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LJ_JAVA_FS_Collection.pptx

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LJ_JAVA_FS_CORE_JAVA.pptx
LJ_JAVA_FS_CORE_JAVA.pptx
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LJ_JAVA_FS_Collection.pptx

  1. 1.  Introduction  Collections and iterators  Linked list  Array list  Hash set  Tree set  Maps  Collections framework
  2. 2.  Data structures can make a big difference in programming ⚫ Do you need to search quickly? Do you need to rapidly insert and remove element? ...  The first version of Java was supplied with a very small set of classes ⚫ Vector, Stack, Hashtable, BitSet and Enumeration ⚫ Java needed a serious data structure library  Without the complexity of C++ STL(Standard Template Library)  With the benefit of «generic algorithms» of C++ STL ⚫ The collections framework satisfies those needs
  3. 3.  Java collection framework separates interfaces and implementations ⚫ It defines a set of interfaces that represents abstract data structures ⚫ Each interface is then implemented in different ways, each with main focus on some aspects  For example, fast insertion, fast search, fixed memory consumption, ... ⚫ Using interfaces, you can change the implementation simply modifying a single statement // ArrayList optimizes random access, LinkedList modification stmts List<Integer> list = new ArrayList<>(); list = new LinkedList<>()
  4. 4.  The fundamental interface for collection is the Collection interface ⚫ The add method adds an element ot the collection  Returns true if the collection was changed by the addition ⚫ Iterators are used visit elements in the collection  Implements the Iterator design pattern public interface Collection<E> { boolean add(E element); Iterator<E> iterator(); // ... } public interface Iterator<E> { E next(); boolean hasNext(); void remove(); }
  5. 5.  Using iterators we have decoupled a collection from the traversing policies ⚫ The order in which elements are visited depends on the the collection type ⚫ Using the next method the collection is visited one element at time  If the end of the collection is reached an NoSuchElementException is thrown ⚫ Use the hasNext method to check if the collection has more elements to visit ⚫ Think of Java iterators as being between elements  The iterator jumps over the next element
  6. 6. Think of Java iterators as being between elements While moving, the iterator returns the current element
  7. 7.  Using next and hasNext it is possibile to traverse the collection ⚫ As of Java 5, there is an elegant shortcut to looping a collection using iterators ⚫ To use the for each loop the data structure must implement Iterable<E> interface for (String element : c) { // do something with element } Collection<String> c = /* ... */; Iterator<String> iter = c.iterator(); while (iter.hasNext()) { String element = iter.next(); // do something with element }
  8. 8.  Using an iterator it possibile to remove elements from a collection ⚫ The remove method removes the element that was returned by the last call to next  There is a strong dependency between next and remove: it is illegal to call remove if it wasn’t preceded by a call to next ⚫ This is the only way to safely modify a collection after the creation of an iterator for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) { // Point the iterator to the current element String string = iterator.next(); iterator.remove(); }
  9. 9.  All concrete collections implement Collection or Map interfaces ⚫ We will introduce only few of them Collection type Description ArrayList An indexed sequence that grows and shrinks dynamically LinkedList An ordered sequence that allows efficient insertion and removal at any location HashSet An unordered collection that rejects duplicates TreeSet A sorted set HashMap A data structure that stores key/value associations
  10. 10.  A LinkedList<E> is an ordered data structure that stores each object in a link ⚫ Each link store a reference to the next link of the seq.  In Java, a linked list is always doubly linked Implements List<E> interface
  11. 11.  Very efficient for remove and add operations ⚫ These ops are made through an Iterator  Other elements in the list have not to be repositioned after removal of an element
  12. 12.  Also di add operation is made efficiently through an iterator of type ListIterator<E> ⚫ Use List.listIterator method to get one ⚫ New element is added before the current position of the iterator  Be carefull of concurrent modification using iterators  Linked list are very inefficient in random access interface ListIterator<E> extends Iterator<E> { void add(E element); E previous() boolean hasPrevious() } for (int i = 0; i < list.size(); i++) // do something with list.get(i);
  13. 13. Adding a new element changes at most two references
  14. 14.  An ArrayList<E> is an ordered data structure that is very efficient in random access ops. ⚫ It encapsulate a dynamically reallocated array of objects ⚫ Adding and removing operation are not so efficient  Reallocation of elements is needed ⚫ Use ArrayList instead of Vector  More efficient due to its not synchronized methods
  15. 15.  A Set<E> is a data structure that doesn’t care about element’s ordering ⚫ A set contain the same element only once ⚫ Search operation performs very efficiently ⚫ A set is a Collection  An HashSet<E> uses hash codes to distinguish among elements ⚫ An hashCode is a number that can be derived from object data  You must provide an hash function to your classes  The function must be compatible with the equals method
  16. 16. If the hash function produceds values that are randomly distributed, collision should be rare buckets
  17. 17.  An HashSet is implemented using an hash table ⚫ The contains method is very efficient, because it has to lookup the element only in one bucket ⚫ An iterator to hash set visit each bucket in turn  Because of scattering, they are visited in a seemingly random order ⚫ The add method adds an element if it is not already present  Don’t mutate an element in a set once inserted ⚫ If the hash code of an element were to change, the element would no longer be in the correct position
  18. 18.  A TreeSet<E> is a sorted set ⚫ While iterating over the collection, the elements are presented in sorted order ⚫ It uses a red-black tree to store data  Insertion is slower than insertion in an hash table, but it is still much faster than insertion in an array or linked list  ...but a tree set automatically sorts the elements ;) ⚫ The type of the elements may implement Comparable<T> public interface Comparable<T> { // It returns a value that is < 0, = 0 or > 0 int compareTo(T other); }
  19. 19.  What if elements do not implement Comparable or if you need more than on compation alg? ⚫ Provide a Comparator during set construction  The compare method acts like the compareTo method  Function object (lambda anyone?!)  Using an HashSet or a TreeSet? ⚫ There must be a total ordering defined on elements  You have to implement also the Comparator interface ⚫ Do you neeed elements to be sorted? public interface Comparator<T> { // Defines how two elements of type T have to be compared int compare(T a, T b); }
  20. 20.  A Map<K,V> is a data structure that allows you to search an element using a key ⚫ A map stores key/value pairs  Key must be unique: If you call the put method twice with the same key, the second value replaces the first one.  If no info is associated with a key, get returns null  In sets to find an element you must have a copy of it ⚫ An HashMap hashes the keys // HashMap implements Map Map<String, Employee> staff = new HashMap<>(); Employee harry = new Employee("Harry Hacker"); staff.put("987-98-9996", harry); String s = "987-98-9996"; e = staff.get(s); // gets harry
  21. 21.  The collection framework does not consider a map itself as a Collection ⚫ It is possible to obtain views of the map  If you are interested in all the values of a map, loop over the enumeration of its entries  Iterators on views cannot add elements to the map  An UnsupportedOperationException is thrown Set<K> keySet() // Set of keys Collection<K> values() // Collection of values Set<Map.Entry<K, V>> entrySet() // Set of pairs (key,value) for (Map.Entry<String, Employee> entry : staff.entrySet()) { String key = entry.getKey(); Employee value = entry.getValue(); // do something with key, value }
  22. 22.  A framework is a set of classes that form the basis for building advanced functionality ⚫ The Collection framework defines classes to implement collections ⚫ The main interfaces are Collection and Map  Insertion interfaces are different between the two types  To get the elements from a Collection, just iterate over it  To get a value from a Map, use the get method // To insert an element in a Collection boolean add(E element) // To store a key/value pair in a Map V put(K key, V value) V get(K key)
  23. 23.  A List is an ordered collection ⚫ There is the concept of position of an element  A list provides random access methods  Lists provides a specialized iterator, ListIterator  A Set is a Collection with no duplicates ⚫ The add method can reject a value if already present ⚫ Methods equals and hashCode are used to maintain elements inside the set void add(int index, E element) E get(int index) void remove(int index) void add(E element)
  24. 24.  The framework provides some wrappers
  25. 25.  The framework have some companion objects ⚫ The Arrays type allows to trasform arrays into List  And so on... static <T> List<T> asList(T... a)  The list returned is a view on the array: it is not possible to change the size of the list; elements are the same ⚫ The Collections type have a bunch of utilities  The method nCopies builds an illusory immutable list List<String> settings = Collections.nCopies(100, "DEFAULT");  Object is stored only once  Method singleton returns an illusory set with one element Collections.singleton(anObject)
  26. 26. EXAMPLES https://beginnersbook.com/java-collections-tutorials/

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