Collections In Java

Collections in Java
• Arrays
n Has special language support
• Iterators
n Iterator (i)
• Collections (also called containers)
n Collection (i)
n Set (i),
u HashSet (c), TreeSet (c)
n List (i),
u ArrayList (c), LinkedList (c)
n Map (i),
u HashMap (c), TreeMap (c)

OOP: Collections 1

Array
• Most efficient way to hold references to objects.

data Car Car Car Car
index 0 1 2 3 4 5 6 7

• Advantages
n An array know the type it holds, i.e., compile-time type checking.
n An array know its size, i.e., ask for the length.
n An array can hold primitive types directly.
• Disadvantages
n An array can only hold one type of objects (including primitives).
n Arrays are fixed size.

OOP: Collections 2

Array, Example
class Car{}; // minimal dummy class
Car[] cars1; // null reference
Car[] cars2 = new Car[10]; // null references

for (int i = 0; i < cars2.length; i++)
cars2[i] = new Car();

// Aggregated initialization
Car[] cars3 = {new Car(), new Car(), new Car(), new Car()};
cars1 = {new Car(), new Car(), new Car()};

• Helper class java.util.Arrays
n Search and sort: binarySearch(), sort()
n Comparison: equals() (many overloaded)
n Instantiation: fill() (many overloaded)
n Conversion: asList()
OOP: Collections 3

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.
n array, linked 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.

OOP: Collections 4

Collection Interfaces
• Collections are primarily defined through a set of interfaces.
n Supported by a set of classes that implement the interfaces

[Source: java.sun.com]

• Interfaces are used of flexibility reasons
n Programs that uses an interface is not tightened to a specific
implementation of a collection.
n It is easy to change or replace the underlying collection class with
another (more efficient) class that implements the same interface.
OOP: Collections 5

Collection Interfaces and Classes

OOP: Collections [Source: bruceeckel.com] 6

The Iterator Interface
• The idea: Select each element in a collection
n Hide the underlying collection

Collection Iterator
list
isEmpty() hasNext()
add() next()
remove() remove()
...

• Iterators are fail-fast
n Exception thrown if collection is modified externally, i.e., not via the
iterator (multi-threading).

OOP: Collections 7

The Iterator Interface, cont.
// the interface definition
Interface Iterator {
boolean hasNext();
Object next(); // note "one-way" traffic
void remove();
}

// an example
public static void main (String[] args){
ArrayList cars = new ArrayList();
for (int i = 0; i < 12; i++)
cars.add (new Car());

Iterator it = cats.iterator();
while (it.hasNext())
System.out.println ((Car)it.next());
}

OOP: Collections 8

The Collection Interface
public interface Collection {
// Basic Operations
int size();
boolean isEmpty();
boolean contains(Object element);
boolean add(Object element); // Optional
boolean remove(Object element); // Optional
Iterator iterator();

// Bulk Operations
boolean containsAll(Collection c);
boolean addAll(Collection c); // Optional
boolean removeAll(Collection c); // Optional
boolean retainAll(Collection c); // Optional
void clear(); // Optional

// Array Operations
Object[] toArray();
Object[] toArray(Object a[]);
}

OOP: Collections 9

The Set Interface
• Corresponds to the mathematical definition of a set (no
duplicates are allowed).

• Compared to the Collection interface
n Interface is identical.
n Every constructor must create a collection without duplicates.
n The operation add cannot add an element already in the set.
n The method call set1.equals(set2) works at follows
u set1 ⊆ set2, and set2 ⊆ set1

OOP: Collections 10

Set Idioms
• set1 ∪ set2
n set1.addAll(set2)
• set1 ∩ set2
n set1.retainAll(set2)
• set1 − set2
n set1.removeAll(set2)

OOP: Collections 11

HashSet and TreeSet Classes
• HashSet and TreeSet implement the interface Set.

• HashSet
n Implemented using a hash table.
n No ordering of elements.
n add, remove, and contains methods constant time complexity
O(c).

• TreeSet
n Implemented using a tree structure.
n Guarantees ordering of elements.
n add, remove, and contains methods logarithmic time complexity
O(log (n)), where n is the number of elements in the set.

OOP: Collections 12

HashSet, Example
// [Source: java.sun.com]
import java.util.*;
public class FindDups {
public static void main(String args[]){
Set s = new HashSet();
for (int i = 0; i < args.length; i++){
if (!s.add(args[i]))
System.out.println("Duplicate detected: " +
args[i]);
}
System.out.println(s.size() +
" distinct words detected: " +
s);
}
}

OOP: Collections 13

The List Interface
• The List interface corresponds to an order group of elements.
Duplicates are allowed.

• Extensions compared to the Collection interface
n Access to elements via indexes, like arrays
u add (int, Object), get(int), remove(int),
set(int, Object) (note set = replace bad name for the method)
n Search for elements
u indexOf(Object), lastIndexOf(Object)
n Specialized Iterator, call ListIterator
n Extraction of sublist
u subList(int fromIndex, int toIndex)

OOP: Collections 14

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
taken into consideration.
• Extra requirements to the method hashCode.
n list1.equals(list2) implies that
list1.hashCode()==list2.hashCode()

OOP: Collections 15

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);
// Optional

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

// Iteration
ListIterator listIterator();
ListIterator listIterator(int index);

// Range-view
List subList(int from, int to);
}

OOP: Collections 16

ArrayList and LinkedList Classes
• The classes ArrayList and LinkedList implement the
List interface.

• ArrayList is an array based implementation where elements
can be accessed directly via the get and set methods.
n Default choice for simple sequence.

• LinkedList is based on a double linked list
n Gives better performance on add and remove compared to
ArrayList.
n Gives poorer performance on get and set methods compared to
ArrayList.

OOP: Collections 17

ArrayList, Example
// [Source: java.sun.com]
import java.util.*;

public class Shuffle {
public static void main(String args[]) {
List l = new ArrayList();
for (int i = 0; i < args.length; i++)
l.add(args[i]);
Collections.shuffle(l, new Random());
System.out.println(l);
}
}

OOP: Collections 18

LinkedList, Example
import java.util.*;
public class MyStack {
private LinkedList list = new LinkedList();
public void push(Object o){
list.addFirst(o);
}
public Object top(){
return list.getFirst();
}
public Object pop(){
return list.removeFirst();
}

Car myCar;
MyStack s = new MyStack();
s.push (new Car());
myCar = (Car)s.pop();
}
}

OOP: Collections 19

The ListIterator Interface
public interface ListIterator extends Iterator {
boolean hasNext();
Object next();

boolean hasPrevious();
Object previous();

int nextIndex();
int previousIndex();

void remove(); // Optional
void set(Object o); // Optional
void add(Object o); // Optional
}

OOP: Collections 20

The 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
n put(Object key, Object value)
n remove (Object key)
• Methods for extraction objects
n get (Object key)
• Methods to retrieve the keys, the values, and (key, value) pairs
n keySet() // returns a Set
n values() // returns a Collection,
n entrySet() // returns a set

OOP: Collections 21

The MAP Interface, cont.
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();
// Interface for entrySet elements
public interface Entry {
Object getKey();
Object getValue();
Object setValue(Object value);
}
}
OOP: Collections 22

HashMap and TreeMap Classes
• The HashMap and HashTree classes implement the Map
interface.

• HashMap
n The implementation is based on a hash table.
n No ordering on (key, value) pairs.

• TreeMap
n The implementation is based on red-black tree structure.
n (key, value) pairs are ordered on the key.

OOP: Collections 23

HashMap, Example
import java.util.*;

public class Freq {
private static final Integer ONE = new Integer(1);
Map m = new HashMap();

// Initialize frequency table from command line
for (int i=0; i < args.length; i++) {
Integer freq = (Integer) m.get(args[i]);
m.put(args[i], (freq==null ? ONE :
new Integer(freq.intValue() + 1)));
}

System.out.println(m.size()+
" distinct words detected:");
System.out.println(m);
}
}

OOP: Collections 24

Static Methods on Collections
• Collection
n Search and sort: binarySearch(), sort()
n Reorganization: reverse(), shuffle()
n Wrappings: unModifiableCollection,
synchonizedCollection

OOP: Collections 25

Collection Advantages and Disadvantages
Advantages Disadvantages
• Can hold different types of • Must cast to correct type
objects. • Cannot do compile-time type
• Resizable checking.

OOP: Collections 26

Summary
• Array
n Holds objects of known type.
n Fixed size.

• Collections
n Generalization of the array concept.
n Set of interfaces defined in Java for storing object.
n Multiple types of objects.
n Resizable.

• Queue, Stack, Deque classes absent
n Use LinkedList.

OOP: Collections 27

Collections In Java

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