Constraining Type Variables

Transcription

1 Constraining Type Variables Type variables can be constrained with bounds. It is often necessary to specify what types can be used in a generic class or method. Consider a generic min method that finds the smallest element in an array list of objects. How can you find the smallest element when you know nothing about the element type? You need to have a mechanism for comparing array elements. One solution is to require that the elements belong to a type that implements the Comparable interface. In this situation, we need to constrain the type variable. public static <E extends Comparable> E min(e[] a) { E smallest = a[0]; for (int i = 1; i < a.length; i++) if (a[i].compareto(smallest) < 0) smallest = a[i]; return smallest; You can call min with a String[] array but not with a Rectangle[] array the String class implements Comparable, but Rectangle does not. Very occasionally, you need to supply two or more type bounds. Then you separate them with the & character, for example <E extends Comparable & Cloneable> The extends keyword, when applied to type variables, actually means extends or implements. The bounds can be either classes or interfaces, and the type variable can be replaced with a class or interface type.

2 Example import java.util.*; public class PairTest2 { public static void main(string[] args) { GregorianCalendar[] birthdays = { new GregorianCalendar(1906, Calendar.DECEMBER, 9), new GregorianCalendar(1815, Calendar.DECEMBER, 10), new GregorianCalendar(1903, Calendar.DECEMBER, 3), new GregorianCalendar(1910, Calendar.JUNE, 22), ; Pair<GregorianCalendar> mm = ArrayAlg.minmax(birthdays); System.out.println("min = " + mm.getfirst().gettime()); System.out.println("max = " + mm.getsecond().gettime()); class ArrayAlg { public static <T extends Comparable> Pair<T> minmax(t[] a) { if (a == null a.length == 0) return null; T min = a[0]; T max = a[0]; for (int i = 1; i < a.length; i++) { if (min.compareto(a[i]) > 0) min = a[i]; if (max.compareto(a[i]) < 0) max = a[i]; return new Pair<T>(min, max);

3 Wildcards It is often necessary to formulate subtle constraints of type variables. Wildcard types were invented for this purpose. There are three kinds of wildcard types: Name Syntax Meaning Wildcard with lower bound? extends B Any subtype of B Wildcard with upper bound? super B Any supertype of B Unbounded wildcard? Any type Because you can t use CustomHolder<Object> as if it were a super-type of CustomHolder<String>, you can t write a method that would process both CustomHolder<Object> and CustomHolder<String>. There is, however, a special way to accomplish this. As part of the generics syntax, a wildcard is introduced, which, when used, basically means any type parameter. Using the wildcard allows you to pass in either stringholder or objectholder. You can read the method parameter type CustomerHolder<?> as a CustomHolder of any type as opposed to of Object type or of String type.

4 Another Example Pair<? extends Employee> denotes any generic Pair type whose type parameter is a subclass of Employee, such as Pair<Manager>, but not Pair<String>. Example: public static void printbuddies(pair<employee> p) { Employee first = p.getfirst(); Employee second = p.getsecond(); System.out.println(first.getName()+ " and " + second.getname() + " are buddies."; you cannot pass a Pair<Manager> to that method. But the solution is simple: use a wildcard type: public static void printbuddies(pair<? extends Employee> p) The type Pair<Manager> is a subtype of Pair<? extends Employee> Subtype Covariance

5 Type of a parameter Type of field Return type Wildcard usages Cannot be used as a type argument for a generic method invocation, a generic class instance creation, a supertype or in the header of reference types Node<?> anynode = new Node<?>(2006, null); //no class QuestionableNode<?> //no, header class SubNode extends Node<?> //no, supertype class ONode implements Comparable<Node<?>> //ok, nested wildcard are permited

6 Subtype covariance and contravariance

7 Unbounded wildcards public static void printlist(list<object> list) { for (Object elem : list) System.out.println(elem + " "); System.out.println(); public static void printlist(list<?> list) { for (Object elem : list) System.out.println(elem + " "); System.out.println();

8 Assignment and Type Erasure Because of type erasure, it is possible to assign a generic class reference to a reference of its nongeneric (legacy) version. Therefore, the following code compiles without error: Vector oldvector; Vector<Integer> intvector; oldvector = intvector; // valid However, though not an error, assigning a reference to a nongeneric class to a reference to a generic class will cause an unchecked compiler warning.

9 Restrictions and Limitations (I) Primitive Types You cannot substitute a primitive type for a type parameter. Thus, there is no Pair<double>, only Pair<Double>. Runtime Type Inquiry Objects in the virtual machine always have a specific nongeneric type. Therefore, all type inquiries yield only the raw type. For example, if (a instanceof Pair<String>) // same as a instanceof Pair really only tests whether a is a Pair of any type. The same is true for the test if (a instanceof Pair<T>) // T is ignored or the cast Pair<String> p = (Pair<String>) a;// can only test that a is a Pair! In the same spirit, the getclass method always returns the raw type. For example, Pair<String> stringpair =...; Pair<Employee> employeepair =...; if (stringpair.getclass() == employeepair.getclass()) // they are equal Arrays You cannot declare arrays of parameterized types, such as Pair<String>[] table = new Pair<String>[10];//ERROR the type is Pair[] You can try to convert it to Object[]: Object[] objarray = table; but if you try to store an element of the wrong type, an ArrayStoreException exception is thrown: objarray[0] = "Hello"; // ERROR--component type is Pair The erasure renders this mechanism ineffective for generic types. For this reason, arrays of parameterized types are outlawed.

10 Restrictions and Limitations (II) Instantiation of Generic Types You cannot instantiate generic types. For example, the following Pair<T> constructor is illegal: public Pair() { first = new T(); second = new T(); // ERROR Type erasure would change T to Object, and surely you don't want to call new Object(). Similarly, you cannot make a generic array: public <T> T[] minmax(t[] a) { T[] mm = new T[2];... // ERROR Type erasure would cause this method to always construct an array Object[2]. However, you can construct generic objects and arrays through reflection, by calling the Class.newInstance and Array.newInstance methods. Static Contexts You cannot reference type variables in static fields or methods. For example, the following clever idea won't work: public class Singleton<T> { public static T getsingleinstance() { // ERROR private static T singleinstance; // ERROR if (singleinstance == null) construct a new instance of T return singleinstance;

11 Implications Subsignature, override-equivalent subsignature: a() subsignature of b() if a() has same signature or signature of a() is same as the erasure of signature of b() Overriding overriding method has a subsignature of the overrided method Overloading Two methods are overloaded if they have the same name but are not override equivalent. static <T> void merge (MyStack<T> s1, MyStack<T> s2) static <T> void merge (MyStack<T> s1, MyStack<? extends T> s2) static <T> void merge (MyStack<T> s1, MyStack<? super T> s2)

12 Collections in Java The Collections Framework provides a well-designed set of interfaces and classes for storing and manipulating groups of data as a single unit, a collection. The framework provides a convenient API to many of the abstract data types familiar from computer science data structure curriculum: maps, sets, lists, trees, arrays, hashtables and other collections. 12

13 Mathematical Background A collection is the same as the mathematical concept of a set. A set is just a group of unique items, meaning that the group contains no duplicates. The Collections Framework in fact includes a Set interface, and a number of concrete Set classes. Some real-world examples of sets include the following: The set of uppercase letters 'A' through 'Z' The set of nonnegative integers {0, 1, 2... A set of people (friends, employees, clients,...) The set of records returned by a database query The set of Component objects in a Container The set of all pairs The empty set {. These examples show the basic properties of sets: Sets contains only one instance of each item Sets may be finite or infinite Sets can define abstract concepts. A map is a special kind of set. Mathematically speaking, it is a function from one set, the key set, to another set, the value set or a set of pairs. Some examples of maps are: The map of IP addresses to domain names (DNS) A map from keys to database records A dictionary (words mapped to meanings) The conversion from base 2 to base 10 Key set Set of pairs Value set

14 Collection Framework <<interface>> Iterator produce <<interface>> Collection produce <<interface>> Map <<abstract>> Dictionary <<interface>> ListIterator <<interface>> List <<interface>> Set <<abstract>> AbstractMap <<interface>> SortedMap Hashtable <<interface>> Comparator <<abstract>> AbstractCollection HashMap TreeMap Properties <<interface>> Comparable <<abstract>> AbstractList <<abstract>> AbstractSet <<interface>> SortedSet Legend HashSet TreeSet Hashtable Classes in versions 1.0, 1.1 Vector ArrayList <<abstract>> AbstractSequentialList <<interface>> Map HashMap Fundamental interfaces Concrete classes Stack LinkedList

15 Interfaces and Classes for Collections All classes and interfaces of the Collections Framework are generic! Main Interfaces Collection Map Set List SortedMap Concrete Classes SortedSet Interface Implementation Previous classes Set HashSet SortedSet TreeSet List ArrayList LinkedList Vector Stack Map HashMap Hashtable Properties SortedMap TreeMap

16 The java.util.collection Interface Methods Single object operations boolean add(object o) boolean remove(object o) boolean contains(object o) Iterator iterator() int size() boolean isempty() Object groups operations boolean addall(collection<?> c) boolean containall(collection<?> c) boolean removeall(collection<?> c) boolean retainall(collection<?> c) void clear() Array operations Object[] toarray() <T> T[] toarray( T[] a) Meaning Ensures that this collection contains the specified element (optional operation). Returns true if this collection changed as a result of the call. (Returns false if this collection does not permit duplicates and already contains the specified element.) Removes a single instance of the specified element from this collection, if it is present (optional operation). Returns true if this collection contained the specified element. Returns true if this collection contains the specified element. Returns an iterator over the elements in this collection. There are no guarantees concerning the order in which the elements are returned. Returns the number of elements in this collection. Returns true if this collection contains no elements. Adds all of the elements in the specified collection to this collection. The behavior of this operation is undefined if the specified collection is modified while the operation is in progress. Returns true if this collection contains all of the elements in the specified collection. Removes all of this collection's elements that are also contained in the specified collection. After this call returns, this collection will contain no elements in common with the specified collection. Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection. Removes all of the elements from this collection (optional operation). The collection will be empty after this method returns. Returns an array containing all of the elements in this collection. If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order. Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this collection.

17 Methods Methods boolean hasnext() Object next() void remove() The java.util.iterator Interface The iterator() method of the Collection interface returns an Iterator object. With the Iterator interface methods, you can traverse a collection from start to finish and safely remove elements from the underlying collection. The remove() method is optionally supported by the underlying collection. When called, and supported, the element returned by the last next() call is gets-inventory-items-from removed. InventoryBrowser Collection coll =...; Iterator iterator=coll.iterator(); while (iterator.hasnext()) { Object element = iterator.next(); if (removalcheck(element)) { iterator.remove(); Meaning Returns true if the iteration has more elements. (In other words, returns true if next() would return an element rather than throwing an exception.) Returns the next element in the iteration. Removes from the underlying collection the last element returned by this iterator. This method can be called only once per call to next(). The behavior of an iterator is unspecified if the underlying collection is modified while the iteration is in progress in any way other than by calling this method <<Interface>> Iterator hasnext() : boolean next (): Object InventoryIterator 1..1 creates_an_inventoryiterator 1..1 <<Interface>> Collection iterator() : Iterator gets-inventory-items-from * display * InventoryItem * InventoryCollection iterator() : Iterator

18 Method Declaration Access E get(int index) boolean add(int index, E element) E remove(int index) E set(int index, E element) boolean addall(int index, Collection<? Extends E> c) Object Searching int indexof(object o) int lastindexof(object o) Iteration ListIterator<E> listiterator() ListIterator<E> listiterator(int index) The java.util.list Interface Meaning Returns the element at the specified position in this list. Inserts the specified element at the specified position in this list. Shifts the element currently at that position (if any) and any subsequent elements to the right (adds one to their indices). Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts one from their indices). Returns the element that was removed from the list. Replaces the element at the specified position in this list with the specified element (optional operation). Inserts all of the elements in the specified collection into this list at the specified position. Shifts the element currently at that position (if any) and any subsequent elements to the right (increases their indices). The new elements will appear in this list in the order that they are returned by the specified collection's iterator. The behavior of this operation is undefined if the specified collection is modified while the operation is in progress. Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element. Returns a list iterator over the elements in this list (in proper sequence). Returns a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list. The specified index indicates the first element that would be returned by an initial call to next(). An initial call to previous() would return the element with the specified index minus one. Global Operation List<E> sublist(int fromindex, int toindex) Returns a view of the portion of this list between the specified fromindex, inclusive, and toindex, exclusive. The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa. The returned list supports all of the optional list operations supported by this list.

19 ArrayList ArrayList = Concrete class that implements the List interface using an array with dynamic extension. The random access is relatively fast. Adds methods to the List interface: ArrayList<E>(Collection<E>) Object clone() void ensurecapacity(int) void trimtosize() Allows simulation of object queues, stacks and generally data structures ended by markers: ArrayList<Employee> elements = new ArrayList<Employee>(); elements.add(queueend++, employee); Employee boss = elements.remove(queueinit++); Queue of employees Simulation of a queue with an ArrayList<Employee> Array elements queueinit queueend

20 LinkedList A linked list is a data structure used for collecting a sequence of objects, which allows efficient addition and removal of elements in the middle of the sequence. A linked list consists of a number of nodes, each of which has a reference to which has a reference to the next node. Visiting the elements of a linked list in sequential order is efficient, but random access is not. The Java library provides a LinkedList class. It is a generic class in the java.util package, just like the ArrayList class. The nodes of the LinkedList class store two links: one to the next element and one to the previous one. Such a list is called a doubly linked list. You can use a ListIterator (a generic class) to access elements inside a linked list. You should think of the iterator as pointing between two elements.

21 LinkedList LinkedList = Concrete class that provides an optimal sequential access with removing and insertion operations. The random access is relatively slow (if the randon access is frequent it is better to use an ArrayList). Adds methods to the List interface: addfirst( ) addlast( ) getfirst( ) getlast( ) removefirst( ) removelast( ) Allows simulation of object queues and stacks. Queue of E Simulation of a Queue with a LinkedList LinkedList<E> queue =...; queue.addfirst(element); E object = queue.removelast(); head info next info next info next tail info next

22 Linked Lists In Java double concatenated lists (linked lists) can be used where the object link contains a reference to the previous object, too. LinkedList staff = new LinkedList();..... Iterator it = staff.iterator(); for (int i=0; i<100; i++) System.out.println(it.next()); it.remove(); ListIterator has a method add different from that in Collection: ListIterator iter= staff.listiterator(); iter.next(); iter.add( Mary ); iter.next(); Object oldvalue = iter.next(); iter.set(newvalue); LinkedList first Link Link Link Link data next prev data next prev.... data next prev data next prev The objects Iterator know when the list changes. LinkedList lista =.... ; ListIterator it1 = lista.listiterator(); ListIterator it2 = lista.listiterator(); it1.next(); it1.remove(); it2.next(); 22

23 ListIterator 1. Initially, the iterator points before the first element. 2. You can move the iterator position with the next method: iterator.next(); The next method throws a NoSuchElementException if you are already past the end of the list. 3. You should always call the method hasnext before calling next - it returns true if there is a next element. if (iterator.hasnext()) iterator.next(); You traverse all elements in a linked list of strings with the following loop: while (iterator. hasnext()) { String name = iterator.next(); Do something with name If your loop simply visits all elements of the linked list, you can use the for each loop: for (String name : employeenames) { Do something with name The add method adds an object after the iterator, then moves the iterator position past the new element. iterator.add("juliet"); The remove method removes the object that was returned by the last call to next or previous, but you cannot call it immediately after a call to add.

24 Output Dick Harry Juliet Nina Tom Expected: Dick Harry Juliet Nina Tom Example import java.util.linkedlist; import java.util.listiterator; /** A program that tests the LinkedList class. */ public class ListTester { public static void main(string[] args) { LinkedList<String> staff = new LinkedList<String>(); staff.addlast( Dick ); staff.addlast( Harry ); staff.addlast( Romeo ); staff.addlast( Tom ); ListIterator<String> iterator = staff.listiterator(); // DHRT iterator.next(); // D HRT iterator.next(); // DH RT // Add more elements after second element iterator.add( Juliet ); // DHJ RT iterator.add( Nina ); // DHJN RT iterator.next(); // DHJNR T // Remove last traversed element iterator.remove(); // DHJN T // Print all elements System.out.print(iterator.next() + ); System.out.println(); System. out. println( Expected: Dick Harry Juliet Nina Tom );

25 import java.util.nosuchelementexception; public class LinkedList { private Node first; private class Node { public Object data; public Node next; public LinkedList() { first = null; public ListIterator listiterator() { return new LinkedListIterator(); private class LinkedListIterator implements ListIterator { private Node position, previous; public LinkedListIterator(){ position = null; previous = null; public Object next(){ if (!hasnext()) throw new NoSuchElementException(); previous = position; // Remember for remove if (position == null) position = first; else position = position.next; return position.data; public boolean hasnext() { if (position == null) return first! = null; else return position.next! = null;... public Object getfirst() { if (first == null) throw new NoSuchElementException(); return first.data; public void addfirst(object element){ Node newnode = new Node(); // 1 newnode.data = element; // 2 newnode.next = first; // 3 first = newnode; // 4 public Object removefirst() { if (first == null) throw new NoSuchElementException(); Object element = first.data; // 1 first = first.next; // 2 return element; // 3... Adding a Node to the Head of a Linked List A Concrete View of a Linked List Implementing Linked Lists (I) Removing the first node from a Linked List

26 Implementing Linked Lists (II) Removing a Node from the Middle of a Linked List with ListIterator Adding a Node to the Middle of a Linked List with ListIterator

27 Sets The Set<T> interface has the same methods of its super-interface Collection<T>. A set is an unordered collection of distinct elements. Elements can be added, located, and removed. Sets don't have duplicates. Adding a duplicate of an element that is already present is silently ignored. There are two different data structures for this purpose, called hash tables and trees. The HashSet and TreeSet classes both implement the Set interface. Set<String> names = new HashSet<String>(); names.add("romeo"); names.add("juliet"); names.remove( Romeo"); if(names.contains( Juliet )... We can use the next and hasnext methods to step through the set: Iterator<String> iter = names.iterator(); while (iter.hasnext()){ String name = iter.next(); Do something with name Warning. A set iterator does not visit the elements in the order in which you inserted them. The set implementation rearranges the elements so that it can locate them quickly. We can also use the for each loop to step through the set : while (String name : names){ Do something with name

28 Set 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( Verificato un duplicato: "+args[i]); System.out.println(s.size()+" distinct words identified: "+s); import java.util.*; public class FindDups2 { public static void main(string args[]) { Set uniques = new HashSet(); Set dups = new HashSet(); for (int i=0; i<args.length; i++) if (!uniques.add(args[i])) dups.add(args[i]); uniques.removeall(dups); // Destructive set-difference System.out.println("Unique words: " + uniques); System.out.println("Duplicate words: " + dups); >java FindDups2 i came i saw i left Unique words: [came, left, saw] Duplicate words: [i] 28

29 Implementing the Set interface HashSet = concrete class that represents a set where objects belong to a hashtable. HashSet should be used when the number of objects in Set is low. The objects in a collection Set should rewrite the method hashcode() of Object. int h = x.hashcode(); A hash function is a function that computes an integer value, the hash code, from an object, in such a way that different objects are likely to yield different hash codes. A good hash function minimizes collisions identical hash codes for different objects. Hash table Object F Hash code bucket HashSet TreeSet Hash function TreeSet = concrete class that represents an ordered set supported by a red-black tree (an always balanced tree). We can extract ordered sequences that implement Set. The objects we add to a TreeSet must be compatible with ordering. TreeSet haa a constructor that allows to add to the Set object a criterium of comparison between two objects: TreeSet ord = new TreeSet(); ord.add( Claude ); ord.add( Mary ); ord.add( Peter ); Iterator it = ord.iterator(); while (it.hasnext()) System.out.println(it.next()); 29

30 HashSet Example import java.util.*; public class hs { static HashSet hs = new HashSet(); public static void main(string[] arg) { createhs(); showhs(); System.out.println( There are "+hs.size() +" divisions"); public static void createhs() { String s1= Sistems", s2= Data", s3= Marketing"; hs.add( s1 ); hs.add( s2 ); hs.add( s3 ); Output: public static void showhs() { Iterator i = hs.iterator(); while( i.hasnext() ) { System.out.println(" Division: "+ i.next() ); Output if the collection was an TreeSet: java hs Division: Data Division: Marketing Division: Sistems There are 3 divisions java hs Division: Marketing Division: Sistems Division: Data There are 3 divisions

31 import java.util.hashset; import java.util.scanner; import java.util.set; public class SetDemo { public static void main(string[] args) { Set<String> names = new HashSet<String>(); Scanner in = new Scanner(System.in); boolean done = false; while (!done) { System.out.print( Add name, Q when done: ); String input = in.next(); if (input.equalsignorecase( Q )) done = true; else { names.add(input); print(names); done = false; while (!done) { System.out.print( Remove name, Q when done: ); String input = in.next(); if (input.equalsignorecase( Q )) done = true; else { names.remove(input); print(names); private static void print(set<string> s) { System.out.print( { ); for (String element : s) { System.out.print(element); System.out.print( ); System.out.println( ); Example with HashSet Output Add name, Q when done: Dick { Dick Add name, Q when done: Tom { Tom Dick Add name, Q when done: Harry { Harry Tom Dick Add name, Q when done: Tom { Harry Tom Dick Add name, Q when done: Q Remove name, Q when done: Tom { Harry Dick Remove name, Q when done: Jerry { Harry Dick Remove name, Q when done: Q

32 A Hash Table with Buckets to Store Elements with the Same Hash Code

33 TreeSet The object added to a TreeSet should implement the Comparable interface. The objects will be added in a tree in ascendent order. import java.util.*; public class SetExample { public static void main(string args[]) { Set set = new TreeSet(); set.add("clara"); set.add( Gene"); set.add("bernadine"); set.add("elizabeth"); set.add("gene"); System.out.println(set); Bernardine Clara Gene Elizabeth For efficiency, objects added to a HashSet need to implement the hashcode() method in a manner that properly distributes the hash codes. import java.util.*; public class SetExample { public static void main(string args[]) { Set set = new HashSet(); set.add("clara"); set.add( Gene"); set.add("bernadine"); set.add("elizabeth"); set.add("gene"); System.out.println(set); Set sortedset = new TreeSet(set); System.out.println(sortedSet);

34 To sort a collection of coins, the Coin class would need to implement this interface and define a compareto method: public class Coin implements Comparable {... public int compareto(object otherobject){ Coin other = (Coin) otherobject; if (value < other.value) return -1; if (value == other.value) return 0; return 1;... When you implement the compareto method of the Comparable Sorting interface, Coins you must make sure that the method defines a total relationship, with the following three properties: Antisymmetric: if a.compareto(b) 0, then b.compareto(a) 0 Reflexive: a.compareto(a) = 0 Transitive: if a.compareto(b) 0 and b.compareto(c) 0, then a.compareto(c) 0 Once your Coin class implements the Comparable interface, you can simply pass an array of coins to the Arrays.sort method: // Add coins... Arrays.sort(coins); If the coins are stored in an ArrayList, use the Collections.sort method instead; it uses the merge sort algorithm: ArrayList<Coin> coins = new ArrayList<Coin>(); // Add coins... Collections. sort (coins);