Marcin Luckner Warsaw University of Technology Faculty of Mathematics and Information Science

Transcription

1 Marcin Luckner Warsaw University of Technology Faculty of Mathematics and Information Science

2 } Generics allow the programer to create an universal parametric description } The parameter is casted to the given type when an instance is created 2

3 } Stronger type checks at compile time } Elimination of casts } Creation of universal algorithms 3

4 List list = new ArrayList(); list.add("hello"); String s = (String) list.get(0); } List<String> list = new ArrayList<>(); list.add("hello"); } String s = list.get(0); without generics Generics 4

5 } E - Element used extensively by the Java Collections Framework } K - Key } N - Number } T - Type } V - Value } S,U,V etc. - 2nd, 3rd, 4th types mluckner@mini.pw.edu.pl 11/30/16 5

6 public class Box { private Object object; public void set(object object) { this.object = object; } public Object get() { return object; } } public class Box<T> { private T t; public void set(t t) { this.t = t; } public T get() { return t; } } without generics Generics mluckner@mini.pw.edu.pl 11/30/16 6

7 public interface Pair<K, V> { public K getkey(); public V getvalue(); } public class OrderedPair<K, V> implements Pair<K, V> { private K key; private V value; public OrderedPair(K key, V value) { this.key = key; this.value = value; } public K getkey() { return key; } public V getvalue() { return value; } } mluckner@mini.pw.edu.pl 11/30/16 7

8 } Box<Integer> integerbox = new Box<>(); } OrderedPair<String, Integer> p1 = new OrderedPair<>("Even", 8); } OrderedPair<String, String> p2 = new OrderedPair<>("hello", "world"); } OrderedPair<String, Box<Integer>> p = new OrderedPair<>("primes", new Box<Integer>(...)); 8

9 } A raw type is the name of a generic class or interface without any type arguments Box<String> stringbox = new Box<>(); Box rawbox = stringbox; } The following code generates a warning Box rawbox = new Box();// Box<T> Box<Integer> intbox = rawbox; 9

10 } Introduces their own type parameters } Definition: public class Util { public static <K, V> boolean compare(pair<k, V> p1, Pair<K, V> p2) { return p1.getkey().equals(p2.getkey()) && p1.getvalue().equals(p2.getvalue()); } } } Application: Pair<Integer, String> p1 = new Pair<>(1, "apple"); Pair<Integer, String> p2 = new Pair<>(2, "pear"); boolean same = Util.<Integer, String>compare(p1, p2); 10

11 } The generic method ArrayAlg.<String>getMiddle("John", "Q.", "Public"); } Can be replaced by ArrayAlg.getMiddle("John", "Q.", "Public"); } Problem: } ArrayAlg.getMiddle(3.14, 1729, 0); } Generates cast error } It can be cast to Number or Comparable 11

12 } We can limit types of parameters } <U extends Number> accepts only instances of Number or its subclasses } We can add many limitations } <T extends C1 & I2 & I3> accepts only a class that extends C1 and implies interfaces I2 and I3. 12

13 public static <T> int countgreaterthan( T[] anarray, T elem) { } int count = 0; for (T e : anarray) if (e > elem) return count; public static <T extends Comparable<T>> int countgreaterthan( T[] anarray, T elem) { } int count = 0; for (T e : anarray) if (e.compareto(elem) > 0) ++count; return count; źle poprawnie 13

14 } The hierarchy of parameters is not mapped on the hierarchy of Generics classes } All Generics classes are on the same hierarchy level mluckner@mini.pw.edu.pl 11/30/16 14

15 } Generic types cannot be use For primitive types As part of Arrays As static fields With instanceof 15

16 } The primitive types cannot be used Box<Double> - OK. Box<double> - error. 16

17 } We cannot check the type during the execution if (a instanceof Pair<String>) Error if (a instanceof Pair<T>) Error Pair<String> p = (Pair<String>) a; Warning Pair<String> stringpair =...; Pair<Employee> employeepair =...; if (stringpair.getclass() == employeepair.getclass()) true 17

18 } You cannot create an Array of Generic Types Pair<String>[] table = new Pair<String>[10] Error 18

19 } Allows the programet to use undefined types } Pair<? extends Employee> Pair<Employee> Pair<Manager> 19

20 } Allows the programer to addres raw type } Pair<? super Manager> Pair<Employee> Pair<Object> 20

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22 } Pair<?>? getfirst() void setfirst(?) } We can call setfirst method for any Parameter } Pair We cannot call setfirst Unbouted Raw type 22

23 public static boolean hasnulls(pair<?> p) { return p.getfirst() == null p.getsecond() == null; } mluckner@mini.pw.edu.pl 11/30/16 23