Collections in Java

OOP: Collections 1

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 2

Array • Most efficient way to hold references to objects.

• 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.

2

Car 3 4 5

Car 6 7

Car 1

Car 0index data

OOP: Collections 3

Array, Example

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

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()};

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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. 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 5

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

• 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.

[Source: java.sun.com]

OOP: Collections 6

Collection Interfaces and Classes

[Source: bruceeckel.com]

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

Iterator hasNext() next() remove()

Collection isEmpty() add() remove() ...

list

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

OOP: Collections 8

The Iterator Interface, cont.

// 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());

}

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

OOP: Collections 9

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 10

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 11

Set Idioms

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

OOP: Collections 12

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 13

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 14

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 15

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 16

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 17

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 18

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 19

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(); }

    public static void main(String args[]) { Car myCar;        MyStack s = new MyStack(); s.push (new Car()); myCar = (Car)s.pop();    } }

OOP: Collections 20

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 21

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

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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 23

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 24

HashMap, Example

import java.util.*;

public class Freq {    private static final Integer ONE = new Integer(1);    public static void main(String args[]) {        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);    } }

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

OOP: Collections 26

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

OOP: Collections 27

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.