Typing issues JAVA GENERICS. ! Parametric Polimorphism. ! Bounded Polymorphism. ! F- Bounded Polymorphism 10/28/13. Examples and Problems

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1 Examples and Problems JAVA GENERICS Typing issues! Parametric Polimorphism o max: t ((t t) bool) (t t) t) max: given LessThan funceon return max of two args! Bounded Polymorphism o printstring: t < Printable. T String for every subtype of printable funeon from T to string! F- Bounded Polymorphism o max t < Comparable(t). t t t for every subtype of return max of two args 1

2 F- bound subtyping! Generic interface! interface Comparable<T> { public int compareto(t arg) x.compareto(y) = negaeve, 0, posieve if x < = > x! Subtyping! interface Comparable<A> { public int compareto(a arg) public int anothermethod (A arg) <! interface Comparable<A> { public int compareto(a arg) Example! Generic interface! Interface Comparable<T> { public int CompareTo(T arg);! Example! public static <T extends Comparable<T>> T max (Collection<T> coll) { T candidate = coll.iterator().next(); for (T elt: coll) { if (candidate.compareto(elt) < 0) candidate = elt; return candidate; 2

3 Example! Generic interface! Interface Comparable<T> { Object public int CompareTo(T arg);! Example! public static <T extends Comparable<T>> T max (Collection<T> coll) { T candidate = coll.iterator().next(); for (T elt: coll) { if (candidate.compareto(elt) < 0) candidate = elt; return candidate; candidate.compareto: T Object Arrays! Using arrays Integer [] ints = new Integer[]{1, 2, 3; Number [] nums = ints; // the class Number is the superclass // of classes BigDecimal,.. Integer,... nums[2] = 3.14; // runt time exception! Using Lists List<Integer> ints = Array.asList(1,2,3); List<Number> nums = ints; // compile-time error! Reason: List<Integer> is not a subtype of List<Number> 3

4 Java Generics: ImplementaEon! Type erasure o Compile- Eme checking uses generics o Compiler eliminates generics by erasing then in then code o Compile List<T> to List, List to Objets with a lot of casts o Java Generics are not templates! Generic declaraeons are typechecked! Generics are compiled once and for all o No instaeaeons o No code expansions Homogeneous ImplementaEon Same representaeon and code fro all types of data 4

5 Heterogeneous ImplementaEons Specialize the representaeon according to type Heterogeneous ImplementaEons! Compile generic class G<param> o Check use of parameter types o Produce extended forms of bytecode! Store constraints, type paramenters in the bytecode! InstanEate before loading o Replace parameter type by actual classes o Transform bytecode into ordinary bytecode (preprocessing before the class loading) o Heterogeneous implementaeon is possible but it was noy adopted 5

6 QuesEon Class C<A> {A id (A arg) {.. Class D extend C<String>{ Object id(object args) { Is this code legal in Java? QuesEon Class C<A> {A id (A arg) {.. Class D extend C<String>{ Object id(object args) { Is this code legal in Java? NO!!! Subclass method looks like a different method but aier type erasure the signature are the me 6

7 Problem Class myvector<t> extends Vector { void add (Object a) { try { T tmp = (T) a; super.add(tmp); catch (ClassCastedExcepEon c) {.. Does this pamern detect insereon errors? class Container<T> { public T internal; public staec Container lastinstance; Container (T value) { internal = value; lastinstance = this; Problem class Main { public staec void main (String args [ ]) { Container<String> str = new Container<String>( Happy ); Container<Integer> myint = new Container<Integer>(31337); System.out.println(str.lastInstance.internal.toString()); QuesEon: Explain the behaviour 7

8 public class Pair<K, V> { public Pair(K key, V value) { this.key = key; this.value = value; public K getkey(); { return key; public V getvalue(); { return value; public void setkey(k key) { this.key = key; public void setvalue(v value) { this.value = value; private K key; private V value; QuesEon: What is class converted to aier type erasure? public class Pair { public Pair(Object key, Object value) { this.key = key; this.value = value; public Object getkey() { return key; public Object getvalue() { return value; public void setkey(object key) { this.key = key; public void setvalue(object value) { this.value = value; private Object key; private Object value; 8

9 Programming Write a generic method to count the number of elements in a colleceon that have a specific property (for example, odd integers, prime numbers, palindromes) public final class gmethod{ public staec <T> int countif(colleceon<t> c, UnaryPredicate<T> p) { int count = 0; for (T elem : c) if (p.test(elem)) ++count; return count; public interface UnaryPredicate<T> { public boolean test(t obj); 9

10 class OddPredicate implements UnaryPredicate<Integer> { public boolean test(integer i) { return i % 2!= 0; public class Test { public staec void main(string[] args) { CollecEon<Integer> ci = Arrays.asList(1, 2, 3, 4); int count = Algorithm.countIf(ci, new OddPredicate()); System.out.println("Number of odd integers = " + count); Concurrent Object Pool 1) The pool must let clients use an object if any is available. 2) it must reuse the objects once they are returned to the pool by a client. 3) If Required, it must be able to create more objects to satisfy growing demands of the client. 4) It must provide a proper shutdown mechanism, such that on shutdown no memory leaks occur. 10