COP 3503 FALL 2012 Shayan Javed Lecture 7

1. COP 3503 FALL 2012ShayanJavedLecture 7 Programming Fundamentals using Java

2. More With Interfaces

3. Interfaces • Language construct for specifying functionality without implementation • Method specifications but no implementations.

4. Interfaces in Software Engineering • In large projects you have a lot of classes interacting with each other

5. Interfaces in Software Engineering • In large projects you have a lot of classes interacting with each other • Need formal descriptions of what a class does, and how it interacts with other classes.

6. Interfaces in Software Engineering • In large projects you have a lot of classes interacting with each other • Need formal descriptions of what a class does, and how it interacts with other classes. • Interfaces are a way to define expected behavior.

7. Interfaces in Software Engineering • In large projects you have a lot of classes interacting with each other • Need formal descriptions of what a class does, and how it interacts with other classes. • Interfaces are a way to define expected behavior. • Useful when multiple teams working in tandem.

8. Interfaces • Some interfaces already defined in Java • Widely used • Comparable • Comparator • Cloneable

9. The Comparable interface • Defined in the java.langPackage

10. The Comparable interface • Defined in the java.langPackage publicinterface Comparable { publicintcompareTo(Object o); }

11. The Comparable interface publicinterface Comparable { publicintcompareTo(Object o); }

12. The Comparable interface publicinterface Comparable { publicintcompareTo(Object o); } • Comparisons between objects of the same type

13. The Comparable interface publicinterface Comparable { publicintcompareTo(Object o); } • Comparisons between objects of the same type • Object1.compareTo(Object2)

14. The Comparable interface publicinterface Comparable { publicintcompareTo(Object o); } • Comparisons between objects of the same type • Object1.compareTo(Object2) • Return values: • < 0 = Object 1 < Object 2 (usually -1)

15. The Comparable interface publicinterface Comparable { publicintcompareTo(Object o); } • Comparisons between objects of the same type • Object1.compareTo(Object2) • Return values: • < 0 = Object 1 < Object 2 (usually -1) • == 0 = Object 1 == Object 2

16. The Comparable interface publicinterface Comparable { publicintcompareTo(Object o); } • Comparisons between objects of the same type • Object1.compareTo(Object2) • Return values: • < 0 = Object 1 < Object 2 (usually -1) • == 0 = Object 1 == Object 2 • > 0 = Object 1 > Object 2 (usually 1)

17. The Comparable interface • Used commonly.

18. The Comparable interface • Used commonly. • For ex. the String and Date classes.

19. The Comparable interface public String implements Comparable,… { publicintcompareTo(Object o) { // compares the two Strings lexicographically } }

20. The Comparable interface public String implements Comparable,… { publicintcompareTo(Object o) { // compares the two Strings lexicographically } } Ex.: "computer".compareTo ("comparison")

21. The Comparable interface public String implements Comparable,… { publicintcompareTo(Object o) { // compares the two Strings lexicographically } } Ex.: "computer".compareTo ("comparison") Returns 20

22. The Comparable interface "computer".compareTo ("comparison") Returns 20 • Provides the first non-zero difference in ASCII values.

23. The Comparable interface "computer".compareTo ("comparison") Returns 20 • Provides the first non-zero difference in ASCII values. • “c”, “o”, “m”, “p” are all equal

24. The Comparable interface "computer".compareTo ("comparison") Returns 20 • Provides the first non-zero difference in ASCII values. • “c”, “o”, “m”, “p” are all equal • Returns: (int)‘u’ - (int)‘a’

25. The Comparable interface "computer".compareTo ("comparison") Returns 20 • Provides the first non-zero difference in ASCII values. • “c”, “o”, “m”, “p” are all equal • Returns: (int)‘u’ - (int)‘a’ • So “computer” > “comparison”

26. The Comparable interface • Implement it for the Rectangle class

27. The Comparable interface • Implement it for the Rectangle class • Comparisons based on area

28. The Comparable interface • Implement it for the Rectangle class • Comparisons based on area public class Rectangle extendsGeometricObjectimplements Comparable { publicintcompareTo(Object ob) { Rectangle r = (Rectangle)ob; if (this.getArea() > r.getArea()) return 1; else if (r.getArea() > this.getArea()) return -1; else return 0; } }

29. The Comparable interface • What if a non-Rectangle object is passed in?

30. The Comparable interface • What if a non-Rectangle object is passed in? • A ClassCastException is thrown

31. The Comparable interface • What if a non-Rectangle object is passed in? • A ClassCastException is thrown • Will look at it later

32. The Comparable interface • Can also use Comparable as a data type

33. The Comparable interface • Can also use Comparable as a data type String s = “aString”; (s instanceofComparable) // returns true!

34. The Comparable interface • Can also use Comparable as a data type String s = “aString”; (s instanceofComparable) // returns true! Comparable[] compObjects = new Comparable[5];

35. The Comparable interface • So now we can compare objects

36. The Comparable interface • So now we can compare objects • Would be nice if we could sort them using this info

37. Sorting • Always need data sorted by certain requirements

38. Sorting • Always need data sorted by certain requirements • How do you sort arrays?

39. Sorting • Always need data sorted by certain requirements • How do you sort arrays? • Will look at specific sorting algorithms later on.

40. Sorting • Sorting on Amazon’s website

41. Sorting • By different categories

42. Comparing Books public Book implements Comparable{ String title; int popularity; double price, avgCustomerReview; DatepublicationDate; }

43. Comparing Books public Book implements Comparable{ String title; int popularity; double price, avgCustomerReview; DatepublicationDate; // comparison based on price publicintcompareTo(Object o) { Book b = (Book)o; if (price > b.getPrice()) return 1; else if (price <b.getPrice()) return -1; else return 0; } }

44. Arrays.sort() • An easy way to sort an array

45. Arrays.sort() • An easy way to sort an array • Works directly with most of the primitive types – int, char, double, long, etc.

46. Arrays.sort() • An easy way to sort an array • Works directly with most of the primitive types – int, char, double, long, etc. int numbers[] = {4, 1, 19, 8}; Arrays.sort(numbers);

47. Arrays.sort() • An easy way to sort an array • Works directly with most of the primitive types – int, char, double, long, etc. int numbers[] = {4, 1, 19, 8}; Arrays.sort(numbers); numbers = [1, 4, 8, 19]

48. Arrays.sort() • Can also sort arrays of Objects

49. Arrays.sort() • Can also sort arrays of Objects • Arrays.sort(Object[] o)

50. Arrays.sort() • Can also sort arrays of Objects • Arrays.sort(Object[] o) • Make sure that the objects in the array: • Implement Comparable