Algorithm Programming 189-210Java API and Containers

Bar-Ilan University

תשס"ח 2007-2008

by Moshe Fresko

“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

“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

“Object” class methods Overriding Object methods

public String toString( )String representation of this object

protected void finilize( ) throws ThrowableThis 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()

“Object” class methods Overriding Object methods

protected Object clone() throws CloneNotSupportedException When overriding, be aware that …

1. x.clone() != x2. x.clone().getClass() == x.getClass()3. x.clone().equals(x)

public boolean equals(Object obj) When overriding, be aware to keep these characteristics

1. Reflexive: x.equals(x) == true2. Symmetric: x.equals(y) == y.equals(x)3. Transitive: if (x.equals(y)) and (y.equals(z)) then z.equals(x)4. Consistent: x.equals(y) returns the same value for the same unchanged object

references x and y.5. x.equals(null) == false

Algorithm Programming 1Exception Handling

Moshe Fresko

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).

Catching Exception The “try” Block

try {// Code that might generate exception

}

Exception Handlerstry {

// 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 }

Custom Exceptions// SimpleExceptionDemo.javaclass 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!") ; } }}

// OutputThrow SimpleException from f()Caught it!

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.

Exception Specification Exception Specification

void f() throws TooBig, TooSmall, DivZero { //...

Checked ExceptionsEither 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 thrownvoid f() { //…

Re-throwing an exceptioncatch(Exception e) {

System.err.println("An exception was thrown"); throw e;

}

Standard Java Exceptions

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”

Object

Error

Throwable

Exception

RuntimeExceptionMyException2MyException1

“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

}

Algorithm Programming 189-210Containers in Java

Bar-Ilan University

תשס"ז 2006-2007

by Moshe Fresko

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)

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)

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 smallerGreater then 0, if left object is greater 0, if both objects are equal

In java.utils.Comparatorinterface Comparator {

int compare(Object o1, Object o2) ; }

In java.lang.Comparable interface Comparable {

int compareTo(Object o) ;}

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)

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 ; }

}

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]

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)

Simplified Collections Diagram

Collection methodsboolean add(Object o) : Adds element to the collectionboolean 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 collectionObject[] toArray() :Object[] toArray(Object[] a) :

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

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”…

Map methodsObject put(Object key, Object value) : Adds key-value pair to mapvoid putAll(Map t) :Object get(Object key) : Get the value for the given keyvoid 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 emptyObject remove(Object key) : Removes the key from mapint size() : Returns the number of elements in the mapSet entrySet() :Set keySet() :Collection values() :

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)

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.

Iterator

Iterator – Example Structure

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).

Iterator – General Structure

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

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.

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) ;}

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()); } }