Interfaces. Interfaces. Interface Animal

Transcription

1 Interfaces January 31, Interfaces An abstract class has both methods with no bodies and methods with bodies. An interface is a class that has no fields and no implemented methods. In short, it is simply a collection of method headers. An interface becomes useful when it is used with a class that implements the methods defined in the interface. January 31, Interface Animal public interface Animal { public void isborn(); public void moves(); public void reproduce(); public void die(); Nothing is implemented. This interface all by itself is useless. January 31,

2 Bird implements Animal public class Bird implements Animal { // Bird has its own fields and own methods. protected String feathertype;... // This implements Animal s method reproduce(). public void reproduce() { System.out.println( This bird is laying eggs. ); In Bird, methods defined in Animal are implemented. January 31, Classes, abstract classes and interfaces In a class, all methods are implemented. In an abstract class, some methods are implemented, but some methods are abstract without implementation. In an interface, only method headers are given, and no method is implemented. January 31, Accessibility Classes, fields and methods must be declared with access modifiers. Classes and methods are usually declared as public accessible by anyone from anywhere. Fields are usually declared as private accessible only within the same object. January 31,

3 Accessibility For superclasses (including abstract classes), private is too restrictive. In such classes, methods are usually declared as public accessible by anyone from anywhere. fields are usually declared as protected accessible within the same class object and its subclass objects. January 31, References Let A be a class and B be a subclass of A. A reference variable of type A may refer to an object of type either A or B. A reference variable of type B may refer to only an object of type B. January 31, References If PoliceDog is a subclass of a class Dog, then the following are both legal: Dog lucy = new Dog(); Dog rusty = new PoliceDog(); The following, however, is not: PoliceDog winston = new Dog(); January 31,

4 References If Cat is a subclass of an abstract class Mammal, then the following are both legal: Cat abe = new Cat(); Mammal hudson = new Cat(); The following, however, is not: Cat fancy = new Mammal(); January 31, References If Bird is a class that implements an interface Animal, then the following are both legal: Bird coco = new Bird(); Animal kiwi = new Bird(); The following, however, is not: Bird angel = new Animal(); January 31, Polymorphism 4

5 What is polymorphism? In English, polymorphism means having many forms. In biology, polymorphism closely related to biodiversity, genetic variation and adaptation. In computing, polymorphism refers to the notion of a single expression having multiple types. January 31, Python example Since Python is weakly-typed (unlike Java), polymorphism is implicit and everywhere: x = Trinity print x x = 1823 print x Here, the type of x in print x is not explicit and thus has multiple possibilities. January 31, Mammal baby; baby = new Dog(); baby.breathe(); baby = new Cat(); baby.breathe(); Simple example These baby.breathe(); statements look identical, but they have different effects, invoking different methods. January 31,

6 Sorting example def sort(list): for i in range(len(list)): minindex = i for j in range(i + 1, len(list)): if list[j] < list[minindex]: minindex = j if minindex!= i: swap(list, i, minindex) L = [4, 1, 5, 9, 3, 2, 6, 7] sort(l) print L January 31, Sorting example def sort(list): for i in range(len(list)): minindex = i for j in range(i + 1, len(list)): if list[j] < list[minindex]: minindex = j if minindex!= i: swap(list, i, minindex) L = [ red, fun, hot,..., interesting ] sort(l) print L January 31, Polymorphic < The function sort is able to sort lists of any type because < is polymorphic; that is, the same < operator can be used to compare integers, words, etc... January 31,

7 How about Java? public void sort(int A[]) { for (int i = 0; i < A.length - 1; i++) { int minindex = i; for (int j = i + 1; j < A.length; j++) if (A[j] < A[minIndex]) minindex = j; if (minindex!= i) swap(a, i, minindex); This only works for int arrays. January 31, How about Java? public void sort(int A[]) { for (int i = 0; i < A.length - 1; i++) { int minindex = i; for (int j = i + 1; j < A.length; j++) if (A[j] < A[minIndex]) minindex = j; if (minindex!= i) swap(a, i, minindex); In Java, < is not polymorphic. January 31, How about Java? public void sort(comparable A[]) { for (int i = 0; i < A.length - 1; i++) { int minindex = i; for (int j = i + 1; j < A.length; j++) if (A[j].compareTo(A[minIndex]) < 0) minindex = j; if (minindex!= i) swap(a, i, minindex); Instead, we use compareto(). January 31,

8 Comparable Many Java classes (such as String) support compareto(). You can also define your own Java class with your own implementation of compareto(). Any Java class that implements the interface Comparable supports compareto(). January 31, Interface Comparable public interface Comparable { public int compareto(object o); This interface imposes an ordering on objects. Any class that implements this interface must support the method compareto(). For example, String is such a class. January 31, Dog implements Comparable public class Dog implements Comparable {... public int compareto(object o) { return weight - ((Dog) o).getweight(); This defines lucy < rusty if lucy weights less than rusty. January 31,

9 Multiple inheritance public class Dog implements Animal, Comparable {... public int compareto(object o) { return weight - ((Dog) o).getweight(); In Java, multiple inheritance is allowed with interfaces. January 31, Sorting Comparable objects public void sort(comparable A[]) { for (int i = 0; i < A.length - 1; i++) { int minindex = i; for (int j = i + 1; j < A.length; j++) if (A[j].compareTo(A[minIndex]) < 0) minindex = j; if (minindex!= i) swap(a, i, minindex); Here, A is an array of Comparable objects. January 31, Sorting Comparable objects Here are some examples of sorting Comparable objects in Java. Integer N[] = {4, 1, 5, 9, 3, 2, 6, 7; String S[] = { red, fun,..., interesting ; sort(n); sort(s); January 31,