Cloning Enums. Cloning and Enums BIU OOP

Transcription

1

2 Table of contents 1 Cloning 2 Integer representation Object representation Java Enum

3 Cloning

4 Objective We have an object and we need to make a copy of it. We need to choose if we want a shallow copy or a deep copy.

5 Shallow VS Deep Shallow copy Copy primitive members values. Copy the references to objects (Referenced objects will not be duplicated). Deep copy Copy primitive members values. Create copies of referenced objects (Referenced objects will be duplicated).

6 Motivation Example object Class Diagram:

7 Motivation Example object Class Diagram: Question How can we create a copy of an existing Professional object?

8 Method 1 - Method Special constructor that receives the same class type. Copy all the values as we initialize the new object.

9 Method 1 - Method Special constructor that receives the same class type. Copy all the values as we initialize the new object. Simple class (only primitives and no inheritance) 3 public class Person { 4 5 private String firstname ; 6 private String lastname ; public Person ( Person other ) { 19 this. firstname = other. firstname ; 20 this. lastname = other. lastname ; 21 } }

10 Method 1 - (2) Method Special constructor that receives the same class type. Copy all the values as we initialize the new object. Complex class (contains references and is a subclass) 3 public class Professional extends Person { 4 5 private int yearsofexperience ; 6 private Diploma diploma ; public Professional ( Professional other ) { 22 super ( other ); this. yearsofexperience = other. yearsofexperience ; 25 // deep copy of diploma 26 this. diploma = new Diploma ( other. diploma ); 27 } }

11 Method 1 - (3) Pros Fast. Simple to implement.

12 Method 1 - (3) Pros Fast. Simple to implement. Cons Assumes s available for super classes. Need to know the exact type during cloning.

13 Method 2 - Method A method that returns a copy of the object.

14 Method 2 - Method A method that returns a copy of the object. Simple class (only primitives and no inheritance) 3 public class Person { 4 5 private String firstname ; 6 private String lastname ; public Person copy (){ 14 return new Person ( getfirstname (), getlastname ()); 15 } }

15 Method 2 - (2) Method A method that returns a copy of the object.

16 Method 2 - (2) Method A method that returns a copy of the object. Complex class (contains references and is a subclass) 3 public class Professional extends Person { 4 5 private int yearsofexperience ; 6 private Diploma diploma ; public Professional copy (){ 16 return new Professional ( 17 getfirstname (), 18 getlastname (), 19 getyearsofexperience (), 20 getdiploma (). copy () // deep copy 21 ); 22 } }

17 Method 2 - (3) Pros Simple to implement. Don t have to know the exact type of object to copy it.

18 Method 2 - (3) Pros Simple to implement. Don t have to know the exact type of object to copy it. Cons Can t reuse the copy() implementation of the super class. Slower than the copy constructor. Additional method call is required.

19 Method 3 - Method A special method that preforms an efficient shallow copy.

20 Method 3 - Method A special method that preforms an efficient shallow copy. 1 public class Object { protected native () throws CloneNotSupportedException ; } How to use? 1 Override the method in your implementation. 2 Change method visibility to public (instead of protected). 3 implement the empty interface Cloneable. 4 call super.clone() for shallow copy. 5 Manually set non primitive fields if deep copy is required.

21 Method 3 - (2) Method A special method that preforms an efficient shallow copy. Simple class (only primitives and no inheritance) 3 public class Person implements Cloneable { 4 5 private String firstname ; 6 private String lastname ; public Person clone () { 23 try { 24 return ( Person ) super. clone (); 25 } catch ( CloneNotSupportedException e) { 26 throw new AssertionError (" Failed cloning Person "); 27 } 28 } 29 }

22 Method 3 - (3) Method A special method that preforms an efficient shallow copy. Complex class (contains references and is a subclass) 3 public class Professional extends Person implements Cloneable { 4 5 private int yearsofexperience ; 6 private Diploma diploma ; public Professional clone () { 25 Professional copy = ( Professional ) super. clone (); 26 copy. diploma = this. diploma. clone (); // deep copy 27 return copy ; 28 } }

23 Method 3 - (4) Pros Fastest for shallow copy or classes with many primitives. Don t have to know the exact type of object to copy it.

24 Method 3 - (4) Pros Fastest for shallow copy or classes with many primitives. Don t have to know the exact type of object to copy it. Cons Complex implementation, easy to forget something. Not necessary faster for deep copy. All classes in inheritance hierarchy must implement Cloneable. Constructor of copied objects is not called. (Can cause logical problems in your implementation)

25 Cloning Summary No single best solution. Avoid unless optimization is really needed. Use copy method if performance is not an issue because it s most flexible.

26 Cloning Summary Note No single best solution. Avoid unless optimization is really needed. Use copy method if performance is not an issue because it s most flexible. The copy method approach is also known as the Prototype creational design pattern.

27 Integer representation Object representation Java Enum

28 Integer representation Object representation Java Enum Objective We need to create a type that has a small number of possible assignments. We want to minimize the risk of assigning values that have no meaning. Example: The days of the week: Sunday, Monday,...

29 Integer representation Object representation Java Enum Solution 1 - Represent as Integer Definition 6 public class WeekDay { 7 8 public static final int SUNDAY = 0; 9 public static final int MONDAY = 1; 10 public static final int TUESDAY = 2; 11 public static final int WEDNESDAY = 3; 12 public static final int THURSDAY = 4; 13 public static final int FRIDAY = 5; 14 public static final int SATURDAY = 6; } Usage 8 int day = WeekDay. SATURDAY ; 9 10 // simple equality 11 if( WeekDay. FRIDAY == day ) { 12 System.out. println ("The day is Friday "); 13 }

30 Integer representation Object representation Java Enum Solution 1 - Represent as Integer (2) Operations 6 public class WeekDay { public static int nextday ( int weekday ) { 30 return ( weekday + 1) % 7; 31 } } Usage 16 int nextday = WeekDay. nextday (day ); System.out. println (" Actual day name :" + WeekDay. tostring (day )); 19 System.out. println ("Next day name :" + WeekDay. tostring ( nextday ));

31 Integer representation Object representation Java Enum Solution 1 - Represent as Integer (3) Cons Type is unclear from reading the code (uses int but means something else). No type safety, can accidentally hold an invalid value (out of meaningful range). Type operations are global (as static methods), not really object oriented.

32 Integer representation Object representation Java Enum Solution 2 - Objects & Exceptions Definition 6 public class WeekDay { 7 8 public static final int SUNDAY_INDEX = 0; 9 public static final int MONDAY_INDEX = 1; 10 public static final int TUESDAY_INDEX = 2; 11 public static final int WEDNESDAY_INDEX = 3; 12 public static final int THURSDAY_INDEX = 4; 13 public static final int FRIDAY_INDEX = 5; 14 public static final int SATURDAY_INDEX = 6; private int index ; public WeekDay ( int index ) { 19 if( index > 6 index < 0) { 20 throw new IllegalArgumentException (" Weekday index must be in range 0-6, provided :" + index ); 21 } this. index = index ; 24 } }

33 Integer representation Object representation Java Enum Solution 2 - Objects & Exceptions (2) Operations 26 public WeekDay nextday () { 27 return new WeekDay (( this. index + 1) % 7); 28 } Usage 11 if( WeekDay. FRIDAY_INDEX == day. getindex ()) { 12 System.out. println ("The day is Friday "); 13 } // operations on type 16 WeekDay nextday = day. nextday (); System.out. println (" Actual day name :" + day ); 19 System.out. println ("Next day name :" + nextday ); 20 } }

34 Integer representation Object representation Java Enum Solution 2 - Objects & Exceptions (3) Pros Clear Type, easier reading. Operations as methods.

35 Integer representation Object representation Java Enum Solution 2 - Objects & Exceptions (3) Pros Clear Type, easier reading. Operations as methods. Cons Type safety only at Runtime (through RuntimeException). Need to use equals() for comparing with another object. Comparing with literal through value of getter. User is aware that internal representation is via an integer index. (Encapsulation is broken)

36 Integer representation Object representation Java Enum Solution 3 - Objects & Singleton Values Definition 7 public class WeekDay { 8 9 public static final WeekDay SUNDAY = new WeekDay (0) ; 10 public static final WeekDay MONDAY = new WeekDay (1) ; 11 public static final WeekDay TUESDAY = new WeekDay (2) ; 12 public static final WeekDay WEDNESDAY = new WeekDay (3) ; 13 public static final WeekDay THURSDAY = new WeekDay (4) ; 14 public static final WeekDay FRIDAY = new WeekDay (5) ; 15 public static final WeekDay SATURDAY = new WeekDay (6) ; private WeekDay ( int index ) { 30 this. index = index ; 31 } }

37 Integer representation Object representation Java Enum Solution 3 - Objects & Singleton Values (2) Operations 7 public class WeekDay { private static final WeekDay [] values = new WeekDay []{ 18 SUNDAY, 19 MONDAY, 20 TUESDAY, 21 WEDNESDAY, 22 THURSDAY, 23 FRIDAY, 24 SATURDAY 25 }; public WeekDay nextday () { 34 return values [( this. index + 1) % 7]; 35 } }

38 Integer representation Object representation Java Enum Solution 3 - Objects & Singleton Values (3) Usage 8 WeekDay day = WeekDay. SUNDAY ; 9 10 // equality 11 if( WeekDay. FRIDAY == day ) { 12 System.out. println ("The day is Friday "); 13 } // operations on type 16 WeekDay nextday = day. nextday (); System.out. println (" Actual day name :" + day ); 19 System.out. println ("Next day name :" + nextday );

39 Integer representation Object representation Java Enum Solution 3 - Objects & Singleton Value (4) Pros Clear Type, easier reading. Operations as methods. Full Type safety. Comparison using the == operator. Internal structure is completely hidden from the user.

40 Integer representation Object representation Java Enum Solution 3 - Objects & Singleton Value (4) Pros Clear Type, easier reading. Operations as methods. Full Type safety. Comparison using the == operator. Internal structure is completely hidden from the user. Cons - Minimal! Must implement equals()/hashcode() to be used in Collections. Can t be used in a switch case. Note This is an excellent solution for other object oriented languages that don t have a special enum type.

41 Integer representation Object representation Java Enum Solution 4 - Java Enum Definition 6 public enum WeekDay { 7 8 SUNDAY (" Sunday "), 9 MONDAY (" Monday "), 10 TUESDAY (" Tuesday "), 11 WEDNESDAY (" Wednesday "), 12 THURSDAY (" Thursday "), 13 FRIDAY (" Friday "), 14 SATURDAY (" Saturday "); private String stringrepresentation ; private WeekDay ( String stringrepresentation ) { 19 this. stringrepresentation = stringrepresentation ; 20 } }

42 Integer representation Object representation Java Enum Solution 4 - Java Enum (2) Operations 6 public enum WeekDay { public WeekDay nextday () { 23 return values () [( this. ordinal () + 1) % 7]; 24 } public WeekDay previousday () { 27 return values () [( this. ordinal () + 6) % 7]; 28 } public String tostring () { 32 return this. stringrepresentation ; 33 } }

43 Integer representation Object representation Java Enum Solution 4 - Java Enum (3) Usage 11 WeekDay day = WeekDay. SUNDAY ; // equality 14 if( WeekDay. FRIDAY == day ) { 15 System.out. println ("The day is Friday "); 16 } // operations on type 19 WeekDay nextday = day. nextday (); System.out. println (" Actual day name :" + day ); 22 System.out. println ("Next day name :" + nextday );

44 Integer representation Object representation Java Enum Solution 4 - Java Enum (4) Pros All the advantages of the previous solution. Can be used with a switch case statement. Has super efficient EnumSet implementation (as a bit mask). Has super efficient EnumMap implementation for Enum keys.

45 Integer representation Object representation Java Enum Solution 4 - Java Enum (4) Pros All the advantages of the previous solution. Can be used with a switch case statement. Has super efficient EnumSet implementation (as a bit mask). Has super efficient EnumMap implementation for Enum keys. Example 25 Set <WeekDay > workdays = EnumSet.of( 26 WeekDay. SUNDAY, WeekDay. MONDAY, WeekDay. TUESDAY, WeekDay. WEDNESDAY, WeekDay. THURSDAY 27 ); // super fast set operations ( set stored as bit array ) 30 if( workdays. contains (day )){ 31 System.out. println (" Better get to work "); 32 }

46 Integer representation Object representation Java Enum Enum Summary Avoid using primitives to represent types that only have a small range of possible values. make the code clear and enforce safety. (and enum collections) are very efficient at run time.

47 Integer representation Object representation Java Enum Enum Summary Note Avoid using primitives to represent types that only have a small range of possible values. make the code clear and enforce safety. (and enum collections) are very efficient at run time. The Objects + Singleton Values approach provides most of the functionality and can be very useful to emulate enum behaviour in languages that don t provide an enum type.

48 Integer representation Object representation Java Enum Summary 1 Cloning 2 Integer representation Object representation Java Enum