11 1 Collections and Generics

8/14/2019 11 1 Collections and Generics

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2Java

Autoboxing with collections10

ArrayList9

List interface methods8

Classes implementing List interface7

Iterations6

Methods of Collection interfaces5Ordered and Sorted collection4

Collection Classes3

Collection interfaces2

Collection framework1

Contents


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3Java

Comparator interface20

Methods19Constructors18

PriorityQueue17

LinkedList continues16

Queue interface methods15

Classes implementing Queue interface14

Linked List13

Stack12

Vector11

Contents


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4Java

Classes implementing Map interface30

Map interface methods29Methods28

SortedSet and TreeSet27

LinkedHashSet26

Contract25

hashCode()24

Constructors and methods23

HashSet22

Set interface methods21

Contents


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5Java

Converting Collection into arrays38

Converting arrays into Collection37

Arrays36

Collections35

equals()34

Methods converting Map into Collection33

LinkedHashMap and TreeMap32

HashMap and Hashtable31

Contents


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6Java

Know

The Collection framework

Collection interfaces and classes


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7Java

Be Able To

Use Collections in Java programs


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8Java

Collection framework

A collection in java is an object that can hold

multiple objects (like an array) and that will grow

dynamically.

A collection framework is a common architecturefor representing and manipulating all the

collections.


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9Java

Collection interfaces

Collection

List Set

Map

Queue

List is a collection of objects.Set is a collection of objects that does not allow duplicate

objects.

Queue is a collection of objects that arranges objects in FIFO

order by comparing the objects and has queue like methods.Map contains pairs of objects (each pair comprising of one

object representing a key and other representing a value ).

java.util package

SortedSetSortedMap


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10Java

Collection Classes

HashSet

LinkedHashSet

TreeSet

1. Set

SortedSet

ArrayList

Vector

StackLinkedList

1. List

Implementation ClassesInterface


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11Java

LinkedList

PriorityQueue

1. Queue

HashtableHashMap

LinkedHashMap

TreeMap

1.Map

SortedMap

Implementation ClassesInterface

LinkedList implements List also

All the above classes are serializable.


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12Java

Ordered and Sorted

collection All collection classes that implement List andQueue interface, LinkedHashSet andLinkedHashMap are ordered. The order is based

on the sequence in which the user enters theobject.

Hashtable, HashMap, HashSet areunordered collections.

All the classes that implements SortedMap orSortedSet are sorted. PriorityQueue is alsosorted.


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13Java

Methods of Collection interfaces

boolean add(E o)void clear()

boolean remove(Object o)

boolean removeAll( Collection c)

boolean retainAll( Collection c)

int size()

boolean isEmpty()

boolean contains(Object o)

T[] toArray()

Iterator iterator()

For now replace with Object

Just Collection

Iterator


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14Java

Iterations

void method(Collection coll){

for(Object c:coll){

System.out.println(c.toString());}}

void method(Collection coll){

for(Iterator i = coll.iterator();i.hasNext(); ){

Object o= i.next();

System.out.println(o.toString()); }

1.4 way using Iterator interface

1.5 way using for-each loop


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15Java

Classes implementing List

interface


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16Java

List interface methods

All the methods of Collection interface and thefollowing methods:

void add(int index, E element)

E remove(int index)int indexOf(Object o)

int lastIndexOf(Object o)

E get(int index)

E set(int index, E element)

List subList(int fromIndex,inttoIndex)

Replace all E by Object

List


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17Java

ArrayListThis class implements all the methods of List interface.

Constructors:

ArrayList()

ArrayList(int initialCapacity)

Capacity 10

Collection class implementations use dynamicarray- Then why are we talking about capacity ?

This collection class (and some others)

internally use array. Capacity is the size

of the array. As elements are added, if the

capacity is not sufficient the array is

reallocated.


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18Java

Exampleimport java.util.*;

class Test{

public static void main(String[] s){

ArrayList a= new ArrayList();for(String x:s)

a.add(x);

for(Object o:a)

System.out.println(o);

} }

adding

iterating

Folder 1


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19Java

Autoboxing with collections

import java.util.*;class Test{

public static void main(String[] s){

ArrayList a= new ArrayList();

a.add(1);

a.add("Mary");

a.add(1.78);

for(Object o:a)System.out.println(o);

} }

Boxed into Integer

wrapper class

Boxed into Double

wrapper class

Folder 2


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20Java

I get a warning when Icompile the previousprograms:Note: Test.java uses

unchecked or unsafeoperations

Note: Recompile with

-Xlint:unchecked for

details.

A single collection object can holdmany objects of different types(likethe example in the previous slide).

In 1.5, we can create type-safecollections where we can restrict acollection to hold only particulartype of object.(The syntax toachieve this will be covered in

generics.) The 1.5 compiler gives awarning message if type safecollection is not used.


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21Java

Vector

The Vector class is exactly same as ArrayList classexcept that the Vector class methods are thread-safe.

Constructor

Vector()

Vector(int initialCapacity)Vector(int initialCapacity,int capacityIncrement)

And all the methods of List interface are implemented.


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22Java

I was just looking at the API

documentation, none of themethods of Vector class aremarked as synchronized !

You wont find it in the API documentation. Butif you are curious you could look at the sourcecode. In your Java home folder look for azipped file called src. Extract the Vector classand look at the source code. The collections

that have synchronized implementation areVector and its subclasses and Hashtable and

its subclasses.


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23Java

Stack Class that represents a stack where objects are

inserted in LIFO manner. Inherits from the Vector class.

Constructor

Stack()

Methods (apart from Vector methods) E pop()

E push(E item)

E peek()boolean empty()

int search(Object o)



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24Java

Linked List

LinkedList implements List and Queue.

All the List classes we have seen so far use

arrays. LinkedList class behaves like doubly-linked list.

The methods in the LinkedList allow it to be

used as a stack, queue, or double-ended

queue.


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25Java

Classes implementing

Queue interface


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26Java

Queue interface methods

E element()

E peek()

boolean offer(E o)

E poll()

E remove()

Methods indicate feature similar to queue.


NoSuchElementExceptionif queue is empty


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27Java

LinkedList continues

Constructor: LinkedList()

Methods:Apart from methods of List and Queuesome more convenient methods are added.

E getFirst()

E getLast()

E removeFirst()

E removeLast()

void addFirst(E o)

void addLast(E o)

Replace all E by Object for now


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28Java

PriorityQueue

It is a sorted collection new in 1.5 thatimplements Queue

Sorting is based on

the compareTo() method of

Comparable interface(also called naturalorder).

the Comparator object passed viaconstructor.

A PriorityQueue does not permit nullelements.


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29Java

Constructors

PriorityQueue()

PriorityQueue(int initialCapacity)

PriorityQueue(int initialCapacity,

Comparator


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30Java

Yes!

I see capacity in the constructor.

Does that mean that thePriorityQueue uses arrays?


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31Java

Methods

All the methods of Queue interface (dont forget

the Queue interface extends Collection interface)

and

Comparator


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32Java

import java.util.*;

public class NaturalOrder {public static void main(String str[])

throws general.InvalidNameException {

student.Student s1= new

student.Student("Rama");


student.Student("Sita");

student.Student s3= newstudent.Student("Hunuman");


student.Student("Ravana");

Example with natural ordering

Folder 3


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33Java

PriorityQueue p= new PriorityQueue();

p.offer(s1);

p.offer(s2);p.offer(s3);

p.offer(s4);

while(!p.isEmpty()) {

student.Students=(student.Student)p.poll();

System.out.println(s.getName()+ " "

+s.getRegNo() );

} } }Result: Hunuman 3

Rama 1

Ravana 4

Sita 2

Method declared in Collection

interface. Queue interfaceextends Collection interface

Casting Object instance into

Student instance


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34Java

Comparator interface

A class that implements Comparator interface must

override

int compare(T o1, T o2)

(Replace T by Object)

The next example uses Comparator to change the

ordering of the Student elements in the

PriorityQueue class.


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35Java

import java.util.*;

class StudentComparator implements

Comparator{

public int compare(Object o1, Object

o2){

student.Student s1=(student.Student)o1;

student.Student s2=(student.Student)

o2;

return s1.getCurrentSemester() -s2.getCurrentSemester();

}

}Folder 4


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import java.util.*;

public class ComparatorOrder {

public static void main(String str[]) throws

general.InvalidNameException {


student.Student("Rama");

s1.setCurrentSemester(1);

student.Student s2= newstudent.Student("Sita");



student.Student("Hunuman");s3.setCurrentSemester(2);


student.Student("Ravana");



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PriorityQueue p= new PriorityQueue(4,new

student.StudentComparator());

p.offer(s1); p.offer(s2); p.offer(s3); p.offer(s4);

while(!p.isEmpty()) {

student.Student s=(student.Student)p.poll();

System.out.println(s.getName()+ " in semester" +s.getCurrentSemester() );

} }}

Result:

Rama in semester 1

Hunuman in semester 2

Ravana in semester 3

Sita in semester 4


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38Java

Set interface methods

As stated earlier a set cannot contain duplicate

elements.

It can contain at most one null element.

Set does not add any new methods apart from

what it gets from Collection interface.

Classes implementing Set must return false if

an attempt is made to add duplicate element.


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39Java

HashSet

HashSet is an unordered and unsorted collection.

This class uses hashCode() method of the object

added in the collection to determine how the

element should be ordered in the collection. Positioning elements using hashCode() helps in

faster retrieval. So, more efficient the hashCode(),

better the performance.

Object class implements hashCode() method that

returns distinct integers for distinct objects.


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40Java

When I say they areunordered andunsorted, I mean itfrom end user pointof view.

Wait a minute-you say thatthe HashSet collection isordered

according tohashCode() method on onehand and on the other handyou say that it is unorderedand unsorted collection. ??!!


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41Java

Constructors and methods

HashSet()

HashSet(int initialCapacity)

and all the Set methods

h hC d ()


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42Java

hashCode() For efficient performance with HashSet collection,

the hashCode() method for the object(that is goingto be added to the collection) must be overridden.

LJIF

0 1 2 24 25

new Student("Raja)

new Student("Rani")

new Student("Maha)

new Teacher("Shree"

called buckets

Overriding hashCode() in Person class such that persons whose name start

with A go inside one bucket and persons whose name start with B go inside

another bucket and so on..


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43Java

Contract

Whenever hashCode() is invoked on the sameobject more than once during an execution of a

Java application, the hashCode method must

consistently return the same integer, provided

no information used in equals comparisons onthe object is modified. This integer need not

remain consistent from one execution of an

application to another execution of the same

application.


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44Java

If two objects are equal according to theequals(Object) method, then calling the hashCode

method on each of the two objects must producethe same integer result.

It is not required that if two objects are unequal

according to the equals(java.lang.Object) method,then calling the hashCode method on each of thetwo objects must produce distinct integer results.However, the programmer should be aware thatproducing distinct integer results for unequalobjects may improve the performance ofhashtables.


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45Java

package general;public abstract class Person{

.

public int hashCode(){

return name.charAt(0);}

}

Folder 5


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46Java

import java.util.*;

public class TestHashSet {

public static void main(String str[])

throws Exception {teacher.Teacher s1= new

teacher.Teacher("Guru");


student.Student("Shree");

teacher.Teacher s3= new

teacher.Teacher("Kumar");

student.Student s4= newstudent.Student(Sheela");

HashSet set= new HashSet();

set.add(s1); set.add(s2);

set.add(s3); set.add(s4);


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47Java

Iterator i= set.iterator();

while(i.hasNext()){

general.Person p= (general.Person)

i.next();

System.out.println(p.getName());

} } }

Result:

Sheela

ShreeKumar

Guru

Into one bucket


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48Java

LinkedHashSet

Maintains the insertion-order and does not allowduplicates. Insertion order is not affected if anelement is re-inserted into the set.

It also uses hashing technique internally for

searching and retrieving. Constructor and methods:

LinkedHashSet()

And all the Set interface methods.


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49Java

SortedSetand TreeSet

Classes implementing this interfaceguarantees that while traversing the order willbe either

B. in natural order (using compareTo() ofComparable interface)

or

by using a Comparator provided at creation

time. There is only one class implementing this

interface: TreeSet

M th d


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50Java

MethodsE first()

E last()

Comparator


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51Java

Map interface methods

Methods:boolean containsKey(Object key)

boolean containsValue(Object value)

V get(Object key)V put(K key, V value)

V remove(Object key)

Set keySet()Collection values()

Replace all V by Object. And ignore


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52Java

Classes implementing Map

interface


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53Java

HashMap and Hashtable

Both of the classes arrange the pair of objectswith respect to hashCode() of the key and the

keys map to a value.

Constructors:

HashMap()

HashMap(int initialCapacity)

Hashtable()

Hashtable (int initialCapacity)

The only difference between HashMap and

Hashtable is that Hashtable is thread-safe.


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54Java

import java.util.*;

public class MyDictionary{public static void main(String str[]) {

HashMap h= new HashMap();

// adding words and their meanings

h.put("benevolent",new String[]{"kind",

"generous","warm-hearted"});

h.put("chastity",new String[]{"purity","self-restraint","warm-hearted"});

h.put("dingy", new String[]{"dark",worn"});Folder 6


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55Java

h.put("gait",new

String[]{"walk","step","stride"});

// reading the word from the console

Scanner scan= new Scanner(System.in);

String word=scan.next().toLowerCase();

String meaning[]=(String [])h.get(word);

for(String

m:meaning)System.out.println(m);

}

}


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56Java

LinkedHashMap and TreeMap

LinkedHashMap isparallel to

LinkedHashSet andTreeMap is parallel to

TreeSet. Do you see thepattern? Try answering

the questions in thenext slide to test your

understanding.


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57Java

Methods converting Map into

Collection

Set keySet() Collection values() Set entrySet()

Map.Entry is an interface with methods:K getKey()V getValue()

V setValue(V value)


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58Java

Questions

1. Which among LinkedHashMap and TreeMapa) is a ordered collection

b) is a sorted collection

c) uses hashingd) has synchronized implementation

e) uses Comparable or Comparator

equals()


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59Java

equals()

equals() method for all the classes in collection

framework is overridden.boolean equals(Object o)

The equals() methods can be used to compare theequality of collection of similar type rater than just its

own type. For example for Lists, it returns true if and only if the

specified Object is also a List, both Lists have thesame size, and all corresponding pairs of elements

in the two Lists are equal. Same is for the classes which implement Map and

Set.

C ll ti


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60Java

Collections This class does some general operations with

collections like searching, filling, copying, gettingthe maximum and minimum of collection etc. Allthe methods in this class are static.

Searching Methods:

static int binarySearch(List


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61Java

static int binarySearch(List


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62Java

Filling Method:

static void fill(List list, Object o)

Maximum and Minimum:

static Object max(Collection coll)

static Object max(Collection coll, Comparator comp)

static Object min(Collection coll)

static Object min(Collection coll, Comparator comp)

Copystatic void copy(List dest, List src)


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63Java

Sort:

static void sort(List list)

static void sort(List list,Comparator c )

Swap

static void swap(List list, int i, int j)Reverse, Replace and Shuffle

static void reverse(List list)

static boolean replaceAll(List list, Object oldVal,Object newVal)

static void shuffle(List list)

Arrays


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64Java

Arrays Like Collections, this class also is a general utility

class providing methods to fill, sort, search but in thiscase it works on arrays instead of a Collection

object.

Methods:static int binarySearch(xxx[] a,

byte key)

xxx: any primitive type or Object

static int binarySearch(T[] a,

T key, Comparator


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65Java

static void sort(xxx[] a)

static void fill(xxx[] a, xxx val)

static boolean equals(xxx[] a, xxx[] a2)

xxx: any primitive type or Object

Converting arrays into


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66Java

Converting arrays into

Collection

In Arrays class: From array into List

static List asList(T... a)

static List asList(Object[] a)For now

Converting Collection into


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Converting Collection into

arrays

In Collection interface: From Collection

into array

Object[] toArray()

T[] toArray(T[] a)

We will come to this at the end ofgenerics

11 1 Collections and Generics

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