ECE122 L4: Creating Objects February 8, 2007 ECE 122 Engineering Problem Solving with Java Lecture 4 Creating and Using Objects

ECE122 L4: Creating Objects February 8, 2007

ECE 122

Engineering Problem Solving with Java

Lecture 4

Creating and Using Objects


Outline

° Problem: How do I create multiple objects from a class• Java provides a number of ‘built-in’ classes for us

° Understanding referencing• Requires understanding of how computer memory works

° Garbage collection• Java automatically “cleans up unused items”

° Generating random numbers in Java programs• Why is this important?


Objects in the computer

° Abstractions for the parts of a problem that matter

° Objects contain data items

• Data description: data value

• My car has a model year, an integer, with value 1999

• Data values are stored in variables

• Variables storing the data values for a particular object are the object’s instance variables

• The values of the object’s instance variables form the object’s state


Classes

° Class: a template for creating objects• Bank account class

• String class

• Student class

° Every Java object is an instance of some class

° The class of an object determines its data description and functionality


English to Java dictionary

° Piece of data: value

° Kind of data: type

° Place to store a value: variable

° Structured group of variables: object

° Kind of object: class

° Object of a particular class: class instance


Creating Objects

° A variable holds either a primitive type or a reference to an object

° A class name can be used as a type to declare an object reference variable

String title;

° No object is created with this declaration

° An object reference variable holds the address of an object

° The object itself must be created separately


Creating Objects

° Generally, we use the new operator to create an object

title = new String ("Java Software Solutions");

° Creating an object is called instantiation

° An object is an instance of a particular class


Invoking Methods

° We've seen that once an object has been instantiated, we can use the dot operator to invoke its methods

count = title.length()

° A method may return a value, which can be used in an assignment or expression

° A method invocation can be thought of as asking an object to perform a service


Reference Variables

204720482049205020512052205320542055

HH

ee

llll

oo

..

° What happens when we create a string?

String testStr = new String ("Hello.");

1. Locations are allocated in memory for the string2. 8 bit characters fill up the location in the memory3. Address of first character saved in another

location called ‘testStr”

So, we can say that “testStr” is a pointer to a seriesof characters in memory


Reference Variables

204720482049205020512052205320542055

HH

ee

llll

oo

..

154415551556155715581559156015611562

testStr


References

° Primitive variable contains the value itself, but an object variable contains the address of the object

° An object reference can be thought of as a pointer to the location of the object

° Rather than dealing with arbitrary addresses, we often depict a reference graphically

"Steve Jobs"name1

num1 38


Assignment Revisited

° The act of assignment takes a copy of a value and stores it in a variable

° For primitive types:

num1 38

num2 96Before:

num2 = num1;

num1 38

num2 38After:


Reference Assignment

° For object references, assignment copies the address:

name2 = name1;

name1

name2Before:

"Steve Jobs"

"Steve Wozniak"

name1

name2After:

"Steve Jobs"


Aliases

° Two or more references that refer to the same object are called aliases of each other

° One object can be accessed using multiple reference variables

° Aliases can be useful, but should be managed carefully

° Changing an object through one reference changes it for all of its aliases

• There is really only one object


Classes° A class can contain data declarations and method

declarations

int acctNumber;float acctBalance;

Data declarations

Method declarations


Bank Account Example

acct1 72354acctNumber

102.56balance

acct2 69713acctNumber

40.00balance


Garbage Collection

° If object no longer has any valid references to it

• It can no longer be accessed by the program

° The object is useless, and therefore is called garbage

° Java performs automatic garbage collection periodically

• Returns an object's memory to the system for future use

° In other languages, the programmer is responsible for performing garbage collection


The String Class

° Because strings are so common, we don't have to use the new operator to create a String object

title = "Java Software Solutions";

° This is special syntax that works only for strings

° Each string literal (enclosed in double quotes) represents a String object

title = new String(“Java Software Solutions”);

alternative


String Methods

° Once a String object has been created, neither its value nor its length can be changed

° Thus we say that an object of the String class is immutable

° Several methods of the String class return new String objects

• They are modified versions of the original


String Class


String Indexes

° It is occasionally helpful to refer to a particular character within a string

° This can be done by specifying the character's numeric index

° The indexes begin at zero in each string

° In the string "Hello", the character 'H' is at index 0 and the 'o' is at index 4


Class Libraries

° A class library is a collection of classes that we can use when developing programs

° The Java standard class library is part of any Java development environment

° Its classes are not part of the Java language, but we rely on them heavily

° Various classes we've already used (System , Scanner, String) are part of the Java standard class library

° Other class libraries can be obtained through third party vendors, or you can create them yourself


Packages° The classes of the Java standard class library are

organized into packages

° Some of the packages in the standard class library are:

Package

java.langjava.appletjava.awtjavax.swingjava.netjava.utiljavax.xml.parsers

Purpose

General supportCreating applets for the webGraphics and graphical user interfacesAdditional graphics capabilitiesNetwork communicationUtilitiesXML document processing


The import Declaration

° All classes of the java.lang package are imported automatically into all programs

° It's as if all programs contain the following line:

import java.lang.*;

° That's why we didn't have to import the System or String classes explicitly in earlier programs


Random Numbers

• Streams of numbers that have been produced by a process that is believed to be random• Truly random numbers.

• One example might be the spinning of a roulette wheel.

• Most truly random activities involve some sort of physical process which is difficult to replicate

• Unfortunately, computers are not very effective at generating truly random numbers

Use pseudorandom number generator


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


java.util.Random ° The java.util.Random class — a more controlled way to

generate random numbers.

° If we set the same “seed,” we get the same “random” sequence.

Random generator = new Random();

Random generator2 = new Random(seed); long seed;


java.util.Random

° Methods:

int k = generator.nextInt ();

int k = generator.nextInt (n);

double x = generator.nextDouble ();

All 232 possible int values are produced with (approximately) equal probability

0 k < n

0 x < 1


Summary

° Very important to understand the difference between objects and classes

° The “new” keyword creates (instantiates) objects in memory.

• A reference (pointer) is used to indicate the location of the object

• Note that there may be multiple references to the same object

° Strings are “immutable”. They cannot be lengthened once they are “instantiated”

° Random number are useful. Java provides a class which generates pseudorandom numbers

Documents

ECE122 L4: Creating Objects February 8, 2007 ECE 122 Engineering Problem Solving with Java Lecture 4 Creating and Using Objects