Algorithm Programming 1 89-210 Java API and Containers

1. Algorithm Programming 189-210Java API and Containers Bar-Ilan University 2007-2008 תשס"ח by Moshe Fresko

2. “Object” class • “java.lang.Object” class is the root of all class hierarchy • Methods of “Object” that can be overridden: • public boolean equals(Object obj) • public String toString() • public int hashCode() • protected void finilize()throws Throwable • protected Object clone() throws CloneNotSupportedException

3. “Object” class • Other methods of “Object” class • public final Class getClass( ) Runtime class information • public final void wait(…) Causes current thread to wait • public final void notify( ) Wakes up a single thread that is waiting on this object’s monitor • public final void notifyAll( ) Wakes up all threads that are waiting

4. “Object” class methods • Overriding Object methods • public String toString( ) String representation of this object • protected void finilize( )throws Throwable This function is called by the Garbage Collector before the physical destruction of the object. • public int hashCode( ) • Hash Code to be used by containers like HashSet or HashMap. 1. It must be consistent within an object: x.hashCode()==x.hashCode() 2. It must be the same for “equal” objects. If x.equals(y), then x.hashCode()==y.hashCode()

5. “Object” class methods • Overriding Object methods • protected Object clone() throws CloneNotSupportedException • When overriding, be aware that … • x.clone() != x • x.clone().getClass() == x.getClass() • x.clone().equals(x) • public boolean equals(Object obj) • When overriding, be aware to keep these characteristics • Reflexive: x.equals(x) == true • Symmetric: x.equals(y) == y.equals(x) • Transitive: if (x.equals(y)) and (y.equals(z)) then z.equals(x) • Consistent: x.equals(y) returns the same value for the same unchanged object references x and y. • x.equals(null) == false

6. Algorithm Programming 1Exception Handling Moshe Fresko

7. Basic Exceptions • float div(float x,float y) throws ArithmeticException { if (y==0.0f) throw new ArithmeticException(“Division by zero in div()”) ;} • void apply(Object t) throws NullPointerException { if (t==null) throw new NullPointerException(“Null pointer in apply()”) ;} • “throw” returns up the newly created object immediately from the function (or scope), even many levels … • “Throwable” : Exception root class. • Throwable() • Throwable(String) • “Exception” extends “Throwable” • All program exception classes must directly/indirectly inherit from “Exception” • Information on exception might be both in the object’s class, or in the String information inside (or other internal data).

8. Catching Exception • The “try” Block try { // Code that might generate exception } • Exception Handlers try { // Code that might generate exceptions } catch(Type1 id1) { // Handle exceptions of Type1 } catch(Type2 id2) { // Handle exceptions of Type2 } catch(Type3 id3) { // Handle exceptions of Type3 }

9. Custom Exceptions // SimpleExceptionDemo.java class SimpleException extends Exception { } public class SimpleExceptionDemo { public void f() throws SimpleException { System.out.println("Throw SimpleException from f()") ; throw new SimpleException(); } public static void main(String[] args) { SimpleExceptionDemo sed = new SimpleExceptionDemo() ; try { sed.f() ; } catch (SimpleException e) { System.err.println("Caught it!") ; } } } // Output Throw SimpleException from f() Caught it!

10. Catching Exception • Catching any exception catch(Exception e) { System.err.println("Caught an exception"); } • Getting information on Exception (“Throwable” methods) • String getMessage() • String toString() • void printStackTrace() • void printStackTrace(PrintStream) • Etc.

11. Exception Specification • Exception Specification void f() throws TooBig, TooSmall, DivZero { //... • Checked Exceptions Either you must take care of exceptions in your code or specify it in Exception Specification • When you don’t tell of any exception, only RuntimeException can be thrown void f() { //… • Re-throwing an exception catch(Exception e) { System.err.println("An exception was thrown"); throw e; }

12. Standard Java Exceptions Object • Throwable : Root of all Exceptions/Errors “Error” : We don’t need to take care of it “Exception” : The Root of all exceptions • Unchecked Exceptions • “RuntimeException” Throwable Error Exception RuntimeException MyException1 MyException2

13. “finally” keyword • Performing clean-up try { // The guarded region: Dangerous activities // that might throw A, B, or C } catch(A a1) { // Handler for situation A } catch(B b1) { // Handler for situation B } catch(C c1) { // Handler for situation C } finally { // Activities that happen every time }

14. Algorithm Programming 189-210Containers in Java Bar-Ilan University 2006-2007 תשס"ז by Moshe Fresko

15. Array utilities • In “java.utils.Arrays” class there are some static utility functions: • List asList(Object[] a) ; • boolean equals(char[] a, char[] b) boolean equals(Object[] a, Object[] b) • void fill(char[] a, char val) void fill(Object[] a, Object val) void fill(char[] a, int fromIdx, int toIdx, char val) void fill(Object[] a, int fromIdx, int toIdx, Object val)

16. Array Utilities int binarySearch(char[] a, char key) int binarySearch(Object[] a, Object key) int binarySearch(Object[] a, Object key, Comparator c) void sort(char[] a) void sort(Object[] a) void sort(Object[] a, Comparator c) void sort(char[] a, int fromIdx, int toIdx) void sort(Object[] a, int fromIdx, int toIdx) void sort(Object[] a, int fromIdx, int toIdx, Comparator c)

17. Array Utilities • For sort and binarySearch of Objects • Either a new Comparator c must be given • Or the given objects must implement Comparable • In both cases the comparison function returns Less then 0, if left object is smaller Greater then 0, if left object is greater 0, if both objects are equal • In java.utils.Comparator interface Comparator { int compare(Object o1, Object o2) ; } • In java.lang.Comparable interface Comparable { int compareTo(Object o) ; }

18. Design Pattern : “STRATEGY” • “Strategy” Pattern: • Define a family of algorithms, encapsulate each one, and make them interchangeable. • Strategy lets the algorithm vary independently from clients that use it. • Example: • Arrays.sort(Object[] a, Comparator c) • binarySearch(Object[] a, Object key, Comparator c)

19. Example : sort() import java.util.* ; class A { public final int i ; A(int i) { this.i = i ; } } class Ascending implements Comparator { public int compare(Object o1, Object o2) { A a1=(A)o1, a2=(A)o2 ; if (a1.i<a2.i) return -1 ; if (a1.i>a2.i) return +1 ; return 0 ; } } class Descending implements Comparator { public int compare(Object o1, Object o2) { A a1=(A)o1, a2=(A)o2 ; if (a1.i<a2.i) return +1 ; if (a1.i>a2.i) return -1 ; return 0 ; } }

20. Example “sort()” public class S { public static void main(String[] args) { A[] asc = { new A(1), new A(5), new A(3), new A(2) } ; A[] dsc = (A[]) asc.clone() ; Arrays.sort(asc,new Ascending()) ; Arrays.sort(dsc,new Descending()) ; System.out.println("Ascending : "+arrStr(asc)) ; System.out.println("Descending: "+arrStr(dsc)) ; } static String arrStr(A[] a) { String s = "[" + a[0].i ; for (int i=1;i<a.length;++i) s += ","+a[i].i ; s+="]" ; return s ; } } // Output : Ascending : [1,2,3,5] // Descending: [5,3,2,1]

21. Java Containers • In Java two types of Containers • Collection : A group of individual elements • List : Keeps elements in a particular sequence. • Set : Cannot have duplicate elements • Map : A set of key-value pairs. ( Also known as Associative arrays)

22. Simplified Collections Diagram

23. Collection methods boolean add(Object o) : Adds element to the collection boolean add(Collection c) : void clear() : Clears the collection. Size will be 0. boolean contains(Object o) : boolean containsAll(Collection c) : boolean isEmpty() : Returns true if collection is empty. Iterator iterator() : boolean remove(Object o) : Removes the element. boolean removeAll(Collection c) : boolean retainAll(Collection c) : int size() : Returns the number of elements in collection Object[] toArray() : Object[] toArray(Object[] a) :

24. List and Set implementations • “Collection” interface • “List” interface : ( Order of insertion is kept ) • ArrayList : Implemented as an array of elements. Fast Random Access, but slow insertion and deletion from the middle. • LinkedList : Implemented as a double linked list. Insertion and Deletion in the middle is fast, but Slow in Random Access. • “Set” interface : ( Unique elements ) • HashSet : For sets where look-up time is important. Inserted objects must implement hashCode() method. • TreeSet : A sorted list can easily be extracted

25. List and Set methods • interface “List” : Addition to “Collection” Object get(int index) Object set(int index, Object element) void add(int index, Object element) Object remove(int index) int indexOf(Object o) … • Interface “Set” : Addition to “Collection” …

26. Map methods Object put(Object key, Object value) : Adds key-value pair to map void putAll(Map t) : Object get(Object key) : Get the value for the given key void clear() : Clears the map. Size will be 0. boolean containsKey(Object key) : Checks if the key exists. boolean containsValue(Object value) : boolean isEmpty() : Returns true if map is empty Object remove(Object key) : Removes the key from map int size() : Returns the number of elements in the map Set entrySet() : Set keySet() : Collection values() :

27. Map implementations • “Map” interface • HashMap : Implemented using hash tables. Key objects must implement hashCode() method. • LinkedHashMap : Like HashMap., but keeps the order of insertion • TreeMap : Implemented by a red-black tree. You get the results in sorted order. (Determined by Comparable or Comparator)

28. Iterator Pattern • Intent • Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. • Motivation • An aggregate object such as a list should give you a way to access its elements without exposing its internal structure. Moreover you might want to traverse the list in different ways. • We cannot fill the List interface with different traversals we can need. • We may want a couple of traversals pending on the same time.

29. Iterator

30. Iterator – Example Structure

31. Iterator • Use iterator pattern … • To access an aggregate object’s contents without exposing its internal representation. • To support multiple traversals of aggregate objects. • To provide a uniform interface for traversing different aggregate structures (to support polymorphic iteration).

32. Iterator – General Structure

33. Iterator • Participants • Iterator • Defines an interface for accessing and traversing elements • ConcreteIterator • Implements the iterator interface • Keeps track of the current position in the traversal • Aggregate • Defines an interface method that creates an iterator object • ConcreteAggregate • Implements the iterator creation method, and returns an instance of the proper ConcreteIterator

34. Iterator • Consequences • It supports variants in the traversal of an aggregate • Iterators simplify the Aggregate interface • More then one traversal can be pending on an aggregate • Implementation • Who controls the iteration? • Client controls the iteration. (called External Iterator) • Iterator controls the iteration. (called Internal Iterator) • Who defines the traversal algorithm? • The aggregate: This is called a cursor. • The iterator. • How robust is the iterator? • Modifying an aggregate while traversing it will be dangerous for iterator. • Robust iterator will not be effected by changes.

35. Java Iterators interface Collection { … Iterator iterator(); … } interface Set extends Collection { … Iterator iterator(); … } interface List extends Collection { … Iterator iterator(); ListIterator listIterator(); ListIterator listIterator(int index); … } Interface Iterator { boolean hasNext() ; Object next() ; void remove() ; } Interface ListIterator extends Iterator { boolean hasNext() ; Object next() ; boolean hasPrevious() ; Object previous() ; int nextIndex() ; int previousIndex() ; void remove() ; void set(Object o) ; void add(Object o) ; }

36. Java Iterator Example import java.util.*; public class IteratorExample { public static void main(String[] args) { List ints = new ArrayList(); for(int i = 0; i < 10; i++) ints.add(new Integer(i)); Iterator e = ints.iterator(); while(e.hasNext()) System.out.println(((Integer)e.next()).intValue()); } }