Practical Session 4 Java Collections

Transcription

1 Practical Session 4 Java 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 Operation + perform(arg: int) + getarg() : int TextArea -text: String -size: int -color: int + getters + setters

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

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

22 The Collections class The Collections class contains static methods that 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);