Practical Session 4 Java Collections

1. Practical Session 4Java Collections

2. Outline Working with a Collection The Collection interface The Collection hierarchy Case Study: Undoable Stack The Collections class Wrapper classes

3. Collection A group of elements. The group size can be changed during run-time. • A collection object has many useful methods for manipulating the collection: • Inserting elements • Deleting elements • Copying the collection • Iterating over the elements • Computing subsets of elements • … Various types of collections are defined in the standard library: Vector, LinkedList, Stack, ArrayDequeue, PriorityQueue, TreeSet,…

4. Collection • Why shouldn’t we just use arrays? • Arrays are not objects! • You cannot invoke methods on arrays. • Arrays have fixed size. • Arrays are less convenient for representing certain types of collections: • Sets • Double-ended queues • Dictionaries • …

5. Working with a collection Definition syntax: Collection <type>colName = new Collection <type>() Example: import java.util.Vector; … Vector <Car> carsVec = new Vector <Car>(); Car Volvo = new Car(2.0); Car BMW = new Car(2.2); carsVec.add(Volvo); carsVec.add(BMW); for (Car c: carsVec) System.out.println(c);

6. Working with a collection Example - continued: Car temp = carsVec.elementAt(0); carsVec.set(0,carsVec.elementAt(1)); carsVec.set(1, temp); for (int i=0; i < carsVec.size(); i++) System.out.println(carsVec.elementAt(i));

7. The Collection Interface Collection is an interface in the standard java library. The Collection interface is generic. It accepts a type as a parameter. public interface Collection<E> extends Iterable<E> { int size(); boolean isEmpty(); boolean contains(Object element); boolean add(E element); //optional boolean remove(Object element); //optional Iterator<E> iterator(); Object[] toArray(); <T> T[] toArray(T[] a); …

8. The Collection Hierarchy (Partial illustration) Collection List Set Queue

9. The Collection Hierarchy (Partial illustration) Collection List Set Queue Vector

10. The Collection Hierarchy (Partial illustration) Collection List Set Queue Vector LinkedList PriorityQueue Stack

11. Case Study: Undoable Stack Various programs allow the user to undo his operations. The undo operations are performed in reverse order. In such a program, we need to add each operation with its parameters onto the stack.

12. Program Stack

13. Program Stack Resize 36

14. Program Stack Resize 36 Recolor 4 Resize 36

15. Program Stack Resize 24 Recolor 4 Resize 36 Undo: Resize(36)

16. Program Stack Resize 24 Recolor 4 Resize 36 Undo: Resize(36) Recolor 4 Resize 36

17. Undoable Stack Hierarchy

18. Undoable Stack Hierarchy TextArea -text: String -size: int -color: int + getters… + setters…

19. Undoable Stack Hierarchy TextArea -text: String -size: int -color: int + getters… + setters… Operation + perform(arg: int) + getArg() : int

20. Undoable Stack Hierarchy TextArea -text: String -size: int -color: int + getters… + setters… Operation + perform(arg: int) + getArg() : int Recolor - color: int + perform(arg: int) + getArg() : int Resize - size: int + perform(arg: int) + getArg() : int

21. Undoable Stack Hierarchy TextArea -text: String -size: int -color: int + getters… + setters… Operation + perform(arg: int) + getArg() : int 0..n Recolor - color: int + perform(arg: int) + getArg() : int Resize - size: int + perform(arg: int) + getArg() : int UndoStack + add (op :Operation) + undo()

22. The Collections class • The Collections class contains static methodsthat operate on collections: • min • max • reverse • sort • … Example: import java.util.Collections; import java.util.Vector; … Vector <Car> carsVec = new Vector <Car>(); … Collections.reverse(carsVec);

23. Wrapper Classes What happens if we want to create a collection of a primitive type? Collections can be made only of objects. Primitive types are not objects. We use wrapper classes instead. A wrapper class is an object representing a primitive type.

24. Wrapper Classes Integer Float Double Character Boolean int float double char boolean Example: Integer intObj = new Integer(10); int number = intObj.intValue();

25. A collection of wrapper objects Import java.util.Vector; … Vector <Integer> numVec = new Vector <Integer>(); int size = 10; for (int i = size; i >0; i--) numVec.add(new Integer( i )); Collections.sort(numVec); for (Integer i: numVec) System.out.println( i);