CIS3023: Programming Fundamentals for CIS Majors II Summer 2010

1. CIS3023: Programming Fundamentals for CIS Majors II Summer 2010 Interfaces (Part II) “A picture is worth a thousand words. An interface is worth a thousand pictures.” -Ben Shneiderman Course Lecture Slides28June 2010 GaneshViswanathan

2. Interface • Language construct that specifies functionality without any hint at implementation. • An interface contains method specifications but no method implementations.

3. Comparable Interface // Interface defined in java.lang package public interface Comparable { public intcompareTo(Object o); } • compareTo( ) • Compares this object with the specified object for order. • Returns a negative integer, zero or a positive integer when this object is less than, equal to or greater than the specified object. • Throws:ClassCastExceptionif the specified object's type prevents it from being compared to this Object.

4. Declaring Classes to Implement Comparable

5. public class ComparableRectangle extends Rectangle implements Comparable { public ComparableRectangle(double width, double height){ super(width, height); } // Implement the compareTo method defined in Comparable public intcompareTo(Object o) { ComparableRectanglecr = (ComparableRectangle)o; if (getArea() > cr.getArea()) return 1; else if(getArea() < cr.getArea()) return -1; else return 0; } }

6. Using Arrays.sort() • If you have an array of objects of some class C and you want to use Arrays.sort(Object[] a) or Arrays.sort(Object[] a, intfromIndex, inttoIndex) to sort this array, you must make class C implement Comparable interface • The above sort() methods use the compareTo() method to compare two array elements of reference type

7. Arrays.Sort() • What if you want to sort this array of objects in order of different attributes of class C at different times? • Solution: Use Arrays.sort(Object[] a, Comparator c) Or Arrays.sort(Object[] a, intfromIndex, inttoIndex, Comparator c)

8. Comparator Interface public interface Comparator { public intcompare(Object o1, Object o2) } Returns: a negative integer, zero, or a positive integer when the first argument is less than,equal to, or greater thanthe second. Throws:ClassCastException - if the arguments' types prevent them from being compared by this Comparator.

9. Solution • Create a class that implements the Comparator interface • Code the compare() method to define the ordering between objects of this class • Pass an instance of this class to the Arrays.Sort() method

10. Another scenario • What if • you are using a class C which does not implement the Comparable interface • You do not have access to C.java file • You only have access to the C.class file • And you want to sort an arrays of objects of class C in order of some attribute of the class

11. Solution • Create a class that implements the Comparator interface • Code the compare() method to define the ordering between objects of this class

12. Another Advantage of using interfaces as data types public class Max { public static Comparable max(? o1, ? o2) { if (o1.compareTo(o2) > 0) return o1; else return o2; } } ComparableRectangle rectangle1 = new ComparableRectangle(4, 5); ComparableRectangle rectangle2 = new ComparableRectangle(3, 6); System.out.println(Max.max(rectangle1, rectangle2));

13. Another Advantage of using interfaces as data types public class Max { public static Comparable max(Comparable o1, Comparable o2) { if (o1.compareTo(o2) > 0) return o1; else return o2; } } ComparableRectangle rectangle1 = new ComparableRectangle(4, 5); ComparableRectangle rectangle2 = new ComparableRectangle(3, 6); System.out.println(Max.max(rectangle1, rectangle2));

14. Interfaces, cont • Max method in (a) is more robust.

15. Get more info! • Tutorial: • http://lkamal.blogspot.com/2008/07/java-sorting-comparator-vs-comparable.html • Comparable vs. Comparator: • http://grdurand.com/static/presentation_four/comparable.html • Using comparable: • http://onjava.com/pub/a/onjava/2003/03/12/java_comp.html

16. Use Comparable(implement compareTo( ) method): • if you want to compare based on the default natural ordering. • if the object is in your control. • Use Comparator (implement compare( ) method): • if you want comparing behavior different from the expected default (which is specified by Comparable). • if the object is outside your control and you cannot make them implement Comparable.

17. “Object Ordering”Reference: Java Docs See supporting code (TestComparator.java)

18. Object Duplication • Shallow Copy and Deep Copy • Using Java’s Cloneable interface

19. Shallow Copy • Bit-wise copy of an object. • A new object created with an exact copy of the values in the original object. • If any of the fields of the object are referencesto other objects, only the references are copied. Thus, if the object you are copying contains references to yet other objects, a shallow copy refers to the same sub-objects.

20. Deep Copy • Complete duplicate copy of an object. • If an object has references to other objects, complete new copies of those objects are also made. • A deep copy generates a copy not only of the primitive values of the original object, but copies of all sub-objects as well, all the way to the bottom. To get a true, complete copy of the original object, you will need to override the default object.clone() to implement a full deep copy for the object.

21. The Cloneable Interface • It is a Marker Interface i.e. • it does not contain constants or methods. • Defined in the java.lang package as follows: package java.lang; public interface Cloneable { }

22. Examples Many classes (e.g., Date and Calendar) in the Java library implement Cloneable. Thus, the instances of these classes can be cloned.For example, the following code outputs • Calendar calendar = new GregorianCalendar(2003, 2, 1); • Calendar calendarCopy = (Calendar)calendar.clone(); • System.out.println("calendar == calendarCopy is " + • (calendar == calendarCopy)); • System.out.println("calendar.equals(calendarCopy) is " • + calendar.equals(calendarCopy)); • calendar == calendarCopy is false • calendar.equals(calendarCopy) is true

23. Implementing Cloneable Interface • A class that implements the Cloneable interface must override the clone() method • in the Object class.

24. public class House implements Cloneable{ private int id; private double area; public House(int id, double area) { this.id = id; this.area = area; } public double getId() { return id; } public double getArea() { return area; } public Object clone() throws CloneNotSupportedException { return super.clone(); } }

25. public class House implements Cloneable{ private int id; private double area; private java.util.DatewhenBuilt; public House(int id, double area) { this.id = id; this.area = area; whenBuilt = new java.util.Date(); } public double getId() { return id; } public double getArea() { return area; } public java.util.DategetWhenBuilt(){ return whenBuilt; } public Object clone() throws CloneNotSupportedException { return super.clone(); } }

26. Shallow vs. Deep Copy House house1 = new House(1, 1750.50); House house2 = (House)house1.clone();

27. public class House implements Cloneable{ private int id; private double area; private java.util.DatewhenBuilt; // other code public Object clone() throws CloneNotSupportedException { House house = (House)super.clone(); house.whenBuilt = (java.util.Date)(whenBuilt.clone()); return house; } } All mutable fields should be cloned to get a deep copy. Strings are immutable, so there is no need to clone String type attributes separately.

28. public class House implements Cloneable{ private int id; private double area; private String name; public House(int id, double area) { this.id = id; this.area = area; whenBuilt = new java.util.Date(); } public double getId() { return id; } public double getArea() { return area; } public java.util.Date getWhenBuilt(){ return whenBuilt; } public Object clone() throws CloneNotSupportedException { return super.clone(); } } Is the clone() method doing deep copy?

29. Can the clone() method be written as follows? class House implements Cloneable { private intnumberOfRooms; private intsquareFeet; //... public Object clone() throws CloneNotSupportedException { return new House(numberOfRooms, squareFeet); } }

30. What happens when the following code is executed? public class MultiStoryHouse extends House { private intnumberOfStories; public MultiStoryHouse(intnumRooms, intsqFootage, intnumStories) { super(numRooms, sqFootage); numberofStories = numStories; } // no clone method defined } public class Driver { public static void main(String[] args) { MultiStoryHousemsh = new MultiStoryHouse(6, 3000, 2); MultiStoryHouse other = (MultiStoryHouse)msh.clone(); } } //Exception

31. Get more info! • Java docs: Cloneable • http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Cloneable.html • Wikipedia: Clone • http://en.wikipedia.org/wiki/Clone_(28Java_method) • Clone( ) method explained! • http://geekexplains.blogspot.com/2008/06/what-is-cloning-how-clone-method-works.html

32. Things to do: this July 4th • 32nd Annual Melon Run (3Miler) • at Westside Park, Gainesville. See Florida Track Club (FTC) website for details. • Celebration and fireworks (in Alachua) at • Hal Brady Recreation Complex(14300 NW 146th Terr.) • See “Venture outside Gainesville this July 4th” article at Gainesville.com