clone()

1. clone() • Defined in Object • Creates an identical copy • Copies pointers to fields (does not copy fields of fields) • Makes a shallow copy • if the object’s class doesn’t implement Cloneable, a CloneNotSupportedException is thrown • x.clone() != x; • Modification to X no longer affect X.clone( ) • x.clone.getClass() == x.getClass(); • Usually,x.equals(x.clone()) == true; • However, this is not a requirement

2. Why Override clone()? • First, Object’s clone is a protected method • Its visibility can be increased: • Sometimes a deeper copy of a field is required publicclass X implements Cloneable {public Object clone () throws CloneNotSupportedException { return super.clone(); } } publicclass X implements Cloneable { private A a;public Object clone () throws CloneNotSupportedException { X x = (X)super.clone(); if(a != null) x.a = (A)a.clone(); return x; } }

3. Pitfalls in Overriding clone() • Implementing clone() using a constructor publicclass X implements Cloneable {public Object clone () throws CloneNotSupportedException { return new X(); } } publicclass Y extends X {//use clone from X } Clone returns wrong type! publicclass Y extends X {public Object clone () throws CloneNotSupportedException { Y y = (Y)super.clone(); … } } Exception!

4. Pitfalls in clone() - Cont • Using Constructors to copy subobjects • if field a refersto an object of a subclass of A, the cloned object will be different from the original publicclass X implements Cloneable {private A a; public Object clone () throws CloneNotSupportedException { X x = (X)super.clone(); if(a != null) x.a = new A(); return x; } }

5. equals(), hashCode() • Are defined in Object • equals() • Used for state equality testing • as opposed to operator == used for reference equality • Used for test containment of object in collection • Used for storing an object in hash-based collection • hashCode() • Returns an int value representing the object • Used for storing an object in hash-based collections

6. hashCode() in insertion publicclass SimpleHashSet { private List<Integer> hashCodes = new ArrayList<Integer>(); private List<List<Object>> lists = new ArrayList<ArrayList<Object>>();   publicvoid add(Object o) { List<Object> lst = null; for(int i=0; lst==null && i<hashCodes.size(); ++i) if(hashCodes.get(i) == o.hashCode()) lst = objects.get(i); if(lst == null) { hashCodes.add(o.hashCode()); lists.add(lst = new ArrayList<Object>()); }   lst.add(o); }   }

7. equals, hashCode in containment test publicclass SimpleHashSet { … public boolean exists(Object o) { List<Object> lst = null; for(int i=0; lst==null && i<hashCodes.size(); ++i) if(hashCodes.get(i) == o.hashCode()) lst = objects.get(i); if(lst == null) returnfalse; for(Object elem : lst) if(elem.equals(o)) returntrue; returnfalse; }   }

8. Overriding equals() • Default implementation of equals() • is based on the == operator • Two objects are equal if and only if they are the same object • Sometimes a programmer specific notion of identity is required: Date d1 = new Date(2007, 7, 7); Date d2 = new Date(2007, 7, 7); d1 != d2; d1.equals(d2) == true;

9. equals() Contract • The equals() method implements an equivalence relation: • It is reflexive • a.equals(a) is true • It is symmetric • a.equals(b)⇔b.equals(a) • It is transitive • a.equals(b) and b.equals(c)⇒a.equals(c) • It is consistent • repeated calls to the method must yield the same result unless the arguments are modified in-between • No object equals null • a.equals(null) = false

10. equals() Contract • The contract is defined in Java documentation: • http://java.sun.com/j2se/1.3/docs/api/java/lang/Object.html#equals(java.lang.Object) • The contract is not enforced by the compiler or the runtime system!

11. A Naïve Example class Point{ private finalint x; private finalint y; publicboolean equals(Object o){ if (!(o instanceof Point)) returnfalse; return ((Point)o).x == x && ((Point)o).y == y; } } Meets all contract’s demands (as long as inheritance is not involved…)

12. Extending the Point Class classColorPointextends Point{ private final Color color; publicboolean equals(Object o){ if (!(o instanceofColorPoint)) returno.equals(this); returnsuper.equals(o) && ((ColorPoint)o).color == color; } } ColorPoint p1 = new ColorPoint(1, 2, Color.RED); Point p2 = new Point(1, 2); p1.equals(p2); // ??? P2.equals(p1); // ???

13. Another try classColorPointextends Point{ final Color color; publicboolean equals(Object o){ if (!(o instanceof Point)) returnfalse; if (!(o instanceofColorPoint)) returno.equals(this); returnsuper.equals(o) && ((ColorPoint)o).color == color; } } ColorPoint p1 = new ColorPoint(1, 2, RED); Point p2 = new Point(1, 2); ColorPoint p3 = new ColorPoint(1, 2, BLUE); p1.equals(p2); // ??? p2.equals(p3); // ??? p3.equals(p1); // ???

14. A Safe Solution classColorPointextends Point{ final Color color; publicboolean equals(Object o){ if (o.getClass != ColorPoint.class) returnfalse; returnsuper.equals(o) && ((ColorPoint)o).color == color; } } • Meets all the contract’s requirements • May be too strict • An object is never equals to an instance of its super class • Even if the derived class doesn’t add fields

15. Another Solution • The idea: two objects must agree on their equality class Point{ … protectedboolean eq(Object o) { if(!(o instanceof Point)) returnfalse; return ((Point)o).x == x && ((Point)o).y == y; } publicboolean equals(Object o){ return (this.eq(o) && o.eq(this)); } } class ColorPoint extends Point{ … publicboolean eq (Object o){ if (!(o instanceof ColorPoint)) return false; returnsuper.eq(o) && ((ColorPoint)o).color == color; } }

16. hashcode() Contract • It is consistent • repeated calls to the method must yield the same int unless the arguments are modified in-between • Must generate equal values for equal objects We see that hashcode() is closely related to equals() • if you override equals, you must override hashCode

17. hashcode / equals contract violation class Point{ finalprivateint x, y; publicboolean equals(Object o){ if (!(o instanceof Point)) returnfalse; return ((Point)o).x == x; } publicinthashCode() { return 19 * y; { } publicstaticvoid main(String[] args) { Set<Point> s = newHashSet<Point>(); Point p1 = new Point(1, 2); s.add(p1); Point p2 = new Point(1, 3); //p1.equals(p2) is true System.out.println(s.contains(p2)); // false }

18. Comparable Interface • Why? • equals() can compare objects of different classes • equals() is boolean – can’t be used for sorting • A problem • we want compareTo to take a parameter of the same type as the receiver • requires a covariant parameter • The Solution • Using generics: publicinterface Comparable<T> { int compareTo(T o); }

19. Implementing Comparable publicclass Person implements Comparable<Person> { public String name; public int age;   // Order by age then by name public int compareTo(Person p) { int diff = this.age - p.age; if(diff != 0) return diff;   // Use compareTo of String returnthis.name.compareTo(p.name); } }

20. Using compareTo() publicstatic <T extends Comparable<T>> void sort(List<T> lst) { int top = lst.size(); for(int i = 0; i < top; ++i) { for(int j = 1; j < top; ++j) { T a = lst.get(j-1); T b = lst.get(j); if(a.compareTo(b) > 0) { lst.set(j-1 ,b); lst.set(j, a); } } } }

21. Comparator Interface • Used for comparing objects of a class that does not implement Comparable publicInterface Comparator<T> { publicintcompare(T o1, T o2); }   publicclassUtils { publicstatic<T> boolean isOrdered(Comparator<T> comp, T... ts) { for(inti = 1; i < ts.length; ++i) if(comp.compare(ts[i], ts[i-1]) < 0) returnfalse; returntrue; } }