© 2004 Pearson Addison-Wesley. All rights reserved3-1 Collaboration Policy Programming Tests are...

© 2004 Pearson Addison-Wesley. All rights reserved 3-1

Collaboration Policy• Programming Tests are individual work.• You may not work with others in the class.• You may get help from the TA’s and from the

instructor.• Any other help on any part of the assignment is

forbidden.• Be sure to protect your files, your password, your

log on, and your stray papers.• Do not submit an assignment to my physical

mailbox or to the pocket outside of my door.• Sliding a stapled PT under my door is acceptable.


Additional• If you get help from a TA, that help must be cited

in the acknowledgement section with the TA name and what they helped you with.

• If you do background research, the source of your information must be cited in the document. Background research on coding is not acceptable.

• For example, if you wanted to find a conversion factor for rods to miles directly, you could find that on the web, but then must state where you found that information.


Collaboration Policy• In the Resources section of Blackboard, you will

find a link to the collaboration policy for this course.

• Please read the collab_intro document which describes programming and programming tasks and the types of assistance that you might get.

• Then read the document for our class which specifies exactly what is permitted and not for programming assignments.

• Also, read the JMU honor code.• If you have not done so, read also the course

policies.


Finally• There is an assignment on Blackboard for you to

signify that you have read and understood these documents. When you have finished with the reading and have had any questions you have answered, write the declaration statement and submit it with the assignment. Note: you will not be attaching a document, but will be making the declaration in the Student Comment section of the assignment.

Chapter 3

Using Classes and Objects


OutlineCreating Objects

The String Class

Packages

Formatting Output

Wrapper Classes


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

DecimalFormat fmt;

• No object is created with this declaration

• An object reference variable holds the address of an object (hence the term reference…it refers to the object itself).

• The object itself must be created separately


Creating Objects• Generally, we use the new operator to create an object

fmt = new DecimalFormat(“0.###");

This calls the DecimalFormat constructor, which isa special method that sets up the object

• Creating an object is called instantiation

• An object is an instance of a particular class


Note• With Strings, you do not need to make the “new”

object in general, since by assigning any kind of String value, you are causing the String variable to reference the String value:Examples

String myName;myName = “John Smith”;

String message;message = Keyboard.readString();


Invoking Methods• We've seen that once an object has been

instantiated, we can use the dot operator to invoke its methods

String msgmsg = fmt.format(12.6548);

• 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


References• Note that a 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"


For now• Just be aware that reference typed variables

contain a pointer or address of where the object is really stored.

• Be aware that they will behave differently than primitive types.

• Variables of primitive types store data directly in the variable.

• Variables of reference types store data in some other location; the variable stores the address directly in the variable.

• There are some important ramifications of this property that we will explore later in the semester.



The String Class

Packages

Formatting Output

Wrapper Classes


The String Class• Because strings are so common, we don't have to

use the new operator to create a String object

String title;title = "Java Software Solutions";

• This is special syntax that works only for strings

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


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

• However, several methods of the String class return new String objects that are modified versions of the original

• See the list of String methods on page 119 and in Appendix M


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

• See StringMutation.java (page 120)



The String Class

Packages

Formatting Output

Wrapper Classes


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 per se, but we rely on them heavily

• Various classes we've already used (System , 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• When you want to use a class from a package, you

could use its fully qualified name

java.text.DecimalFormat

• Or you can import the class, and then use just the class name

import java.text.DecimalFormat;

• To import all classes in a particular package, you can use the * wildcard character

import java.text.*;


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

• The DecimalFormat class, on the other hand, is part of the java.text package, and therefore must be imported


Keyboard• Currently, you are downloading the Keyboard

class into your working directory.• Keyboard is a part of the cs1 package that was

provided by your authors.• This package is available for you to download onto

your computers, so that you can access it using the import statement and don’t have to have multiple copies of the Keyboard class.

• The instructions tell you where to store the cs1 package to make it available to your compiler and interpreter.


Lab machines• cs1 is not yet installed on the lab machines. • When it is, you will begin to use the import

statement to import the Keyboard class into your programs.

• Import will work on the Linux machines. They will also recognize the Keyboard methods without import.



The String Class

Packages

Formatting Output

Wrapper Classes


Formatting Output• It is often necessary to format values in certain

ways so that they can be presented properly

• The Java standard class library contains classes that provide formatting capabilities

• The NumberFormat class allows you to format values as currency or percentages

• The DecimalFormat class allows you to format values based on a pattern

• Both are part of the java.text package


Formatting Output• The DecimalFormat class can be used to format a

floating point value in various ways

• For example, you can specify that the number should be truncated to three decimal places

• The constructor of the DecimalFormat class takes a string that represents a pattern for the formatted number

• See CircleStats.java (page 134) - examples

© 2004 Pearson Addison-Wesley. All rights reserved3-1 Collaboration Policy Programming Tests are...

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