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Nataliia Romanenko

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Agenda Data Structure What is JCF? The Collection Interfaces Collections Implementations Ordering and Sorting The Legacy Collections Type The Collections Toolbox Other implementations in the API

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Lecture Objectives To understand the concepts of Java collections Framework To be able to implement Java programs based on Collections

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Store of Data Structure Arrays - a linear data structure and it's mainly used to store similar data. An array is a particular method of storing elements of indexed data.

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Store of Data Structure (cont.) Linked list is a data structure consisting of a group of nodes. Each node is composed of a datum and a reference to the next node in the sequence. This structure allows for efficient insertion or removal of elements from any position in the sequence.

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Store of Data Structure (cont.) If each node has a reference to the next and previous nodes it’s called Doubly Linked List.

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Store of Data Structure (cont.) Binary tree is a tree data structure in which each node has at most two child nodes, usually distinguished as "left" and "right".

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Store of Data Structure (cont.) A hash table, or a hash map, is a data structure that associates keys with values.

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The Limitation of Arrays An array is a very useful type in Java but it has its restrictions: once an array is created it must be sized, and this size is fixed; it contains no useful pre-defined methods. Java comes with a group of generic collection classes that grow as more elements are added to them, and these classes provide lots of useful methods. This group of collection classes are referred to as the Java Collections Framework.

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What is a Collections Framework? A unified architecture for representing and manipulating collections. Includes: – Interfaces: A hierarchy of abstract data types. – Implementations – Algorithms: The methods that perform useful computations, such as searching and sorting, on objects that implement collection interfaces.

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Hierarchy of interfaces

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Exception Conventions UnsupportedOperationException ClassCastException IllegalArgumentException IllegalStateException NoSuchElementException NullPointerException IndexOutOfBoundsException

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Iterators Iterator is an object that enables a programmer to traverse a container, particularly lists. public interface Iterator { boolean hasNext(); E next(); void remove(); //optional }

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Collection public interface Collection extends Iterable { // Basic operations int size(); boolean isEmpty(); boolean contains(Object element); boolean add(E element); //optional boolean remove(Object element); //optional Iterator iterator(); // Bulk operations boolean containsAll(Collection c); boolean addAll(Collection c); //optional boolean retainAll(Collection c); //optional void clear(); //optional // Array operations Object[] toArray(); T[] toArray(T[] a); }

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Set Set — a collection that cannot contain duplicate elements This interface models the mathematical set abstraction and is used to represent sets, such as the cards comprising a poker hand, the courses making up a student's schedule, or the processes running on a machine

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Set (methods) public interface Set extends Collection { // Basic operations int size(); boolean isEmpty(); boolean contains(Object element); boolean add(E element); //optional boolean remove(Object element); //optional Iterator iterator(); // Bulk operations boolean containsAll(Collection c); boolean addAll(Collection c); //optional boolean retainAll(Collection c); //optional void clear(); //optional // Array Operations Object[] toArray(); T[] toArray(T[] a); }

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List List — an ordered collection (sometimes called a sequence) Lists can contain duplicate elements

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List (methods) public interface List extends Collection { // Positional access E get(int index); E set(int index, E element); //optional boolean add(E element); //optional void add(int index, E element); //optional E remove(int index); //optional boolean addAll(int index, Collection

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ListIterators A ListIterator extends Iterator to treat the collection as a list, allowing – access to the integer position (index) of elements – forward and backward traversal – modification and insertion of elements

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ListIterator (methods) public interface ListIterator extends Iterator { boolean hasNext(); E next(); boolean hasPrevious(); E previous(); int nextIndex(); int previousIndex(); void remove(); //optional void set(E e); //optional void add(E e); //optional }

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ListIterator ListIterator it = list.listIterator(list.size()); while (it.hasPrevious()) { String obj = it.previous(); // … use obj …. }

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Queue Queue — a collection used to hold multiple elements prior to processing. Besides basic Collection operations, a Queue provides additional insertion, extraction, and inspection operations Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner

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Queue (methods) public interface Queue extends Collection { E element(); //throws E peek(); //null boolean add(E e); //throws boolean offer(E e); //null E remove(); //throws E poll(); //null }

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Queue In addition to the inherited core services offered by Collection, queue offers following methods in two flavors:

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Deque A linear collection that supports element insertion and removal at both ends When a Deque is used as a queue, FIFO (First-In-First-Out) behavior results Deques can also be used as LIFO (Last-In-First-Out) stacks

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Deque (methods) public interface Deque extends Queue { element() add(E e) (addLast(E e)) offer(E e) (offerLast(E e)), offerFirst(E e) peek()* (peekFirst()), peekLast() getFirst(), getLast() remove() (removeFirst()), removeFirstOccurrence(Object o), removeLast(), removeLastOccurrence(Object o) poll() (pollFirst()), pollLast() contains(Object o) iterator() descendingIterator() size() pop() (removeFirst()) push(E e) (addFirst(E e)) } from Queue, from Stack

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SortedSet SortedSet — a Set that maintains its elements in ascending order. Several additional operations are provided to take advantage of the ordering. Sorted sets are used for naturally ordered sets, such as word lists and membership rolls

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SortedSet (methods) public interface SortedSet extends Set { // Range-view SortedSet subSet(E fromElement, E toElement); SortedSet headSet(E toElement); SortedSet tailSet(E fromElement); // Endpoints E first(); E last(); // Comparator access Comparator

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Map An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value The Map interface provides three collection views, which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings

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Map (methods) public interface Map { // Basic operations V put(K key, V value); V get(Object key); V remove(Object key); boolean containsKey(Object key); boolean containsValue(Object value); int size(); boolean isEmpty(); // Bulk operations void putAll(Map

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Map (methods) void clear(); // Collection Views public Set keySet(); public Collection values(); public Set entrySet(); // Interface for entrySet elements public interface Entry { K getKey(); V getValue(); V setValue(V value); } }

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Map.Entry public interface Entry { K getKey(); V getValue(); V setValue(V value); } Map map = new HashMap(); map.put("1", "a"); map.put("2", "b"); map.put("3", "c"); for( Entry entry : map.entrySet() ) { if( "2".equals( entry.getKey() ) ) entry.setValue( "x" ); }

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SortedMap SortedMap — a Map that maintains its mappings in ascending key order. This is the Map analog of SortedSet. Sorted maps are used for naturally ordered collections of key/value pairs, such as dictionaries and telephone directories

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SortedMap (methods) public interface SortedMap extends Map{ SortedMap subMap(K fromKey, K toKey); SortedMap headMap(K toKey); SortedMap tailMap(K fromKey); K firstKey(); K lastKey(); Comparator

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Implementations JDK provides implementations of each interface. All implementations permit null elements, keys and values All are Serializable, and all support a public clone method Each one is unsynchronized If you need a synchronized collection, the synchronization wrappers allow any collection to be transformed into a synchronized collection

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HashSet, TreeSet, LinkedHashSet The two general purpose Set implementations are HashSet and TreeSet (and LinkedHashSet which is between them) HashSet is much faster but offers no ordering guarantees. If in-order iteration is important use TreeSet. Iteration in HashSet is linear in the sum of the number of entries and the capacity. It's important to choose an appropriate initial capacity if iteration performance is important. The default initial capacity is 101. The initial capacity may be specified using the int constructor. To allocate a HashSet whose initial capacity is 17: Set s= new HashSet(17);

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Set Implementation Comparisons

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ArrayList and LinkedList The two general purpose List implementations are ArrayList and LinkedList . ArrayList offers constant time positional access, and it's just plain fast, because it does not have to allocate a node object for each element in the List, and it can take advantage of the native method System.arraycopy when it has to move multiple elements at once If you frequently add elements to the beginning of the List, or iterate over the List deleting elements from its interior, you might want to consider LinkedList. These operations are constant time in a LinkedList but linear time in an ArrayList. Positional access is linear time in a LinkedList and constant time in an ArrayList

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HashMap, TreeMap, LinkedHashMap The two general purpose Map implementations are HashMap and TreeMap. And LinkedHashMap (similar to LinkedHashSet) The situation for Map is exactly analogous to Set If you need SortedMap operations you should use TreeMap; otherwise, use HashMap

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The Legacy Collection Types Enumeration Analogous to Iterator. Vector Analogous to ArrayList, maintains an ordered list of elements that are stored in an underlying array. Stack Analogous of Vector that adds methods to push and pop elements. Dictionary Analogous to the Map interface, although Dictionary is an abstract class, not an interface. Hashtable Analogous HashMap. Properties A subclass of Hashtable. Maintains a map of key/value pairs where the keys and values are strings. If a key is not found in a properties object a “default” properties object can be searched.

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Implementations Each of the implementations offers the strengths and weaknesses of the underlying data structure. What does that mean for: Hashtable Resizable array Tree LinkedList Hashtable plus LinkedList Think about these tradeoffs when selecting the implementation!

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Ordering and Sorting There are two ways to define orders on objects. Each class can define a natural order among its instances by implementing the Comparable interface. Arbitrary orders among different objects can be defined by comparators, classes that implement the Comparator interface.

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The Comparable Interface The Comparable interface consists of a single method: public interface Comparable { public int compareTo(T o); } The compareTo method compares the receiving object with the specified object, and returns a negative integer, zero, or a positive integer as the receiving object is less than, equal to, or greater than the specified Object.

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Comparator Comparator is another interface (in addition to Comparable) provided by the Java API which can be used to order objects. You can use this interface to define an order that is different from the Comparable (natural) order.

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Comparator A Comparator is an object that encapsulates an ordering. Like the Comparable interface, the Comparator interface consists of a single method: public interface Comparator { int compare(T o1, T o2); } The compare method compares its two arguments, returning a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.

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Live Code . Comparable class HDTV implements Comparable { private int size; private String brand; public HDTV(int size, String brand) { this.size = size; this.brand = brand; } public int getSize() { return size;} public void setSize(int size) {this.size = size; } public String getBrand() { return brand;} public void setBrand(String brand) {this.brand = brand; } @Override public int compareTo(HDTV tv) { if (this.getSize() > tv.getSize()) return 1; else if (this.getSize() < tv.getSize()) return -1; else return 0; } }

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Live Code . Comparable public class Main { public static void main(String[] args) { HDTV tv1 = new HDTV(55, "Samsung"); HDTV tv2 = new HDTV(60, "Sony"); HDTV tv3 = new HDTV(42, "Panasonic"); ArrayList al = new ArrayList(); al.add(tv1); al.add(tv2); al.add(tv3); Collections.sort(al); for (HDTV a : al) { System.out.println(a.getBrand()); } } }

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Live Code . Comparator class HDTV { private int size; private String brand; public HDTV(int size, String brand) { this.size = size; this.brand = brand; } public int getSize() { return size;} public void setSize(int size) {this.size = size; } public String getBrand() { return brand;} public void setBrand(String brand) {this.brand = brand; } }

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Live Code . Comparator class SizeComparator implements Comparator { @Override public int compare(HDTV tv1, HDTV tv2) { int tv1Size = tv1.getSize(); int tv2Size = tv2.getSize(); if (tv1Size > tv2Size) { return 1; } else if (tv1Size < tv2Size) { return -1; } else { return 0; } } } class BrandComparatorDescOrder implements Comparator { @Override public int compare(HDTV tv1, HDTV tv2) { return tv2.getBrand().compareTo(tv1.getBrand()) } }

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Live Code . Comparator public class Main { public static void main(String[] args) { HDTV tv1 = new HDTV(55, "Samsung"); HDTV tv2 = new HDTV(60, "Sony"); HDTV tv3 = new HDTV(42, "Panasonic"); ArrayList al = new ArrayList(); al.add(tv1); al.add(tv2); al.add(tv3); Collections.sort(al, new SizeComparator()); for (HDTV a : al) { System.out.println(a.getBrand()); } Collections.sort(al, new BrandComparatorDescOrder()); for (HDTV a : al) { System.out.println(a.getBrand()); } } }

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Other implementations in the API Wrapper implementations delegate all their real work to a specified collection but add (or remove) extra functionality on top of what the collection offers. Synchronization Wrappers Unmodifiable Wrappers Convenience implementations are mini-implementations that can be more convenient and more efficient than general-purpose implementations when you don't need their full power List View of an Array Immutable Multiple-Copy List Immutable Singleton Set Empty Set, List, and Map Constants

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Synchronization wrappers The synchronization wrappers add automatic synchronization (thread-safety) to an arbitrary collection. There is one static factory method for each of the six core collection interfaces: public static Collection synchronizedCollection(Collection c); public static Set synchronizedSet(Set s); public static List synchronizedList(List list); public static Map synchronizedMap(Map m); public static SortedSet synchronizedSortedSet(SortedSet s); public static SortedMap synchronizedSortedMap(SortedMap m); Each of these methods returns a synchronized (thread-safe) Collection backed by the specified collection.

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Unmodifiable wrappers Unmodifiable wrappers take away the ability to modify the collection, by intercepting all of the operations that would modify the collection, and throwing an UnsupportedOperationException. The unmodifiable wrappers have two main uses: To make a collection immutable once it has been built. To allow "second-class citizens" read-only access to your data structures. You keep a reference to the backing collection, but hand out a reference to the wrapper. In this way, the second-class citizens can look but not touch, while you maintain full access.

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Unmodifiable wrappers(cont.) There is one static factory method for each of the six core collection interfaces: public static Collection unmodifiableCollection(Collection c); public static Set unmodifiableSet(Set s); public static List unmodifiableList(List list); public static Map unmodifiableMap(Map m); public static SortedSet unmodifiableSortedSet(SortedSet s); public static SortedMap unmodifiableSortedMap(SortedMap m);

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Singleton static SetCollections.singleton(T e) returns an immutable set containing only the element e This is handy when you have a single element but you would like to use a Set operation c.removeAll(Collections.singleton(e)); will remove all occurrences of e from the Collection c

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The Collections Toolbox The collections framework also provides polymorphic versions of algorithms you can run on collections. Sorting Shuffling Routine Data Manipulation Reverse Fill copy etc. Searching Binary Search Composition Frequency Disjoint Finding extreme values Min Max

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Concurrent Collections ConcurrentReaderHashMap An analog of java.util.Hashtable that allows retrievals during updates. ConcurrentHashMap An analog of java.util.Hashtable that allows both concurrent retrievals and concurrent updates. CopyOnWriteArrayList A copy-on-write analog of java.util.ArrayList CopyOnWriteArraySet A java.util.Set based on CopyOnWriteArrayList. SyncCollection A wrapper class placing either Syncs or ReadWriteLocks around java.util.Collection SyncSet A wrapper around java.util.Set SyncSortedSet A wrapper around java.util.SortedSet SyncList A wrapper around java.util.List SyncMap A wrapper around java.util.Map SyncSortedMap A wrapper around java.util.SortedMap

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How to choose which Java collection class to use?

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How to choose which Java collection class to use?

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How to choose which Java collection class to use?

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Open Source Collections Libraries in Java Apache Commons Collections Package Guava-libraries (Google Collections Library) Trove high performance collections for Java The Mango Library