Interfaces, collections and comparisons

Transcription

1 תכנות מונחה עצמים תרגול מספר 4 Interfaces, collections and comparisons

2 Interfaces

3 Overview

4 Overview O Interface defines behavior.

5 Overview O Interface defines behavior. O The interface includes: O Name O Functionality (methods).

6 Overview O Interface defines behavior. O The interface includes: O Name O Functionality (methods). O Classes can implement one or more interfaces.

7 Importance

8 Importance O Interfaces are useful for the following: O Capturing similarities among unrelated classes without artificially forcing a class relationship. O Declaring methods that one or more classes are expected to implement. O Revealing an object's programming interface without revealing its class.

9 Syntax O Syntax of defining java interface is: interface <interface-name>{ //method signatures Interface of entities that can talk: interface Talkable { void sayhello(); void saybye();

10 Talkable example

11 Talkable example public class Cat implements Talkable{

12 Talkable example public class Cat implements Talkable{ public class Dog implements Talkable{

13 Talkable example public class Cat implements Talkable{ public class Dog implements Talkable{ Note that each of the animals can have it own methods

14 Talkable example public class Cat implements Talkable{ public void sayhello(){ public class Dog implements Talkable{ Note that each of the animals can have it own methods

15 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public class Dog implements Talkable{ Note that each of the animals can have it own methods

16 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public void saybye(){ public class Dog implements Talkable{ Note that each of the animals can have it own methods

17 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public void saybye(){ System.out.println("Miau!"); public class Dog implements Talkable{ Note that each of the animals can have it own methods

18 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public void saybye(){ System.out.println("Miau!"); public void rest(){ public class Dog implements Talkable{ Note that each of the animals can have it own methods

19 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public void saybye(){ System.out.println("Miau!"); public void rest(){... public class Dog implements Talkable{ Note that each of the animals can have it own methods

20 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public void saybye(){ System.out.println("Miau!"); public void rest(){... public class Dog implements Talkable{ Note that each of the animals can have it own methods

21 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public void saybye(){ System.out.println("Miau!"); public void rest(){... public class Dog implements Talkable{ Note that each of the animals can have it own methods

22 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public void saybye(){ System.out.println("Miau!"); public void rest(){... public class Dog implements Talkable{ public void sayhello(){ Note that each of the animals can have it own methods

23 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public void saybye(){ System.out.println("Miau!"); public void rest(){... public class Dog implements Talkable{ public void sayhello(){ System.out.println( Hav Hav!"); Note that each of the animals can have it own methods

24 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public void saybye(){ System.out.println("Miau!"); public void rest(){... public class Dog implements Talkable{ public void sayhello(){ System.out.println( Hav Hav!"); public void saybye(){ Note that each of the animals can have it own methods

25 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public void saybye(){ System.out.println("Miau!"); public void rest(){... public class Dog implements Talkable{ public void sayhello(){ System.out.println( Hav Hav!"); public void saybye(){ System.out.println( Hou Hou!"); Note that each of the animals can have it own methods

26 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public void saybye(){ System.out.println("Miau!"); public void rest(){... public class Dog implements Talkable{ public void sayhello(){ System.out.println( Hav Hav!"); public void saybye(){ System.out.println( Hou Hou!"); public void run(){ Note that each of the animals can have it own methods

27 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public void saybye(){ System.out.println("Miau!"); public void rest(){... public class Dog implements Talkable{ public void sayhello(){ System.out.println( Hav Hav!"); public void saybye(){ System.out.println( Hou Hou!"); public void run(){... Note that each of the animals can have it own methods

28 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public void saybye(){ System.out.println("Miau!"); public void rest(){... public class Dog implements Talkable{ public void sayhello(){ System.out.println( Hav Hav!"); public void saybye(){ System.out.println( Hou Hou!"); public void run(){... Note that each of the animals can have it own methods

29 Talkable example public class Cat implements Talkable{ public void sayhello(){ System.out.println("Miau!"); public void saybye(){ System.out.println("Miau!"); public void rest(){... public class Dog implements Talkable{ public void sayhello(){ System.out.println( Hav Hav!"); public void saybye(){ System.out.println( Hou Hou!"); public void run(){... Note that each of the animals can have it own methods

30 What is a Dog (Cat)? O Dog is a Talkable, and also a Dog O Cat is a Talkable, and also a Cat O Every dog is a Talkable but not every Talkable is a Dog!!!

31 Talkable example O At this step, after we defined our different animals we want each of them to say hello public class MainFile { public static void main(string[] args) { Talkable t1=new Dog(); Talkable t2=new Cat(); System.out.print("The dog says Hi:"); t1.sayhello(); System.out.print("The cat says Hi:"); t2.sayhello(); Runtime decision (Talkable vs. dog\cat)

32 Talkable example public class RuntimeTalkable{ public static void main(string[] args) { Talkable t1; if (args[0].equals( Dog )) { t1=new Dog(); else { t1=new Cat(); System.out.print( A random animal says Hi:"); t1.sayhello(); Runtime decision (Talkable vs. dog\cat)

33 Using functions private static void makeitsayhi(talkable t1) { t1.sayhello(); The function gets a Talkable and the output is based on the specific class that t1 belongs to.

34 Collections

35 Collections

36 Collections O Everything in Java is an object.

37 Collections O Everything in Java is an object. O We want to store similar objects together.

38 Collections O Everything in Java is an object. O We want to store similar objects together. O Collections - a group of single items O List stores the objects in a certain order. O Set the same object cannot appear more than once.

39 Collections O Everything in Java is an object. O We want to store similar objects together. O Collections - a group of single items O List stores the objects in a certain order. O Set the same object cannot appear more than once. O Each collection has it own advantages and disadvantages.

40 Collections O Everything in Java is an object. O We want to store similar objects together. O Collections - a group of single items O List stores the objects in a certain order. O Set the same object cannot appear more than once. O Each collection has it own advantages and disadvantages. O Have to choose the collection based on the program s requirements.

41 Collections Collection Set List TreeSet HashSet LinkedList ArrayList Collection, Set and List are interfaces

42 List

43 Overview O List is an interface for an ordered collection, may contain duplicate elements. O Supports Positional access: O methods such as get, set, add, addall, and remove. O Support Search O methods such as indexof and lastindexof.

44 Javadoc Available at: List - JAVADOC

45 Built-in functions O void add(int index, Object element) Inserts the specified element at the specified position index in this list. O boolean add(object o) Appends the specified element to the end of this list. O boolean addall(collection c) Appends all of the elements in the specified collection to the end of this list. O void clear() Removes all of the elements from this list.

46 Built-in functions

47 Built-in functions O Object get(int index) Returns the element at the specified position in this list.

48 Built-in functions O Object get(int index) Returns the element at the specified position in this list. O int indexof(object o) Returns the index in this list of the first occurrence of the specified element, or -1 if the List does not contain this element.

49 Built-in functions O Object get(int index) Returns the element at the specified position in this list. O int indexof(object o) Returns the index in this list of the first occurrence of the specified element, or -1 if the List does not contain this element. O Object remove(int index) Removes the element at the specified position in this list.

50 Built-in functions O Object get(int index) Returns the element at the specified position in this list. O int indexof(object o) Returns the index in this list of the first occurrence of the specified element, or -1 if the List does not contain this element. O Object remove(int index) Removes the element at the specified position in this list. O Object set(int index, Object element) Replaces the element at the specified position in this list with the specified element.

51 Built-in functions O Object get(int index) Returns the element at the specified position in this list. O int indexof(object o) Returns the index in this list of the first occurrence of the specified element, or -1 if the List does not contain this element. O Object remove(int index) Removes the element at the specified position in this list. O Object set(int index, Object element) Replaces the element at the specified position in this list with the specified element. O int size() Returns the number of elements in this list.

52 Implementation O Useful lists: O ArrayList based on an array: O Fast random access. O Slow addition and removal from the middle of the list. O LinkedList based on a two-sided linked list O Slow random access. O Fast addition and removal from the middle of the list.

53 ArrayList - Overview O The ArrayList class implements the List interface. O ArrayList supports dynamic arrays that can grow and shrink as needed.

54 Time complexity Function get(int index) ArrayList O(1) LinkedList O(n) add(e element) O(1) O(1) add(int index, E element) O(n) O(n) remove(int index) O(n) O(n)

55 Time complexity 000,100 items

56 The talkable example public static void main(string[] args) { Talkable t1=new Dog(); Talkable t2=new Cat(); System.out.print("The dog says Hi:"); t1.sayhello(); System.out.print("The cat says Hi:"); t2.sayhello(); System.out.print("The dog says Bye:"); t1.saybye();

57 The talkable example O We want a list of talk able animals: public static void main(string[] args) { List l1 = new ArrayList (); List l2 = new LinkedList (); System.out.println("Using ArrayList"); l1.add(t1); l1.add(t2); System.out.println("Using LinkedList"); l2.add(t1); l2.add(t2);

58 Make them talk O Remember our list of talkable animals? O Now, lets make them make some sounds for (int i = 0; i < l1.size(); i++) { Talkable t=(talkable)l1.get(i); t.sayhello(); We iterated over each object in l1 (the ArrayList) or l2 (the LinkedList) and ask each of them to execute its implementation of sayhello.

59 Shortcut for (int i = 0; i < l1.size(); i++) { Talkable t=(talkable)l1.get(i); t.saybye(); for (Object entity: l1) { ((Talkable) entity).saybye();

60 Comparator

61 Comparator

62 Comparator O It will be really nice to sort our list.

63 Comparator O It will be really nice to sort our list. O Do we know what comes before what?

64 Comparator O It will be really nice to sort our list. O Do we know what comes before what? O How do we sort animals? O By Name O By Size

65 Comparator

66 Comparator O Do we have to define two implementations to sort a list from bottom to top and vice versa?

67 Comparator O Do we have to define two implementations to sort a list from bottom to top and vice versa? No, we can use Strategy pattern.

68 Comparator O Do we have to define two implementations to sort a list from bottom to top and vice versa? No, we can use Strategy pattern. O Based on this pattern, the sorting algorithm will use the external decision regarding how to sort the list.

69 Comparator O Do we have to define two implementations to sort a list from bottom to top and vice versa? No, we can use Strategy pattern. O Based on this pattern, the sorting algorithm will use the external decision regarding how to sort the list. O In order to affect the sorting method with collections we use the comparator.

70 Comparator \ Comparable O Any class can implement the above interfaces: interface Comparator { int compare(object o1, Object o2); interface Comparable { int compareto(object o) ; O Based on the implementation of these interfaces, the collections will decide how to sort the elements.

71 Comparable \ Comparator Parameter Comparable Comparator Sorting logic Implementati on Sorting method Calling method In same class whose objects are being sorted. Hence this is called natural ordering of objects Class whose objects to be sorted must implement this interface. E.g Country class needs to implement comparable to sort collection of country objects by area int compareto(object o1) This method compares this object with o1 object and returns a integer. Its value has following meaning: 1. positive this object is greater than o1 2. zero this object equals to o1 3. negative this object is less than o1 Collections.sort(List) Here objects will be sorted on the basis of CompareTo method In a separate class. Hence we can write different sorting based on different attributes of objects to be sorted. E.g. Sorting using id,name etc. Class whose objects to be sorted do not need to implement this interface. Some other class can implement this interface as well. E.g.- CountrySortByAreaComparator class can implement Comparator interface to sort collection of country objects by area int compare(object o1,object o2) This method compares o1 and o2 objects and returns a integer. Its value has following meaning: 1. positive o1 is greater than o2 2. zero o1 equals to o2 3. negative o1 is less than o2 Collections.sort(List, Comparator) Here objects will be sorted on the basis of Compare method in Comparator Package Java.lang.Comparable Java.util.Comparator

72 Code examples O ComparableMain O ComparatorMain O CountrySortByIdComparator

73 What have we learned today? Using interface Collections Sorting Customized sorting