Chapter 3

	Basic Java structural components

This chapter discusses

	Some Java fundamentals.
	The high-level structure of a system written in Java.
		packages
		compilation units
	Some fundamental “tokens” that make up a Java program.
		identifiers
		literals

Creating a Software System

	Define the classes to which objects belong.
	A class definition determines the features and behavior of the objects that are instances of the class.
	A program source is a collection of class definitions.

Packages

	A system definition is composed of a number of modules called packages.
	A package is a collection of one or more closely related classes.
	public class: a class that is accessible throughout the entire system.

Packages (cont.)

Compilation unit

	A source file containing the definition of one or more classes of a package.
	It can contain the definition of at most one public class.

Compilation unit (cont.)

Identifiers

	Sequences of characters that can be used to name things in a Java program.
		packages
		classes
		objects
		features

Identifiers (cont.)

	A sequence of letters, digits, ‘$’s, and/or ‘_’s.
	Cannot begin with a digit.
	Case sensitive (‘A’ and ‘a’ are considered different!!).

Identifiers (cont.)

	Legal: X   Abc   A_a_x   b$2 aVeryLongIdentifier   b29  a2b $_   $$$   IXLR8
	Illegal: 2BRnot2B   a.b   Hello! A-a  A+a
	All different identifiers:            total   Total   TOTAL   tOtAl

Identifiers used already

Choosing identifiers

	Choose descriptive names.
	Student or Textbook
	not       S or Thing
	Avoid overly long identifiers.
	HoldsTheNumberOfIterationsOfLoop
	Avoid abbreviations; if you abbreviate, be consistent.
	Inconsistent: clientRec and studentRecord

Choosing identifiers (cont.)

	Be as specific as possible.
	Take particular care to distinguish closely related entities.
	Effective        Less-Effective
	newMasterRecord    masterRecord1
	oldMasterRecord     masterRecord2
	Don’t incorporate the name of its syntactic category in its name.
	Less-Effective:StudentClass

Literals

	Sequences of characters that denote particular values.
	We write literals in our programs to denote specific values.

int

	Numbers -- both positive and negative.
	Commas, periods, and leading zeros are not allowed in ‘int’s.
	Legal:
	25 0 123456 -289765 7
	Illegal:
	123,456 25.0 014765

Double

	Numbers including decimal points.
	0.5
	-2.67
	0.00123
	2.
	.6
	Digits before and after the decimal point are preferred.

Exponential Notation

	Can be used to represent doubles.
	0.5e+3
	0.5e-3
	-0.5E3
	5e4
	2.0E-27
	The “e” can be upper or lower case.
	The mantissa need not contain a decimal point.

Character literals

	Single characters between apostrophes (single quotes).
	‘A’ ‘a’ ‘2’ ‘;’ ‘ ’
	3 characters not represented by themselves:
	‘ -> ‘\’’ (apostrophe)
	“ -> ‘\”’ (quotation mark)
	\ -> ‘\\’ (backslash)

Boolean

	Only 2 possible literals:
		true (Not TRUE or True)
		false

General lexical rules

	Files are made up of tokens -- identifiers, keywords, literals, and punctuation marks.
	Spaces and line ends are somewhat arbitrary.
	Spaces are required between words:
		Wrong: publicclass Student

General lexical rules (cont.)

	Spaces are not required, but are permitted, around punctuation.
	All correct examples:
	public class Student{
	public class Student {
	a+b
	a + b
	Extra spaces and line endings are allowed.
	public class Student {

General lexical practices

	Be consistent in spacing and line endings to make your programs as readable as possible.

Comments

	Explanatory remarks are included in a program for the benefit of a human reader, and are ignored by the compiler.
	Use // to treat the rest of the line as a comment.
	Use /* and */ to begin and end a section of comments.
	/* This is a comment */

We’ve covered

	Fundamental structure of a Java program.
		Packages
		Class definitions and compilation units
		Instances of classes
	Lexical structure
		Identifiers
		Literals
	Comments

Glossary

Glossary (cont.)