Basic Java structural components

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Chapter 3

• Basic Java structural components

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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

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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.

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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.

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Packages (cont.)
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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.

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Compilation unit (cont.)
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Identifiers

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

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Identifiers (cont.)

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

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Identifiers (cont.)

• Legal X Abc A_a_x b2 aVeryLongIdentifier
  b29 a2b _ IXLR8
• Illegal 2BRnot2B a.b Hello! A-a Aa
• All different identifiers total
  Total TOTAL tOtAl

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Identifiers used already
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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

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Choosing identifiers (cont.)

• Be as specific as possible.
• Take particular care to distinguish closely
  related entities.
• Effective Less-Effective
• newMasterRecord masterRecord1
• oldMasterRecord masterRecord2
• Dont incorporate the name of its syntactic
  category in its name.
• Less-EffectiveStudentClass

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Literals

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

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int

• Numbers -- both positive and negative.
• Commas, periods, and leading zeros are not
  allowed in ints.
• Legal
• 25 0 123456 -289765 7
• Illegal
• 123,456 25.0 014765

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Double

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

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Exponential Notation

• Can be used to represent doubles.
• 0.5e3
• 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.

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Character literals

• Single characters between apostrophes (single
  quotes).
• A a 2
• 3 characters not represented by themselves
• -gt \ (apostrophe)
• -gt \ (quotation mark)
• \ -gt \\ (backslash)

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Boolean

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

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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

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General lexical rules (cont.)

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

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General lexical practices

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

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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 /

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Weve covered

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

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Glossary
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Glossary (cont.)