Modulo I Java Generics

Transcription

1 Modulo I Java Generics Prof. Ismael H F Santos April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 1 Ementa Modulo I - Tópicos em JAVA Generics (incompleto!) April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 2 1

2 Bibliografia Linguagem de Programação JAVA Ismael H. F. Santos, Apostila UniverCidade, 2002 The Java Tutorial: A practical guide for programmers Tutorial on-line: Java in a Nutshell David Flanagan, O Reilly & Associates Just Java 2 Mark C. Chan, Steven W. Griffith e Anthony F. Iasi, Makron Books. Java 1.2 Laura Lemay & Rogers Cadenhead, Editora Campos April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 3 Livros Core Java 2, Cay S. Horstmann, Gary Cornell Volume 1 (Fundamentos) Volume 2 (Características Avançadas) Java: Como Programar, Deitel & Deitel Thinking in Patterns with JAVA, Bruce Eckel Gratuito. April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 4 2

3 POO-Java Generics April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 5 Concepts Generalizing Collection Classes Using Generics with other Java 1.5 Features Integration of Generics with Previous Releases User built generic classes April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 6 3

4 What Are Generics? Generics abstract over Types Classes, Interfaces and Methods can be Parameterized by Types Generics provide increased readability and type safety April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 7 Java Generic Programming Java has class Object Supertype of all object types This allows subtype polymorphism Can apply operation on class T to any subclass S <: T Java do not have templates No parametric polymorphism Many consider this the biggest deficiency of Java Java type system does not let you cheat Can cast from supertype to subtype Cast is checked at run time April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 8 4

5 Generics (Cont d) A class definition with a type parameter is stored in a file and compiled just like any other class. Once a parameterized class is compiled, it can be used like any other class. However, the class type plugged in for the type parameter must be specified before it can be used in a program. Doing this is said to instantiate the generic class. Sample<String> object = new Sample<String>(); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 9 A Class Definition with a Type Parameter April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 10 5

6 A Class Definition with a Type Parameter (Cont d) A class that is defined with a parameter for a type is called a generic class or a parameterized class The type parameter is included in angular brackets after the class name in the class definition heading. Any non-keyword identifier can be used for the type parameter, but by convention, the parameter starts with an uppercase letter. The type parameter can be used like other types used in the definition of a class. April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 11 Tip: Compile with the -Xlint Option There are many pitfalls that can be encountered when using type parameters Compiling with the -Xlint option will provide more informative diagnostics of any problems or potential problems in the code javac Xlint Sample.java April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 12 6

7 Example generic construct: Stack Stacks possible for any type of object For any type t, can have type stack_of_t Operations push, pop work for any type In C++, would write generic stack class template <type t> class Stack { private: t data; Stack<t> * next; public: void push (t* x) { t* pop ( ) { ; What can we do in Java? April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 13 Simple Example Using Interfaces interface List<E> { void add(e x); Iterator<E> iterator(); interface Iterator<E> { E next(); boolean hasnext(); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 14 7

8 What Generics Are Not Generics are not templates Unlike C++, generic declarations are Typechecked at compile time Generics are compiled once and for all Generic source code not exposed to user No bloat The type checking with Java 1.5 Generics changes the way one programs (as the next few slides show) April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 15 Java 1.0 vs Generics class Stack { void push(object o) {... Object pop() { String s = "Hello"; Stack st = new Stack();... st.push(s);... s = (String) st.pop(); class Stack<A> { void push(a a) {... A pop() { String s = "Hello"; Stack<String> st = new Stack<String>(); st.push(s);... s = st.pop(); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 16 8

9 Generic Class Definition: An Example April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 17 Generic Class Definition: An Example (Cont d) April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 18 9

10 Generic Class Usage: An Example April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 19 Generic Class Usage: An Example (Cont d) Program Output: April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 20 10

11 Java generics are type checked A generic class may use operations on objects of a parameter type Example: PriorityQueue<T> if x.less(y) then Two possible solutions C++: Link and see if all operations can be resolved Java: Type check and compile generics w/o linking This requires programmer to give information about type parameter Example: PriorityQueue<T extends...> April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 21 How to Use Generics List<Integer> xs = new LinkedList<Integer>(); xs.add(new Integer(0)); Integer x = xs.iterator.next(); Compare with List xs = new LinkedList(); xs.add(new Integer(0)); Integer x = (Integer)xs.iterator.next(); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 22 11

12 A Generic Constructor Name Has No Type Parameter!!! Although the class name in a parameterized class definition has a type parameter attached, the type parameter is not used in the heading of the constructor definition: public Pair<T>() A constructor can use the type parameter as the type for a parameter of the constructor, but in this case, the angular brackets are not used: public Pair(T first, T second) However, when a generic class is instantiated, the angular brackets are used: Pair<String> pair= new Pair<STring>("Happy", "Day"); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 23 A Primitive Type Cannot be Plugged in for a Type Parameter!!! The type plugged in for a type parameter must always be a reference type: It cannot be a primitive type such as int, double, or char However, now that Java has automatic boxing, this is not a big restriction. Note: Reference types can include arrays. April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 24 12

13 Limitations on Type Parameter Usage Within the definition of a parameterized class definition, there are places where an ordinary class name would be allowed, but a type parameter is not allowed. In particular, the type parameter cannot be used in simple expressions using new to create a new object For instance, the type parameter cannot be used as a constructor name or like a constructor: T object = new T(); T[] a = new T[10]; April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 25 Limitations on Generic Class Instantiation Arrays such as the following are illegal: Pair<String>[] a = new Pair<String>[10]; Although this is a reasonable thing to want to do, it is not allowed given the way that Java implements generic classes April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 26 13

14 Using Generic Classes and Automatic Boxing April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 27 Using Generic Classes and Automatic Boxing (Cont d) Program Output: April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 28 14

15 Collection Class Example HashMap<Intger, Double> hm = new HashMap<Intger, Double> (); // Note Auto-boxing from 15. hm.put (1,2.0); double coeff = hm.get(1); Hashmap 1.4 hm => Hashmap 1.5 <Object, Object> Hashmap 1.4 hm => Hashmap 1.5 <Object, Object> April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 29 List Usage: Without Generics List ys = new LinkedList(); ys.add("zero"); List yss; yss = new LinkedList(); yss.add(ys); String y = (String) ((List)yss.iterator().next()).iterator().next(); // Evil run-time error time error Integer z = (Integer)ys.iterator().next(); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 30 15

16 List Usage: With Generics List<String> ys = new LinkedList<String>(); ys.add("zero"); List<List<String>> yss; yss = new LinkedList<List<String>>(); yss.add(ys); String y = yss.iterator().next().iterator().next(); // Compile-time error much better! Integer z = ys.iterator().next(); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 31 List Implementation w/o Generics class LinkedList implements List { protected class Node { Inner Inner Class Class Object elt; Remember these? these? Node next; Node(Object elt){elt = e; next = null; protected Node h, t; public LinkedList() {h = new Node(null); t = h; public void add(object elt){ t.next = new Node(elt); t = t.next; April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 32 16

17 List Implementation With Generics class LinkedList<E >implements List<E> protected class Node { E elt; Node next; Node(E elt){elt = e; next = null; protected Node h, t; public LinkedList() {h = new Node(null); t = h; public void add(e elt){ t.next = new Node(elt); t = t.next; // April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 33 Recall the Interator Interface class LinkedList<E >implements List<E> // public Iterator<E> iterator(){ example) example) return new Iterator<E>(){ protected Node p = h.next; public boolean hasnext(){return p!= null; public E next(){ E e = p.elt; p = p.next; return e; Anonymous Anonymous Inner Inner Class Class (see (see Swing Swing scribble scribble April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 34 17

18 Multiple Type Parameters A generic class definition can have any number of type parameters. Multiple type parameters are listed in angular brackets just as in the single type parameter case, but are separated by commas. April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 35 Multiple Type Parameters (Cont d) April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 36 18

19 Multiple Type Parameters (Cont d) April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 37 Using a Generic Class with Two Type Parameters Program Output: April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 38 19

20 A Generic Classes and Exceptions It is not permitted to create a generic class with Exception, Error, Throwable, or any descendent class of Throwable A generic class cannot be created whose objects are throwable public class GEx<T> extends Exception The above example will generate a compiler error message April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 39 Methods Can Be Generic Also interface Function<A,B>{ B value(a arg); interface Ntuple<T> { <S> Ntuple<S> map(function<t,s> f); Ntuple<Integer> nti =...; nti.map (new Function<Integer, Integer> { Integer value(integer i) { return new Integer(i.intValue()*2); ); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 40 20

21 Example: Generics and Inheritence 1 st consider this code w/o Generics class ListUtilities { public static Comparable max(list xs) { Iterator xi = xs.iterator(); Comparable w = (Comparable) xi.next(); while (xi.hasnext()) { Comparable x = (Comparable) xi.next(); if (w.compareto(x) < 0) w = x; return w; What dangers lurk lurk here? April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 41 Consider what happens (and when) List xs = new LinkedList(); xs.add(new Byte(0)); Byte x = (Byte) ListUtilities.max(xs); List ys = new LinkedList(); ys.add(new Boolean(false)); // Evil run-time error Boolean y = (Boolean) ListUtilities.max(ys); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 42 21

22 With Generics We Get Compile Check List<Byte> xs = new LinkedList<Byte>(); xs.add(new Byte(0)); Byte x = ListUtilities.max(xs); List<Boolean> ys = new LinkedList<Boolean>(); ys.add(new Boolean(false)); // Compile-time error Boolean y = ListUtilities.max(ys); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 43 Generics and Inheritance Suppose you want to restrict the type parameter to express some restriction on the type parameter This can be done with a notion of subtypes Subtypes (weakly construed) can be expressed in Java using inheritance So it s a natural combination to combine inheritance with generics April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 44 22

23 Priority Queue Example interface Comparable<I> { boolean lessthan(i); class PriorityQueue<T extends Comparable<T>> { T queue[ ] ; void insert(t t) {... if ( t.lessthan(queue[i]) )... T remove() { Said to be bounded Said to be bounded April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 45 Bounded Parameterized Types The <E extends Number> syntax means that the type parameter of MathBox must be a subclass of the Number class We say that the type parameter is bounded new MathBox<Integer>(5); //Legal new MathBox<Double>(32.1); //Legal new MathBox<String>( No good! );//Illegal April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 46 23

24 Bounded Parameterized Types Inside a parameterized class, the type parameter serves as a valid type. So the following is valid. public class OuterClass<T> { private class InnerClass<E extends T> { Syntax note: The <A extends B> syntax is valid even if B is an interface. April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 47 Bounded Parameterized Types Java allows multiple inheritance in the form of implementing multiple interfaces, so multiple bounds may be necessary to specify a type parameter. The following syntax is used then: <T extends A & B & C & > Example interface A { interface B { class MultiBounds<T extends A & B> { April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 48 24

25 Using a Generic Class with Two Type Parameters Program Output: April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 49 Bounds for Type Parameters Sometimes it makes sense to restrict the possible types that can be plugged in for a type parameter T. For instance, to ensure that only classes that implement the Comparable interface are plugged in for T, define a class as follows: public class RClass<T extends Comparable> "extends Comparable" serves as a bound on the type parameter T. Any attempt to plug in a type for T which does not implement the Comparable interface will result in a compiler error message. April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 50 25

26 Bounds for Type Parameters (Cont d) A bound on a type may be a class name (rather than an interface name) Then only descendent classes of the bounding class may be plugged in for the type parameters: public class ExClass<T extends Class1> A bounds expression may contain multiple interfaces and up to one class. If there is more than one type parameter, the syntax is as follows: public class Two<T1 extends Class1, T2 extends Class2 & Comparable> April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 51 Bounds for Type Parameters (Cont d) April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 52 26

27 Another example interface LessAndEqual<I> { boolean lessthan(i); boolean equal(i); class Relations<C extends LessAndEqual<C>> extends C { boolean greaterthan(relations<c> a) { return a.lessthan(this); boolean greaterequal(relations<c> a) { return greaterthan(a) equal(a); boolean notequal(relations<c> a) {... boolean lessequal(relations<c> a) { April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 53 Generics and Subtyping Is the following code snippet legal? List<String> ls = new ArrayList<String>(); //1 List<Object> lo = ls; //2 Line 1 is certainly legal. What about line 2? Is a List of Strings a List of Object. Intuitive answer for most is sure!. But wait! The following code (if line 2 were allowed) would attempt to assign an Object to a String! For For all all types P and and C (i.e (i.ec is is a subtype subtype of of P) P) lo.add(new Object()); // 3 Subtype(P,C)!=>!=> String s = ls.get(0); // 4: Subtype(Generic<P>,Generic<C>) April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 54 27

28 Subclassing a generic class import java.awt.color; public class Subclass extends MyClass<Color> { // You almost always need to supply a constructor public Subclass(Color color) { super(color); public static void main(string[ ] args) { Subclass sc = new Subclass(Color.GREEN); sc.print(color.white); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 55 Generic Interfaces An interface can have one or more type parameters. The details and notation are the same as they are for classes with type parameters. April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 56 28

29 Generic Methods When a generic class is defined, the type parameter can be used in the definitions of the methods for that generic class. In addition, a generic method can be defined that has its own type parameter that is not the type parameter of any class A generic method can be a member of an ordinary class or a member of a generic class that has some other type parameter. The type parameter of a generic method is local to that method, not to the class. April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 57 Generic Methods (Cont d) The type parameter must be placed (in angular brackets) after all the modifiers, and before the returned type: public static <T> T genmethod(t[] a) When one of these generic methods is invoked, the method name is prefaced with the type to be plugged in, enclosed in angular brackets String s = NonG.<String>genMethod(c); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 58 29

30 Inheritance with Generic Classes A generic class can be defined as a derived class of an ordinary class or of another generic class As in ordinary classes, an object of the subclass type would also be of the superclass type Given two classes: A and B, and given G: a generic class, there is no relationship between G<A> and G<B> This is true regardless of the relationship between class A and B, e.g., if class B is a subclass of class A April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 59 A Derived Generic Class: An Example April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 60 30

31 A Derived Generic Class: An Example (Cont d) Program Output: April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 61 Wildcards Consider the problem of writing code that prints out all the elements in a collection before 1.5. void printcollection(collection c) { Iterator i = c.iterator(); for (k = 0; k < c.size(); k++) { System.out.println(i.next()); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 62 31

32 1 st Naïve Try w/ Generics void printcollection(collection<object> c) { for (Object e : c) { System.out.println(e); The problem is that this new version is much less useful than the old one. The old code could be called with any kind of collection as a parameter, The new code only takes Collection<Object>, which, as is not a supertypeof all kinds of collections! April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 63 Correct way Use Wildcards So what is the supertype of all kinds of collections? Pronounced collection of unknown and denoted Collection<?>, A collection whose element type matches anything. It s called a wildcard type for obvious reasons. void printcollection(collection<?> c) { for (Object e : c) { System.out.println(e); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 64 32

33 Using Wildcards Again public class Census { public static void addregistry(map<string,? extends Person> registry) { // Assuming Drivers are a subtype of Person Map<String, Driver> alldrivers =...; Census.addRegistry(allDrivers); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 65 Implementing Generics Type erasure Compile-time type checking uses generics Compiler eliminates generics by erasing them Compile List<T> to List, T to Object, insert casts Generics are not templates Generic declarations are typechecked Generics are compiled once and for all No instantiation No code bloat April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 66 33

34 How Do Generics Affect My Code? They don t except for the way you ll code! Non-generic code can use generic libraries; For example, existing code will run unchanged with generic Collection library April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 67 Erasure Erasure erases all generics type argument information at compilation phase. E.g. List<String> is converted to List E.g. String t = stringlist.iterator().next() is converted to String t = (String) stringlist.iterator().next() As part of its translation, a compiler will map every parameterized type to its type erasure. April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 68 34

35 Erasure List <String> l1 = new ArrayList<String>(); List<Integer> l2 = new ArrayList<Integer>(); System.out.println(l1.getClass() == l2.getclass()); April 05 Prof. Ismael H. F. Santos - ismael@tecgraf.puc-rio.br 69 35