Mutable, Immutable, and Cloneable Objects Chapter 15

2 Chapter Contents Mutable and Immutable Objects Companion Classes Using Inheritance to Form Companion Classes Cloneable Objects A Sorted List of Clones Cloning an Array Cloning a Chain

3 Mutable and Immutable Objects A mutable object belongs to a class that has mutator or set methods for its data fields The client uses set methods to change values of the object's data fields Fig An object and its reference variable chris

4 Mutable and Immutable Objects Fig An object in the list nameList (a) initially; (b) after the reference variable chris is used to change it Done by executing chris.setLast ("Smith");

5 Mutable and Immutable Objects Immutable object belongs to a class that does NOT have mutator or set methods Class said to be read only Placing immutable objects in a sorted list is a way to prevent the client from destroying the order of the list Use an immutable object if it will be shared Use a mutable object if its data will change frequently

6 Companion Classes If it is necessary to alter an immutable object Can be accomplished by having a companion class of corresponding mutable objects Also helpful to have methods that convert an object from one type to another

7 Companion Classes Fig the classes Name and ImmutableName

8 Companion Classes Java's String class is a read-only class Instances of String are immutable Java provides a companion class, StringBuffer Has a constructor that takes an instance of String as an argument Has the toString method that converts a mutable instance of StringBuffer to an immutable instance of String

9 Companion Classes Inheritance can be used to form companion classes Text shows declaration of ImmutableName Then uses this to declare the derived class Name Invokes protected methods of base class Adds mutator methods It is best to derive the mutable class from the immutable class

10 Companion Classes Fig The class Name is derived from the class ImmutableName

11 Cloneable Objects A clone is a copy of an object The Object class contains a protected method clone that returns a copy of an object The implementation of any method can invoke clone Clients cannot invoke clone unless the class overrides it, declares it public public class MyClass implements Cloneable {...

12 Cloneable Objects Fig (a) A shallow clone; (b) a deep clone.

13 Cloneable Objects Fig An instance of Name and its shallow clone.

14 Cloneable Objects Fig A clone after one of its data fields is changed.

15 Cloneable Objects A clone method for class Student that does a deep copy public Object clone() {try {Student theCopy = (Student)super.clone(); theCopy.fullName = (Name)fullName.clone(); return theCopy; } catch (CloneNotSupportedException e) {throw new Error(e.toString()); } } // end clone

16 Cloneable Objects Fig An instance of Student and its clone, including a deep copy of fullName.

17 Cloneable Objects Fig A shallow copy of fullName.

18 Tasks for a clone Method Invoke the clone method of the superclass with super.clone() Enclose call to clone in a try block Write a catch block to handle exception of CloneNotSupportedException Skip if super.clone() invokes a public clone method Clone mutable data fields of object super.clone() returned, when possible Return the clone

19 A Sorted List of Clones Recall problem of placing mutable objects in an ADT (such as a sorted list) If object is cloned before it is added to an ADT Client could access/change the ADT's data only by using ADT operations Requires that object added to the ADT be Cloneable

20 A Sorted List of Clones Fig An ADT and its client after the clone of an object is added to the ADT.

21 A Sorted List of Clones Fig The effect of getEntry if it did not return a clone.

22 A Sorted List of Clones Fig The effect of getEntry when it does return a clone.

23 Cloning an Array To make a deep clone of an array a of cloneable objects The class must implement Cloneable Invoke a.clone () Clone each object in the array Thing[ ] clonedArray = (Thing[ ])myArray.clone(); for (int index = 0; index < myArray.length; index++) clonedArray[index] = (Thing)myArray[index].clone();

24 Cloning a Chain The ADT list class must implement the interface Cloneable public class LList implements ListInterface, Cloneable { private Node firstNode; // reference to first node private int length; // number of entries in list...

25 Cloning a Chain Fig A list that stores its data in a linked chain and its shallow clone.

26 Cloning a Chain Fig A list that stores its data in a linked chain and its deep clone.