11.Collection

8/12/2019 11.Collection

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Java Collection API


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Overview of Collection

A collection is a group of data manipulate as a single

object.Corresponds to a bag. Insulate client programs from the implementation. array, li

list, hash table, balanced binary tree.

Like C++'s Standard Template Library (STL)

Can grow as necessary. Contain only Objects (reference types).

Heterogeneous.

Can be made thread safe (concurrent access).

Can be made not-modifiable


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Collection Interfaces


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The Collection Interface

/ / For Basic Operations

int size();

boolean isEmpty();

boolean contains(Object element);

boolean add(Object element); boolean remove(Object element); //

Iterator iterator();


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The List Interface

The List interface corresponds to an order group of elements

Duplicates are allowed. Extensions compared to the Collection interface

Access to elements via indexes, like arrays

add (int, Object), get(int), remove(int),

set(int, Object) (note set = replace bad name for the method)

Search for elements indexOf(Object), lastIndexOf(Object)

Specialized Iterator, call ListIterator

Extraction of sublist

subList(int fromIndex, int toIndex)


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The List Interface, cont.

Further requirements compared to the Collection

Interface add(Object)adds at the end of the list.

remove(Object)removes at the start of the list.

list1.equals(list2) the ordering of the elements is tainto consideration.

list1.equals(list2) implies that

list1.hashCode()==list2.hashCode()


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The List Interface, cont. public interface List extends Collection {

// Positional Access

Object get(int index); Object set(int index, Object element); // Optional

void add(int index, Object element); // Optional

Object remove(int index); // Optional

abstract boolean addAll(int index, Collection c);

// Search

int indexOf(Object o); int lastIndexOf(Object o);

// Iteration ListIterator listIterator();

ListIterator listIterator(int index);

}


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ArrayList Class The classes ArrayListand LinkedList implement the List

interface.

ArrayList is an array based implementation where elementaccessed directly via the getand setmethods.

Default choice for simple sequence.

It uses a dynamic array for storing the elements.

It can contain duplicate elements.

It maintains insertion order. It is not synchronized.

It allows random access because array works at the index b

Its manipulation is slower because a lot of shifting needs toccurred.


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E l A Li t t i d fi d bj t


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Example: ArrayList storing user defined objectsimportjava.util.*;

classSimple{

publicstaticvoidmain(String args[]){

Student s1=new Student(101,"Sonoo",23);

Student s2=new Student(102,"Ravi",21);Student s2=new Student(103,"Hanumat",25);

ArrayList al=newArrayList();

al.add(s1);

al.add(s2);

al.add(s3);

Iterator itr=al.iterator();

while(itr.hasNext()){

Student st=(Student)itr.next();

System.out.println(st.rollno+" "+st.name+" "+st.age);

}

}

}


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LinkedList Classes

LinkedList is based on a double linked list

Gives better performance on add andremove comparedArrayList.

Gives poorer performance on get andset methods comptoArrayList.

It can contain duplicate elements.

It maintains insertion order.

It is not synchronized. No random access.

It can be manipulated faster because no shifting needs to boccurred.

It can be used as list, stack or queue.

k d l


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LinkedList Exampleimportjava.util.*;

classSimple{


LinkedList al=newLinkedList();

al.add("Ravi");

al.add("Vijay");

al.add("Ravi");

al.add("Ajay");



System.out.println(itr.next());

}

}


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The Set Interface

Corresponds to the mathematical definition of a set (no

duplicates are allowed).

Compared to the Collection interface

Interface is identical.

Every constructor must create a collection without

duplicates.

The operation add cannot add an element already

the set.

The method call set1.equals(set2) works at follows

set1 union set2, and set2 union set1.


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E H hS


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Ex: HashSetimportjava.util.*;

classSimple{


HashSet al=newHashSet();al.add("Ravi");

al.add("Vijay");

al.add("Ravi");

al.add("Ajay");




}

}


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TreeSet

Implemented using a tree structure.

Guaranteesordering of elements.(maintainsascending order)

add, remove, and contains methods

logarithmic time complexityO(log (n)), whern is the number of elements in the set.

E T S


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Ex: TreeSetimportjava.util.*;

classSimple{


TreeSet al=newTreeSet();

al.add("Ravi");

al.add("Vijay");

al.add("Ravi");

al.add("Ajay");





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Map Interface

A Map is an object that maps keys to values. Also called an

associative array or a dictionary.

Methods for adding and deleting

put(Object key, Object value)

remove (Object key)

Methods for extraction objects

get (Object key)

Methods to retrieve the keys, the values, and (key, value) pairs

keySet() // returns a Set

values() // returns a Collection,

entrySet() // returns a set

MAP Interface methods


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MAP Interface methods public interface Map {

// Basic Operations

Object put(Object key, Object value);

Object get(Object key);

Object remove(Object key);

boolean containsKey(Object key);

boolean containsValue(Object value);

int size();

boolean isEmpty();

// Bulk Operations void putAll(Map t);

void clear();

// Collection Views

public Set keySet();

public Collection values();

public Set entrySet();


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HashMap and TreeMap Classes

The HashMap and TreeMap classes implement the Ma

interface.

HashMap

The implementation is based on a hash table.

No ordering on (key, value) pairs.

A HashMap contains values based on the key. It contains only unique elements.

It may have one null key and multiple null values.

EX H hM


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EX: HashMapimportjava.util.*;

classSimple{


HashMap hm=newHashMap();

hm.put(100,"Amit");

hm.put(101,"Vijay");

hm.put(102,"Rahul");

Set set=hm.entrySet();Iterator itr=set.iterator();


Map.Entry m=(Map.Entry)itr.next();

System.out.println(m.getKey()+" "+m.getValue());

}


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TreeMap

The implementation is based on tree structure.

(key, value) pairs are ordered on the key.(ascending)

A TreeMap contains values based on the key.

It contains only unique elements.

It cannot have null key but can have multiple null valu

It is same as HashMap instead maintains ascending o

Ex: TreeMap


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Ex: TreeMapimportjava.util.*;

classSimple{


TreeMap hm=newTreeMap();

hm.put(100,"Amit");

hm.put(102,"Ravi");

hm.put(101,"Vijay");

hm.put(103,"Rahul");

Set set=hm.entrySet();

Iterator itr=set.iterator();


Map.Entry m=(Map.Entry)itr.next();

System.out.println(m.getKey()+" "+m.getValue());

}

Utility methods of Collections


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Utility methods of Collections

All the methods of class Collections are static

Search and sort:binarySearch(), sort() Reorganization: reverse(), shuffle()

Wrappings: unModifiableCollection,

synchonizedCollection

C ll i Ad & di d


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Collection Advantages & disadvantages

Advantages:

Can hold different types of objects. Resizable

Disadvantages

Must cast to correct type

Cannot do compile-time type checking.


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Thank You

Documents

11.Collection