Identifiers, Variables and the JDK

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Identifiers, Variables and the JDK

• Introduction to Software Development
• Week 1 Lecture 2

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Objectives

• In this lecture, we will
• Introduce identifiers and variables
• Compare assignment and initialisation
• Use variables in a Java application
• Discuss downloading the Java Development Kit

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Identifiers

• These are the human readable words or characters
  used in programs
• Every language has a set of identifiers which
  represent some predefined aspect of the language
• These are called keywords
• For example the following are keywords in Java
• class
• public
• static
• void
• In Java identifiers are case sensitive

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Identifiers

• Most languages have rules and recommendations
  about what is or is not valid identifier form
• Rules as to which characters are allowed at the
  start of an identifier
• Limitations on the length of the identifier
• How you should represent different entities such
  as variable types
• Spaces are not permitted in identifier names in
  most programming languages
• User specified identifiers should be meaningful

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Identifiers in Java

• In Java user specified identifiers
• May be any length
• Must start with a letter, underscore (_), or a
  sign
• The remaining characters that make up the
  identifier may be most characters other than
  those used as operators such as - / lt gt
• Cannot contain spaces or tabs
• Cannot be keywords
• The following conventions are used in naming
  identifiers in Java
• The first letter should be lowercase
• Multiple words are concurrent, not separated with
  underscores
• Each word after the first should be capitalised

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Variables and Constants

• Variables and constants are used to represent
  information and data in programs
• A variable is a value in a program which may
  change during the lifetime of the program
• A constant is a value in a program which does not
  change during the lifetime of the program or
  during the execution of a particular algorithm

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Data types

• Most programming languages allow the type of data
  being processed to be specified
• Four main data type categories are
• Numeric
• Integer and Real
• Alphanumeric
• Characters and Strings
• Logical
• Boolean that is True or False
• Abstract
• Stack
• Every data type has a set of symbols which can be
  applied to variables or constants of that type

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Variables in Java

• A variable in Java can be declared as follows
• int num1
• The declaration involves two parts a data type
  and an identifier
• In this example the data type is int
• The variable is associated with enough memory to
  hold a value of type integer
• Java supports several data types these will be
  discussed in detail next week
• The identifier is num1
• In Java variables can be declared almost anywhere
• They must be declared before they are used

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Using Variables

• // Addition Java Application
• public class Add
• public static void main (String args)
• System.out.println ("This program adds three
  numbers")
• int num1, num2, num3
• // input has not been covered yet so the values
  of the
• // numbers are set directly in the program
• num1 10
• num2 20
• num3 30

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Using Variables (continued)

• // calculate the sum of the three numbers
• int sum
• sum num1 num2 num3
• // output the results
• System.out.println ("The sum is " sum)

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Assignment and Initialisation

• The operation of storing some data in a variable
  is referred to as assignment
• int num1 num1 10
• The variable num1 is assigned the value 10
• What value is in the variable num1 initially?
• It is good practice to assign an initial value to
  a variable
• The initial value can be given to the variable as
  it is declared using initialisation
• int num1 10
• The symbol is used for two distinct operations
• Assignment and Initialisation

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Operators

• The symbol seen earlier is an example of an
  operator
• It is a binary operator
• The operation is carried out on two operands
• In the case num1 num2 the two operands are
  num1 and num2
• Java has many operators including - /
• The minus operator can be used as a binary
  operator or a unary operator
• num1 num2 a binary use
• - num2 a unary use
• Operators will be discussed in detail next week

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Working with Real Numbers

• Another data type provided by Java is double
• A variable of type double can hold a real number
• double dNum
• dNum 17.5
• An integer can be assigned to a double variable
• dNum 10
• int num1 7
• dNum num1
• The integer is promoted to a double
• A double CANNOT be assigned to an integer
  variable without loss of accuracy and so fails to
  compile

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Code Fragment

• int num1 5, num2 7, num3 20
• double avg
• avg (num1 num2 num3) / 3.0
• Note the use of 3.0 rather than 3
• What is the difference?
• The literal 3.0 is a real number but the literal
  3 is an integer
• If the two operands of the division operator /
  are integers then integer division takes place
• If the two operands are doubles then real
  arithmetic is used

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Java Development Kit

• All the software required to develop Java
  applications is available from Sun
• Visit the Sun Java website
• http//java.sun.com
• http//java.sun.com/j2se/1.5.0/download.jsp
• In this module the Java Development Kit is used
• Java 1.5
• Versions are available for Windows and Linux

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NetBeans

• NetBeans is an integrated Java development
  environment
• The latest version can be downloaded from Sun
  along with JDK 1.5

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Installing

• Installation is straightforward
• Once the download is complete execute the
  installation file and follow the instructions
• The default location for the files created during
  installation will be a folder called Java
• The folder should be in the program files
  directory
• In order to make using the JDK as simple as
  possible it is useful to set a couple of
  environment variables

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Environment Variables

• First launch Settings Control Panel System
• Then select Environment Variables on the Advanced
  tab

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Environment Variables

• The following screen is displayed
• Three System Variables can be added or edited if
  they already exist
• PATH
• JAVA_HOME
• CLASSPATH

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Environment Variables

• PATH
• Add an entry to the Path identifying where to
  find the bin folder of JDK1.5
• JAVA_HOME
• Set this to the JDK1.5 folder
• CLASSPATH
• Add . and the JDK1.5 bin folder

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Editors

• Notepad is a simple editor to use but it provides
  no support for Java
• Many other editors are available that make
  entering Java applications easier providing
  facilities such as
• Highlighting keywords
• Automatic indenting
• There are numerous websites where free editors
  can be found and downloaded
• For example
• http//java.about.com

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Summary

• In this lecture we have
• Introduced identifiers and variables
• Compared assignment and initialisation
• Used variables in a Java application
• Discussed downloading the Java Development Kit