Binghamton University. CS-140 Fall Interfaces

Transcription

1 Interfaces 1

2 Arrays vs. ArrayLists Similarities Indexed list of data elements of a specific type Allow you to determine the number of elements Allow you to read (get) or write (set) individual elements Differences Arrays are built-in, ArrayLists are objects in the library You can add new elements to an ArrayList 2

3 What can you do with a list? Java has a very strict definition every list in Java must support size() to get the number of items in a list get(n) to get the value of the n th element in the list add(e) to add element e to the list the type of e must match the type of the list add(n,e) insert element e at the n th set(n,e) replace the value at position n with element e and several others ArrayList is a formal list in java Many other classes that are lists (LinkedLists, Collections, ) 3

4 Interface Concept Chapter 10 An interface is a standardized set of methods The interface does not define HOW methods are implemented just that they exist When you define a class in Java, you can tell the compiler: I support interface xyz. If you do so, the compiler will look up interface xyz and make sure that every method in xyz is implemented for your class! Several interface are included in the standard library Your customer knows the standard interfaces using them makes his life easier 4

5 For instance, the list interface See Javadoc under List (Note interfaces are in italic print) Note that interfaces can have super interfaces List has super interfaces of Collection and Iterable An interface includes all the methods in the super interfaces Includes definitions for size(),get(), add(), and set() If you learn these for ArrayList, you already know them for Collection, LinkedList, etc. 5

6 For instance, test1 shapes Test1 included Circle and Rectangle Add RightTriangle All support similar methods void move(double dx,double dy) void grow(double factor) Point getllc() Point getur() String tostring() See xmp_interface 6

7 Creating an Interface package pkg_name; interface interface_name { method_declarations; } 7

8 Example Interface package xmp_interface; interface Shapes { void move(double xd, double yd); void grow(double factor); Point getllc(); Point getur(); } 8

9 Method Privacy Attributes Who is allowed to use your method? Privacy Invoke by Default public Any java code Interface default Code in this package Class private Code in this Class 9

10 Using an Interface package xmp_interface; class Rectangle implements Shapes { } May have to upgrade privacy to public Compiler enforces compliance with the interface! 10

11 Using library interfaces public class mylist implements java.util.list { int get(int idx) { } int set(int idx,int val) { } // Compile error if you forget an interface method } 11

12 Enhanced For Loops The list interface provides all the methods required for the compiler to automatically figure out how to implement an enhanced for loop, so for(int i=0; i<lines.size(); i++) { String line=lines.get(i); Is the same as for(string line : lines) { 12

13 The Comparable Interface Many classes have a natural order It is possible to say this object is less than this object Examples of comparable classes: integers, doubles, Strings, BankAccounts, 13

14 The compareto method Given two comparable objects, a and b, one of the following is true a < b, a == b, a > b The comparable interface requires one compareto method, invoked as: a.compareto(b); The compareto returns a - int if a<b, 0 if a==b, and + if a>b For instance, for Integer: int compareto(integer b) { return this.value b.value; } 14

15 Back to the String class public final class String implements java.io.serializable, Comparable<String>, CharSequence { A class may support MULTIPLE interfaces! The Comparable interface is generic (supports multiple types) Comparable<String> implies a method: int CompareTo(String comp); 15

16 String compareto (simplified) method int compareto(string b) { for(int i=0;i<this.size(); i++) { if (i>=b.size()) { return 1; } if (this.charat(i)<b.charat(i)) { return -1; } if (this.charat(i)>b.charat(i)) { return 1; } } if (this.size()<b.size()) { return -1; } return 0; } 16

17 String compareto (actual) method public int compareto(string anotherstring) { int len1 = value.length; int len2 = anotherstring.value.length; int lim = Math.min(len1, len2); char v1[ ] = value; char v2[ ] = anotherstring.value; int k = 0; while (k < lim) { char c1 = v1[k]; char c2 = v2[k]; if (c1!= c2) { return c1 - c2; } k++; } return len1 - len2; } 17

18 Free Stuff! If your class is comparable Arrays of references to your class can be sorted Lists of references to your class can be sorted Java figures out how to implement all this stuff! You don t have to write any code to sort! Java s algorithm is probably as fast or faster than any you could write. 18

19 Multiple Compare Fields Sometimes compareto uses more than one field to compare.. For instance, suppose a Person class has first and last names int compareto(person that) { } if (0==this.lastName.compareTo(that.lastName) { } return this.firstname.compareto(that.firstname); return this.lastname.compareto(that.lastname); 19

20 The Arrays Class Static utility methods supporting arrays Things like sorting, searching, filling, converting to lists, etc. 20

21 Sorting Arrays String[ ] arr1 = {"This", "is", "a", "simple", "String", "without", any", "punctuation"}; Arrays.sort(arr1); System.out.println(Arrays.toString(arr1)); // [String, This, a, any, is, punctuation, simple, without] double[ ] arr2 = {6.5, 9.7, 4.1, 1.6, 8.3, 2.3, 3.5, 5.8}; Arrays.sort(arr2); System.out.println(Arrays.toString(arr2)); // [1.6, 2.3, 3.5, 4.1, 5.8, 6.5, 8.3, 9.7] 21

22 Arrays of comparables can be sorted LocalDate d1 = LocalDate.of(2015, 2, 1); LocalDate d2 = LocalDate.of(2015, 2, 10); LocalDate d3 = LocalDate.of(2015, 2, 16); LocalDate d4 = LocalDate.of(2015, 1, 31); LocalDate d5 = LocalDate.of(2014, 2, 16); LocalDate[ ] dates = {d1, d2, d3, d4, d5}; Arrays.sort(dates); System.out.println(Arrays.toString(dates)); // [ , , , , ] 22

23 The Collection Interface Most Java data structures implement the collection interface The Java documentation points you to Set, List, SortedSet, HashSet, TreeSet, ArrayList, LinkedList, Vector Collections are bunches of data elements may or may not contain duplicates Collections are iterable Support the Iterable interface Enables enhance for loops Supports add(), remove(), size(), etc. 23

24 The Collections Class Static utility methods supporting collections Enable methods for all classes that support collections Things like sorting, searching, filling, converting to array, etc. 24

25 Collections of comparables can be sorted ArrayList<String> list1 = new ArrayList<>(); list1.add("this"); list1.add("is"); list1.add("a"); list1.add("simple"); list1.add("string"); list1.add("with"); list1.add("some"); list1.add("punctuation"); Collections.sort(list1); System.out.println(list1); // [String, This, a, is, punctuation, simple, some, with] Note: An ArrayList is a Collection 25

26 Other Collections Methods binarysearch fill frequency max min reverse rotate shuffle 26

27 The Collection Family Collection List Queue Set ArrayList Stack LinkedList PriorityQue HashSet TreeSet 27