Grouping objects

Collections and iterators

Main concepts to be covered

Collections Loops Iterators

The requirement to group objects Many applications involve collections of

objects: Personal organizers. Library catalogs. Online shopping.

The number of items to be stored varies. Items added. Items deleted.

A personal notebook

Notes may be stored. Individual notes can be viewed. There is no limit to the number of notes. It will tell how many notes are stored.

Lecture 4. Grouping objects 5 Pablo Romero, Department of InformaticsLecture 4. Grouping objects 5 Pablo Romero, Department of Informatics

import java.util.ArrayList;

/** * ... */public class Notebook{ // Storage for an arbitrary number of notes. private ArrayList<String> notes;  /** * Perform any initialization required for the * notebook. */ public Notebook() { notes = new ArrayList<String>(); }

...}

Field declaration

Field initialisation

Object structures with collections

<String>

Adding a third note

<String>

Using the collectionpublic class Notebook{ private ArrayList<String> notes; ...  public void storeNote(String note) { notes.add(note); }  public int numberOfNotes() { return notes.size(); }

...}

Adding a new note

Returning the number of notes(delegation).

Index numbering

<String>

Index validity checks

Retrieving an object

Retrieve and print the note

public void showNote(int noteNumber){ if(noteNumber < 0) { // This is not a valid note number. } else { if(noteNumber < numberOfNotes()) { System.out.println(notes.get(noteNumber)); } else { // This is not a valid note number. } }}

Removal may affect numbering (deleting the second note)

<String>

Review

Collections allow an arbitrary number of objects to be stored.

Class libraries (packages) usually contain tried-and-tested collection classes.

We have used the ArrayList class from the java.util package.

To use hash maps we would need to use the HashMap class

Review

Items may be added and removed. Each item has an index. Index values may change if items are

removed (or further items added). The main ArrayList methods are add, get, remove and size.

Iteration We often want to perform some actions an

arbitrary number of times. E.g., print all the notes in the notebook. How many

are there? Most programming languages include loop

statements to make this possible. Three sorts of loop statement.

for-each loop while loop for loop

For-each loop pseudo code

for(ElementType element: collection) { loop body}

Loop variablefor keyword

Statements to be repeated

General form of a for-each loop

for each(note in the notes collection) { show the next note}

Pseudo-code example to print every note

Our collection

A Java example

/** * List all notes in the notebook. */public void listNotes(){ for(String note : notes) { System.out.println(note); }}

Loop variable

How many times is this executed?

While loop pseudo code

while(loop condition) { loop body}

Boolean testwhile keyword

Statements to be repeated

General form of a while loop

while(there is at least one more note to be printed) { show the next note}

Pseudo-code example to print every note

A Java example

/** * List all notes in the notebook. */public void listNotes(){ int index = 0; while(index < notes.size()) { System.out.println(notes.get(index)); index++; }}

Increment by one

Local index variable

Exercise

Write the method noteExists(String searchString) that searches the notebook for a specific string and returns true if any of the strings in the notebook contains the parameter and false otherwise. You might need to use a variable of type boolean (which can take the literal values true or false) and the contains method from the String class which takes a parameter of type string and returns true if the parameter is included in the string and false otherwise.

For loop pseudo-code

for(initialization; condition; post-body action) { statements to be repeated}

General form of a for loop

Equivalent in while-loop form

initialization;while(condition) { statements to be repeated post-body action}

Lecture 4. Grouping objects 21 Pablo Romero, Department of InformaticsLecture 4. Grouping objects 21 Pablo Romero, Department of Informatics

for(int index = 0; index < notes.size(); index++) { System.out.println(notes.get(index));}

for loop version

while loop version

public void listNotes(){ int index = 0; while(index < notes.size()) { System.out.println(notes.get(index)); index++; }}

Three types of loops

for-each if you need to iterate over all elements of a collection

while or for if the iteration is not related to a collection for when the number of iterations is known while when number of iterations is

determined on the fly

Iterators

while loop made simple Special classes to iterate over collections Have methods to

Check whether there are more elements Obtain the next element

Iterating over a collection

Iterator<ElementType> it = myCollection.iterator();while(it.hasNext()) { call it.next() to get the next object do something with that object}

java.util.Iterator

Returns an Iterator object

public void listNotes(){

}

Iterator<String> it = notes.iterator(); while(it.hasNext()) { System.out.println(it.next()); }

Review Loop statements allow a block of statements to

be repeated. Three types of loops

for-each while for

Collection classes have special Iterator objects that simplify iteration over the whole collection.