Random, Collections & Loops Chapter 5 Copyright © 2012 Pearson Education, Inc.

Random, Collections & Loops

Chapter 5

Copyright © 2012 Pearson Education, Inc.

Conditionals and Loops• So far, we’ve looked at:

– making decisions with if

– how to compare data

– Boolean expressions

• Today we’ll look at:– Generating lists of random numbers

– Storing lists (collections) of objects

– repeat processing steps in a loop


RANDOM


The Random Class

• The Random class is part of the java.util package

• It provides methods that generate pseudorandom numbers

• A Random object performs complicated calculations based on a seed value to produce a stream of seemingly random values



//********************************************************************// RandomNumbers.java Author: Lewis/Loftus//// Demonstrates the creation of pseudo-random numbers using the// Random class.//********************************************************************

import java.util.Random;

public class RandomNumbers{ //----------------------------------------------------------------- // Generates random numbers in various ranges. //----------------------------------------------------------------- public static void main (String[] args) { Random generator = new Random(); int num1; float num2;

num1 = generator.nextInt(); System.out.println ("A random integer: " + num1);

num1 = generator.nextInt(10); System.out.println ("From 0 to 9: " + num1);

continued


continued

num1 = generator.nextInt(10) + 1; System.out.println ("From 1 to 10: " + num1);

num1 = generator.nextInt(15) + 20; System.out.println ("From 20 to 34: " + num1);

num1 = generator.nextInt(20) - 10; System.out.println ("From -10 to 9: " + num1);

num2 = generator.nextFloat(); System.out.println ("A random float (between 0-1): " + num2);

num2 = generator.nextFloat() * 6; // 0.0 to 5.999999 num1 = (int)num2 + 1; System.out.println ("From 1 to 6: " + num1); }}

Sample RunA random integer: 672981683From 0 to 9: 0From 1 to 10: 3From 20 to 34: 30From -10 to 9: -4A random float (between 0-1): 0.18538326From 1 to 6: 3

Examples – code to outcome


Given a Random object named gen, what range of values are produced by the following expressions?

gen.nextInt(25)

gen.nextInt(6) + 1

gen.nextInt(100) + 10


gen.nextInt(10) – 5


Range?

0 to 24

1 to 6

10 to 109

100 to 149

-5 to 4

12 to 33

Examples – outcome to code


Write an expression that produces a random integer in the following ranges:

gen.nextInt(13)

gen.nextInt(20) + 1

gen.nextInt(6) + 15

gen.nextInt(11) – 10

Range

0 to 12

1 to 20

15 to 20

-10 to 0

COLLECTIONS & LOOPS


Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

Grouping (i.e., collecting) objects• Many applications involve collections of objects:

– Personal organizers.– Library catalogs.– Student-record system.

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


Example: 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.• Consider a Notebook project.


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>(); }

...}

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>(); }

...}

the type of collection

the type of objects in the collection


Object structures with collections


Adding a third note


Features of the collection• It increases its capacity as necessary.• It keeps a private count (size() method).• It keeps the objects in order.• Details of how all this is done are hidden.

– Does that matter? Does not knowing how prevent us from using it?


Using the collection

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

...}

public 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

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. }}

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. }}


Retrieving an object

Index validity checks

Retrieve and print the note


Removal may affect numberingnotes.remove(1);notes.remove(1);

For-each Loops• Simplifies repetitive processing of items in a collection

• For example, suppose bookList is an ArrayList<Book> object:

ArrayList<Book> bookList = new ArrayList<Book>();

• The following loop will print each book:

for (Book myBook : bookList) System.out.println (myBook);


Logic of a for-each loop

Statements to process each item

truefalse

Are there more items in the collection?

Quick Check


Write a for-each loop that prints all of the Student objects in an ArrayList<Student> object called roster.

for (Student student : roster)

System.out.println (student);

Other Collection Types• List (e.g., java.util.ArrayList)

– Ordered elements– Allows duplicates

• Set (e.g., java.util.HashSet)– Unordered elements– No duplicates

• Map (e.g. java.util.HashMap)– Unordered elements, stored as key-value pairs– No duplicate keys (can be duplicate values)


Using collections example


Homework• Finish creating the Song class to prep for next

week’s lab• Work on Project 1• CodingBat


Random, Collections & Loops Chapter 5 Copyright © 2012 Pearson Education, Inc.

Documents

Transcript of Random, Collections & Loops Chapter 5 Copyright © 2012 Pearson Education, Inc.