implementing Cloneable
Classes which implement Cloneable should override clone
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

14. Properties of Object.clone
Objektorienterad programmering d2, förel. 12
Properties of Object.clone
When Object.clone is called for an instance of a class
it will return a field-by-field copy of the whole instance
the copy will have the same dynamic type as the calling instance, this applies also to overridden clone methods in subclasses
Notera den sista punkten!
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

15. Objektorienterad programmering d2, förel. 12
Overiding Clone
public class MyClass implements Cloneable {
private int x = 123;
private boolean y = true;
private char z = 'A';
@Override
public MyClass clone() throws CloneNotSupportedException
return (MyClass)super.clone(); }
...
MyClass x = new MyClass();
MyClass copy = x.clone();
Object.clone copies x,y,z
It returns an object having static type Object, but dynamic type MyClass
- so the type cast will be OK!
Använd ALDRIG new i clone!
Det fungerar inte vid arv.
”implements Cloneable” behöver ej upprepas i subklasser
no need for a cast here
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

16. Objektorienterad programmering d2, förel. 12
Overiding Clone (2)
public class MyClass // implements Cloneable {
private int x = 123;
private boolean y = true;
private char z = 'A';
@Override
public MyClass clone() throws CloneNotSupportedException
return (MyClass)super.clone(); }
...
MyClass cc = new MyClass();
MyClass copy = cc.clone();
Object.clone throws CloneNotSupportedException
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

17. Objektorienterad programmering d2, förel. 12
Overiding Clone (3)
public class MyClass implements Cloneable {
private int x = 123;
private boolean y = true;
private char z = 'A';
@Override
public MyClass clone() {
try {
return (MyClass)super.clone(); }
catch ( CloneNotSupportedException e) {
throw new InternalError();
Om klassen ärver från Object och man får CloneNotSupportedException är något helt fel i JVM:en. Kasta InternalError!
Detta är ett sätt ...
If Object.clone throws CloneNotSupportedException, something is seriously wrong in the JVM
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

18. Objektorienterad programmering d2, förel. 12
Overiding Clone (4)
public class NoCopy {
private int x = 123;
private boolean y = true;
private char z = 'A';
@Override
public NoCopy clone() throws CloneNotSupportedException
throw new CloneNotSupportedException(); }
...
NoCopy nc = new NoCopy();
nc.clone();
Use this scheme if instances of a class may not be cloned
throws CloneNotSupportedException
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

19. Deep and shallow copying
Objektorienterad programmering d2, förel. 12
Deep and shallow copying
Suppose we want a copy of the entire object graph referenced by v1
:C1 …
:C2
:C2
:C2 v1
:C3
:C3
:C3
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

20. The first attempt - Shallow copy
Objektorienterad programmering d2, förel. 12
The first attempt - Shallow copy
A simple assignment of
an object reference is
in most cases inadequate
- some objects will be shared
v2 = v1;
:C1 …
:C2
:C2
:C2 v1
:C3
:C3
:C3 v2
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

21. Objektorienterad programmering d2, förel. 12
The second attempt
v2 = v1.clone();
C1.clone() calls super.clone()
(as in previous slides)
- the list is shared
:C1 v1 …
:C1 v2
:C2
:C2
:C2
:C3
:C3
:C3
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

22. Objektorienterad programmering d2, förel. 12
The third attempt
Inadequately overridden clone in C1
copies the list but not the list elements
- the list elements are shared
v2 = v1.clone();
:C1 … v1 …
:C1 v2
:C2
:C2
:C2
Titta på kloning av listor i API:t
:C3
:C3
:C3
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

23. Objektorienterad programmering d2, förel. 12
The fourth attempt
Overridden clone in C1 copies the
list and the list elements correctly
- but C2 does just a shallow copy!
v2 = v1.clone();
:C1 …
:C2
:C2
:C2 v1
:C3
:C3
:C3 v2
:C2
:C2
:C2
:C1
Här har C1 uppfyllt sitt kontrakt. …
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

24. Objektorienterad programmering d2, förel. 12
The final version
Finally, also C2 overrides clone …
v2 = v1.clone();
:C1
:C2
:C2
:C2 v1
:C3
:C3
:C3
- nothing left to share
:C3
:C3
:C3 v2
:C1
:C2
:C2
:C2 …
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

25. Cloning and inheritance
Objektorienterad programmering d2, förel. 12
Cloning and inheritance
Redefined clone methods in subclasses calls super.clone()
class Sub extends Base {
@Override
public Sub clone() {
Sub result = (Sub)super.clone();
...
return result; }
Sub
Base
Add copies of sub class specific fields to result
Om super.clone är en överskuggning av Object.clone så vet vi att returvärdet har dynamisk typ Sub. Alltså är typomvandligen ok.
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

26. Copying cyclic structures
Objektorienterad programmering d2, förel. 12
Copying cyclic structures
In cyclic structures and structures where parts are shared, care must be taken to preserve the sharing patterns of the original in the copy.
Copy
Original
Wrong!
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

27. Objektorienterad programmering d2, förel. 12
Ex. Angle clone method
public class Angle implements Cloneable {
private int degrees;
private float minutes;
...
@Override
public Angle clone() {
try {
return (Angle)super.clone(); }
catch (CloneNotSupportedException e) {
throw new InternalError();
Explore the gps_tracks project!
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

28. Ex. LatitudeAngle clone method
Objektorienterad programmering d2, förel. 12
Ex. LatitudeAngle clone method
public class LatitudeAngle extends Angle {
private boolean isNorth;
...
@Override
public LatitudeAngle clone() {
return (LatitudeAngle)super.clone(); }
// LongitudeAngle similar
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

29. Ex. GeoPosition clone method
Objektorienterad programmering d2, förel. 12
Ex. GeoPosition clone method
public class GeoPosition implements Cloneable {
private LatitudeAngle latitude;
private LongitudeAngle longitude;
...
@Override
public GeoPosition clone() {
GeoPosition copy = null;
try {
copy = (GeoPosition)super.clone(); }
catch (CloneNotSupportedException e) {
throw new InternalError();
copy.latitude = latitude.clone();
copy.longitude = longitude.clone();
return copy;
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

30. Objektorienterad programmering d2, förel. 12
Ex. Leg clone method
public class Leg implements Cloneable {
private GeoPosition startPos;
private int course;
private float length;
...
@Override
public Leg clone() {
Leg copy = null;
try {
copy = (Leg)super.clone(); }
catch (CloneNotSupportedException e) {
throw new InternalError();
copy.startPos = startPos.clone();
return copy;
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

31. Objektorienterad programmering d2, förel. 12
Ex. Track clone method
public class Track implements Cloneable {
private float totalLength;
private ArrayList<Leg> legs;
...
@Override
@SuppressWarnings("unchecked")
public Track clone() {
Track copy = null;
try {
copy = (Track)super.clone(); }
catch (CloneNotSupportedException e) {
throw new InternalError();
copy.legs = (ArrayList<Leg>)legs.clone();
for ( int i = 0; i < legs.size(); i++ )
copy.legs.set(i,legs.get(i).clone());
return copy;
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

32. String, clone and equality
Objektorienterad programmering d2, förel. 12
String, clone and equality
Java strings are immutable and can therefore be stored in shared memory, so there is no need to copy strings
String does not override Object.clone
The following code creates just one String object – so s1 == s2 is true String s1 = ”abc”; String s2 = ”abc”; s1 s2
’a’
’b’
’c’
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

33. String, clone and equality
Objektorienterad programmering d2, förel. 12
String, clone and equality
String has a constructor taking a String argument. The following code creates two distinct String objects, but they will share the same character array
so s1 == s2 is false but s1.equals(s2) is true String s1 = new String(”abc”); String s2 = new String(”abc”);
Always use the equals method when testing string equality! s1 s2
’a’
’b’
’c’
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1

34. Objektorienterad programmering d2, förel. 12
References
Angelika Langer, Das Kopieren von Objekten in Java,
(In Deutsch) ?
Objektorienterad programmering, DAT050, DAI2, 17/18, lp 1
DAT050, 17/18, lp 1