Chương 4 Iterator - Comparable - Comparator

elements of Java Sets and Maps can't be accessed by index must use a "foreach" loop: Set scores = new HashSet(); for (int score : scores) { System.out.println("The score is " + score); }

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Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level 18/09/2013 ‹#› /XX MÔN: CẤU TRÚC DỮ LIỆU GV: NGUYỄN XUÂN VINH Iterator - Comparable - Comparator Nguyễn Xuân Vinh CẤU TRÚC DỮ LIỆU DATA STRUCTURES [214331] Java Collection Architecture The Differences!!! How to browse element? Elements of List are indexed. int value = list[0]; Elements of LinkedList, Sets and Maps can't be accessed by index How to remove element? index 0 1 2 3 4 5 6 7 8 9 value 3 8 9 7 5 12 0 0 0 0 List Set "the" "to" "from" "we" Linked List Map Examining sets and maps elements of Java Sets and Maps can't be accessed by index must use a "foreach" loop: Set scores = new HashSet(); for (int score : scores) { System.out.println("The score is " + score); } Problem: foreach is read-only; cannot modify set while looping for (int score : scores) { if (score set = new HashSet(); ... Iterator itr = set.iterator(); ... hasNext() returns true if there are more elements to examine next() returns the next element from the collection (throws a NoSuchElementException if there are none left to examine) remove() removes from the collection the last value returned by next() (throws IllegalStateException if you have not called next() yet) Iterator example Set scores = new HashSet(); scores.add(38); scores.add(94); scores.add(87); scores.add(43); scores.add(62); ... Iterator itr = scores.iterator(); while (itr.hasNext()) { int score =; System.out.println("The score is " + score); // eliminate any failing grades if (score scores = new HashMap(); scores.put("Kim", 38); scores.put("Lisa", 94); scores.put("Ryan", 87); scores.put("Morgan", 43); scores.put("Marisa", 62); ... Iterator itr = scores.keySet().iterator(); while (itr.hasNext()) { String name =; int score = scores.get(name); System.out.println(name + " got " + score); // eliminate any failing students if (score scores = new HashSet(); scores.add(38); scores.add(94); scores.add(87); scores.add(43); // unpredictable order scores.add(62); System.out.println(scores); // [62, 94, 43, 87, 38] Set scores = new TreeSet(); scores.add(38); scores.add(94); scores.add(87); scores.add(43); // sorted natural order scores.add(62); System.out.println(scores); // [38, 43, 62, 87, 94] Ordering our own types We cannot make a TreeSet or TreeMap of any arbitrary type, because Java doesn't know how to order the elements. The program compiles but crashes when we run it. Set tags = new TreeSet(); tags.add(new HtmlTag("body", true)); tags.add(new HtmlTag("b", false)); ... Exception in thread "main" java.lang.ClassCastException at java.util.TreeMap.put( at java.util.TreeSet.add( at MyProgram.main( Comparable (10.2) public interface Comparable { public int compareTo(E other); } A class can implement the Comparable interface to define a natural ordering function for its objects. A call of a.compareTo(b) should return: a value 0 if a comes "after" b in the ordering, or 0 if a and b are considered "equal" in the ordering. Comparable example public class Point implements Comparable { private int x; private int y; ... // sort by x and break ties by y public int compareTo(Point other) { if (x other.x) { return 1; } else if (y other.y) { return 1; // same x, larger y } else { return 0; // same x and same y } } } compareTo tricks subtraction trick - Subtracting related numeric values produces the right result for what you want compareTo to return: // sort by x and break ties by y public int compareTo(Point other) { if (x != other.x) { return x - other.x; // different x } else { return y - other.y; // same x; compare y } } The idea: if x > other.x, then x - other.x > 0 if x "04/01" public int compareTo(Date other) { return toString().compareTo(other.toString()); } Comparable and sorting The Arrays and Collections classes in java.util have a static method sort that sorts the elements of an array/list Point[] points = new Point[3]; points[0] = new Point(7, 6); points[1] = new Point(10, 2) points[2] = new Point(7, -1); points[3] = new Point(3, 11); Arrays.sort(points); System.out.println(Arrays.toString(points)); // (3, 11), (7, -1), (7, 6), (10, 2) List points = new ArrayList(); points.add(new Point(7, 6)); ... Collections.sort(points); System.out.println(points); // (3, 11), (7, -1), (7, 6), (10, 2) Arrays class Method name Description asList(value1, ..., valueN) returns a List containing the given values as its elements binarySearch(array, value) returns the index of the given value in a sorted array ( object compared to. Positive number: current object < object compared to. Zero: two objects are equal. Understanding Comparable The natural ordering should be consistent with equals(): Two elements are equal (equals()): compareTo() should return zero. Your class implements Comparable and inconsistent with equals(): won't work properly within a SortedSet or SortedMap. Understanding Comparable Never call the method directly: Collections Framework call the compareTo() method for you when necessary. Not your responsibility to call the method. Comparator Comparator basics Comparable objects: compare multiple instances of themselves. Comparator: compare two other objects. Custom ordering by creating a Comparator’s implement when: Don't like the natural ordering Your class doesn't implement Comparable Comparator basics Ordering is done through the compare () method. Understanding Comparator You have to implement compare(). compare()  compareTo() But, both objects to compare must be passed in as arguments: public int compare(Object obj1, Object obj2) Understanding Comparator There is one predefined Comparator already implemented for you to sort elements in their reverse natural order: Collections .reverseOrder() comparator When should we use Comparable or Comparator? ITERABLE - ITERATOR Iterator Object An iterator is an object that implements the interface Iterator Usage of Iterator Provide a uniform way of accessing collection elements sequentially in earlier versions of Java From JDK 5 will work with anything that implements the interface Iterable Directly implementing Iterable Collection interface was made to extend Iterable  any collection can be the target of foreach To build your own implementation of Iterable Summary Sorting: Comparable, Comparator Iterable, Iterator HỎI ĐÁP