Binghamton University. CS-140 Fall Interfaces

Transcription

1 Interfaces 1

2 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, ) 2

3 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 3

4 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. 4

5 Another example : Shapes Let s make a package about 2d shapes We need a Coordinate class to manage (x,y) coordinates Let s have a triangle class with three points Let s have a circle class with a center and a radius Let s have a rectangle class with a lowerleft (x,y), with and height 5

6 What do we want to do with these shapes Find the perimeter Find the area Move the shape to a different location (translate) Find the min/max x and y coordinates Make a bounding box for the shape 6

7 Or, in terms of methods public double perimeter(); public double area(); public double minx(); public double miny(); public double maxx(); public double maxy(); public void translate(double dx, double dy); public Rectangle boundingbox(); 7

8 Creating an Interface package pkg_name; interface interface_name { method_declarations; } Warning: method declarations in interfaces must be public! 8

9 Example Interface package xmpshapes; interface Shape { public double perimeter(); public double area(); public double minx(); public double miny(); public double maxx(); public double maxy(); public void translate(double dx, double dy); public Rectangle boundingbox(); } 9

10 Using an Interface package xmp_interface; class Rectangle implements Shape { } May have to upgrade privacy to public Compiler enforces compliance with the interface! Classes may implement multiple interfaces 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 Interface as a Type! We can use an interface name like a type name For instance, a reference variable may be declared as an interface type List<Integer> nums = new ArrayList<Integer>(); Implication: nums references an object that supports the List<Integer> interface We can do anything to nums that we can do to lists but if there are ArrayList methods that are NOT in the List interface, we CANNOT use those methods without extra work! 12

13 Interface as a Parameter Type Use an interface as the type of a parameter Then, any object which implements that interface is valid as that parameter static public int firstelement(list<integer> list) { if (list==null) return 0; if (list.size()==0) return 0; return list.get(0); } 13

14 Implication of Interface as Type We can write code that works on a large variety of classes as long as all those classes implement the interface we care about We can re-use code for our new classes as long as these new classes can implement an interface required by old code 14

15 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) { 15

16 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, 16

17 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; } 17

18 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); 18

19 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; } 19

20 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; } 20

21 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. 21

22 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); 22

23 The Arrays Class Static utility methods supporting arrays Things like sorting, searching, filling, converting to lists, etc. 23

24 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] 24

25 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)); // [ , , , , ] 25

26 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 26

27 Other Collections Methods binarysearch fill frequency max min reverse rotate shuffle 27

28 The Collection Family Collection List Queue Set ArrayList Stack LinkedList PriorityQue HashSet TreeSet 28