Practical Session 4Java Collections

OutlineWorking with a Collection

The Collection interface

The Collection hierarchy

Case Study: Undoable Stack

The Collections class

Wrapper classes

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,…

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

• …

Working with a collection

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);

Definition syntax: Collection <type> colName = new Collection <type>()

Example:

Working with a collection

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));

Example - continued:

The Collection InterfaceCollection 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);

…

The Collection Hierarchy

Collection

List Set Queue

(Partial illustration)

The Collection Hierarchy

Collection

List Set Queue

Vector

(Partial illustration)

The Collection Hierarchy

Collection

List Set Queue

Vector

Stack

LinkedList PriorityQueue

(Partial illustration)

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.

Program Stack

Program Stack

Resize 36

Program Stack

Resize 36

Resize 36

Recolor 4

Program Stack

Resize 36

Recolor 4

Resize 24

Undo:

Resize(36)

Program Stack

Resize 36

Recolor 4

Resize 24

Resize 36

Recolor 4

Undo:

Resize(36)

Undoable Stack Hierarchy

Undoable Stack Hierarchy

TextArea

-text: String

-size: int

-color: int

+ getters…

+ setters…

Undoable Stack Hierarchy

Operation

+ perform(arg: int)

+ getArg() : int

TextArea

-text: String

-size: int

-color: int

+ getters…

+ setters…

Undoable Stack Hierarchy

Operation

+ perform(arg: int)

+ getArg() : int

Recolor

- color: int

+ perform(arg: int)

+ getArg() : int

Resize

- size: int

+ perform(arg: int)

+ getArg() : int

TextArea

-text: String

-size: int

-color: int

+ getters…

+ setters…

Undoable Stack Hierarchy

Operation

+ perform(arg: int)

+ getArg() : int

Recolor

- color: int

+ perform(arg: int)

+ getArg() : int

Resize

- size: int

+ perform(arg: int)

+ getArg() : int

TextArea

-text: String

-size: int

-color: int

+ getters…

+ setters…

UndoStack

+ add (op :Operation)

+ undo()

0..n

The Collections classThe 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);

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.

Wrapper Classes

int

float

double

char

boolean

Integer

Float

Double

Character

Boolean

Example:Integer intObj = new Integer(10);int number = intObj.intValue();

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);