Physics Simulation       Lecture 12B Cloning File Read by Applet RandomIO,Append DecimalFormat DecimalFormat-IBM Precision Class-KI Format - Core Java Output to String Console Demo Exercises Lectures 1A: Introduction 1B: Java Intro 2A: BuildingBlocks 2B: Objects 3A: More Objects 3B: Exceptions        & Threads 4A: Waves 4B: Nuclear &        Particle 5A: AWT 5B: More AWT       & Graphics 6A:Detectors &       Simulation 6B: LHC/Atlas &      RandomSims     7A: Swing 7B: Java2D 8A: Java Apps 8B: Dialogs &     MoreClasses 9A: Java I/O 9B: Utilities,         Unicode 10A: More     Threading 10B: File     Handling 11A: Array,Print,     Images 11B: SimplePhysics     Simulation 12A: Tips &     Techniques 12B: More Tips &     Techniques 13A: Satellite     Simulations 13B: Intro to Java     Networking 14A: Java Servers 14B: HTTP Server 15A: ServerClient 15B: ServerClient    Expt.Simulation 16A: Course           Review 16B: ExerciseTest         Discussion     Contacts     Description     Exercises     Index     Outline     Q&A     Resources     StudentInfoForm     Student Pages     What's New

Home : Lectures : Lecture 12B : Cloning Cloning Making a copy of an object seems at first to be a straight forward task: Simply copy the values of all the properties into another instance of the same class. But what about the properties that are references to other objects? Copies of these reference values mean they will point to the same objects as the first class. But maybe that is not what we want. Perhaps we want all the objects referenced by the first object to be copied as well. These two types of object copies are called: shallow copy - exact bit copy of all the attributes deep copy - objects referenced by the original properties are copied as well. The Object() class that is inherited by all Java classes (implicitly) includes the clone() method that will make exact bit copies of all the properties. However, it is a protected method. So a given object can not be cloned by instances of any classes outside the package (unless they are subclasses of the object's class). See the lecture page on visibility. The reason for this is so that the class designer can specify exactly what kind of clones (e.g. shallow or deep) should be made. Java requires classes that want to override the clone() method, to implement the cloneable interface. The clone() method must be made public as well to override the access restrictions. From Exploring Java come the following examples of shallow and deep copies: Sheep.java import java.util.Hashtable; public class Sheep implements Cloneable {   Hashtable flock = new Hashtable();   public Object clone() {     try {       // This clone() from Object makes a bitwise copy       // of the properties (here just "flock")       return super.clone();     } catch (CloneNotSupportedException e ) {       throw new Error("This should never happen!");     }   }   public void main(String [] args) {     Sheep one = new Sheep();     Sheep anotherOne = (Sheep)one.clone();     // anotherOne.flock points at the same HashTable as     // one.flock.   } } DeepSheep.java import java.util.Hashtable; public class DeepSheep implements Cloneable {   Hashtable flock = new Hashtable();   public Object clone() {     try {       // Here also make a bitwise copy first       DeepSheep copy = (DeepSheep)super.clone();       // But also make a new HashTable that is also a       // copy of the original HashTable.       copy.flock = (Hashtable)flock.clone();       return copy;     } catch (CloneNotSupportedException e ) {       throw new Error("This should never happen!");     }   }   public void main(String [] args) {     DeepSheep one = new DeepSheep();     DeepSheep anotherOne = (DeepSheep)one.clone();     // anotherOne.flock now points to a new HashTable in     // a different part of memory than one.flock and is     // a copy of one.flock.   } }

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