1. 1 נתבונן בפונקציה הבאה public static int min(int[] a,int n) { int min = a[0]; for (int i = 1; (i < n ) && (i < a.length) ; i++) if (min > a[i]) min = a[i]; return min; } הפונקציה מחזירה לכל מערך של מספרים שלמים את המספר הנמוך ביותר בו. ניתן כמובן לכתוב פונקציה כזו עבור טיפוסי עצמים רבים ושונים זה מזה : מספרים, מחרוזות, תלמידים וכו '. היינו מעדיפים לכתוב את הקוד פעם אחת.

2. 2 הכלי המאפשר לנו זאת הוא הממשק Comparable (interface). המוגדר על ידי // public int compareTo(Object obj) returns: // a positive integer if this > obj // zero if this = obj // a negative integer if this < obj Public interface Comparable { public int compareTo(Object obj); }

3. 3 public class Util { public static Comparable min(Comparable[] a,int n) { Comparable min = a[0]; for (int i = 1; (i < a.length) && (a[i] != null) && (i < n); i++) if (min.compareTo(a[i]) > 0) min = a[i]; return min; } כעת ניתן לכתוב את הפונקציה מחדש בתוך המחלקה Util:

4. 4 נותר רק ליישם את הממשק ע " י המחלקות השונות לדוגמה public class Integer implements Comparable { : public int compareTo(Object other) { if (! (other instanceof Integer)) { System.out.println(“Cannot compare Integer to “+other); return 10000; } if (intValue() >((Integer) other).intValue()) return 1; if (intValue() <((Integer) other).intValue()) return -1; return 0; } : }

5. 5 או Public class String implements Comparable { : public int compareTo(String other) public int compareTo(Object other) { if (! (other instanceof Integer)) { System.out.println(“Cannot compare Integer to “+other); return 10000; } for (int i = 0; (i < size() )& (i <other.size()); i++) { if (charAt(i) > other.charAt(i)) return 1; if (charAt(i) < other.charAt(i)) return -1; } if (size() >other.size()) return 1; if (size() < other.size()) return –1; return 0; }

6. 6 וגם public class TwoDvector implements Comparable{ private double x,y; public TwoDvector(double x,double y) { this.x = x; this.y = y; } public String toString() { return "( "+x+", "+y+" )"; } public double x() { return x;} public double y() { return y;}

7. 7 public int compareTo(Object compare_me) { if (! (compare_me instanceof TwoDvector)) { System.out.println("cannot compare a "+ "twoDvector to "+compare_me); return 100000; } TwoDvector other = (TwoDvector) compare_me; double my_length = Math.sqrt(x*x+y*y); double other_length = Math.sqrt(other.x()*other.x()+ other.y()*other.y()); if (my_length > other_length) return 1; if (my_length < other_length) return -1; return 0; }

8. 8 public class TwoDvector_driver { public static void main(String[] args) { TwoDvector[] vectors = new TwoDvector[100]; vectors[0] = new TwoDvector(1,2); vectors[1] = new TwoDvector(0.5,2); vectors[2] = new TwoDvector(0.5,1); System.out.println(Tools.min(vectors,3)); } הפלט ( 0.5, 1.0 )

9. 9 ניתבונן כעת בממשק Set public interface Set { public boolean add(Object addMe); public Iterator iterator(); public boolean contains(Object obj); public boolean equals(Set other); public int size(); }

10. 10 ובממשק Iterator public interface Iterator { public boolean hasNext(); public Object next(); }

11. 11 public class SimpleSet implements Set{ public static final int DEFAULT_CAPACITY = 100; private int capacity; private int n_members; private Object[] members; public SimpleSet() { this(DEFAULT_CAPACITY); } public SimpleSet(int capacity) { this.capacity = capacity; members = new Object[capacity]; } נציג ישום אחד שלהם במחלקה SimpleSet

12. 12 public boolean add(Object add_me) { if (contains(add_me)) return false; if (n_members == capacity) { capacity = capacity + DEFAULT_CAPACITY ; Object[] new_members = new Object[capacity]; for (int i = 0; i < n_members; i++) new_members[i] = members[i]; members = new_members; } members[n_members] = add_me; n_members++; return true; }

13. 13 public boolean contains(Object other) { for (int i = 0; i < n_members; i++) if (members[i].equals(other)) return true; return false; } public int size() { return n_members; } public Iterator iterator() { return (Iterator) new SimpleSetIterator(); }

14. 14 public class SimpleSetIterator implements Iterator { private int current; public SimpleSetIterator() { current = 0; } public boolean hasNext() { return current < n_members; } public Object next() { current = current+1; return members[current - 1]; }

15. 15 public boolean equals (Object test_me) { if (! (test_me instanceof Set)) return false; Set other = (Set) test_me; if (size() != other.size()) return false; Iterator it = iterator(); while (it.hasNext()) if (! other.contains(it.next())) return false; return true; } } // class SimpleSet

16. 16 public class Set_driver { public static void main(String[] args) { SimpleSet my_set = new SimpleSet(2); my_set.add(new Integer(1)); my_set.add(new Double(50)); my_set.add("Happy new year"); Iterator it = my_set.iterator(); while (it.hasNext()) System.out.println(it.next());

17. 17 SimpleSet your_set = new SimpleSet(); your_set.add(new Double(50)); your_set.add(new Integer(1)); your_set.add("Happy new year"); it = your_set.iterator(); while (it.hasNext()) System.out.println(it.next()); System.out.println(my_set.equals(your_set)); System.out.println(my_set == your_set); }

18. 18 1 50.0 Happy new year 50.0 1 Happy new year true false פלט