Notes

Outline

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.)