עקרונות תכנות מונחה עצמים תרגול 2 1: Generics

1. עקרונות תכנות מונחה עצמיםתרגול 21: Generics

2. Outline • Generic classes • Generics & Inheritance • Wild Cards characters  • Case study: Generic Graph

3. Generic Classes – Motivation List v = new ArrayList(); v.add("test"); Integer i = (Integer)v.get(0); => // runtime error List<String> v = new ArrayList<String>(); v.add("test"); Integer i = (Integer)v.get(0); => // compilation error

4. Generic Classes • A class is generic if it declares one or more type variables. • A generic class is defined with the following format: class name<T1, T2, ..., Tn> { /* ... */ } • The type parameter section, delimited by angle brackets (<>), follows the class name. It specifies the type parameters (also called type variables) T1,T2, ..., and Tn. • The type variables can be used as: • Method parameters. • Return value. • Local variables.

5. Generic Classes – Example public interface List<E> { void add(E x); Iterator<E> iterator(); } publicinterface Iterator<E> { E next(); booleanhasNext(); } public class ArrayList<E> implements List<E> { private E[] _data; ... }

6. Generic Classes vsConcrete Classes • Generic Class declaration defines a set of parameterized types, one for each possible invocation of the type parameter section. • All compiles to the same .class file. • All uses the same .class file at runtime.

7. Generic Classes VS Concrete Classes (Example) public class ArrayList<E> implements List<E> { private E[] _data; ... } List<String> v; v = new ArrayList<String>(); Generic class Compile to ArrayList.class Concrete class Uses ArrayList.class

8. Generics & Inheritance

9. Assignments roles: • If a type A is of a sub-class of type B we will mark it as: • We can assign (runtime) type A to a (compile time) variable B iff

10. Using wild cards – Motivation Vector<Animal> animals; Vector<Cat> cats = new Vector<Cat>(); animals = cats; animals.add(new Dog()); Cat cat = cats.elementAt(0); // Animal animal; animal = new Dog(); Cat cat = animal; // // compilation error!!! // compilation error!!!

11. Using wild cards • We can read type B from A. • We can write type A to B. • We will use wild card (?) to represent an unknown type.

12. Using wild cards • ? extends A – a variable type which can only be read from. We can write only null to it. • ? super B – a variable type which can only be written to. We can read only Object from it.

13. Using wild cards – example: publicstaticvoidmakeNoise (Vector<Animal> animals) { for (Animal animal:animals) { animal.say(); } { Vector<Cat> cats = new Vector<Cat>(); makeNoise(cats); => Correction: publicstaticvoidmakeNoise (Vector<? extends Animal> animals) // compilation error!!!

14. Using wild cards – example: Vector<? extends Animal> animals = null; Vector<Cat> cats = new Vector<Cat>(); animals = cats; animals.add(new Dog()); Cat cat = cats.elementAt(0); => // compilation error!!! Correction: animals.add(null); - The only one that allows.

15. Using wild cards – examples: Vector<? extends Animal> animals = new Vector<Cat>(); Vector<? extends Cat> cats = new Vector<Animal>(); Vector<? super Animal> animals2 = new Vector<Cat>(); Vector<? super Cat> cats2 = new Vector<Animal>(); animals.add(new Cat()); cats.add(new Cat()); animals.add(null); Cat cat = animals.elementAt(0); Animal animal = cats.elementAt(0); Cat cat2 = animals2.elementAt(0); Animal animal2 = animals2.elementAt(0); Object o = animals2.elementAt(0); // compilation error // compilation error // compilation error // compilation error // compilation error // compilation error // compilation error

16. Question from 2013 test. נתון שהמחלקה Bיורשת מהמחלקה A, והמחלקה G היא גנרית עם פרמטר יחיד. מה צריך להיות הפרמטר X כדי שההשמה הבאה תהיה תקפה ע"פ כללי הטיפוסים של ג'אווה? G<X> g = new G<B>(); א. A ב. extends A ? ג. super A ? ד. super B ? ה. תשובות ב, ד נכונות ו. תשובות א ,ג נכונות

17. Question from 2013 test. נתון שהמחלקה Bיורשת מהמחלקה A, והמחלקה G היא גנרית עם פרמטר יחיד. מה צריך להיות הפרמטר X כדי שההשמה הבאה תהיה תקפה ע"פ כללי הטיפוסים של ג'אווה? G<X> g = new G<B>(); א. A ב. extends A ? ג. super A ? ד. super B ? ה. תשובות ב, ד נכונות ו. תשובות א ,ג נכונות

18. Case study:Generic Graph

19. Graph

20. Graphs can model • The internet • Maps • Social networks • …

21. Graph Representation

22. The vertex class

23. The Graph Class

24. The Graph Class

25. Vertex class - code

26. Graph class - code

27. DFS Tour

28. DFS Tour

29. Execution example

30. The Weighted interface

31. Weight query

32. Usage example

33. Usage example (cont.)

34. Exception handling

35. Exception handling

36. Summary • Generic class ( List<E> ) • ? extends A • ? super A

37. Summary • Generic class ( List<E> ) • ? extends A • ? super A