Lecture 3 Java Basics

1. Lecture 3Java Basics Richard Gesick

2. Java Basics • Java Application Structure • Data Types, Variables, and Constants • Expressions and Arithmetic Operators

3. Typical Program Structure • Input some data • Perform some processing on the data • Output the results • In each execution of the program • The data may be different • The instructions will be the same

4. Java Application Structure • All programs consist of at least one class. • Java source code file must have same name as class with a .java extension.

5. A Skeleton Application 1 /* An application shell 2 Anderson, Franceschi 3 */ 4 public class ShellApplication 5 { 6 public static void main( String [ ] args ) 7 { 8 // write your code here 9 } 10 } See Example 2.1SkeletonApplication.java

6. Identifiers - symbolic names • Identifiers are used to name classes, variables, and methods • Identifier Rules: • Must start with a "Java letter" • A - Z, a - z, _, $, and Unicode letters • Can contain essentially any number of Java letters and digits, but no spaces • Case sensitive!! • Number1 and number1 are different • Cannot be keywords or reserved words • See Appendix A

7. Examples Are these identifiers valid? taxRate Yes. char No. char is a keyword intNumber Yes, The keyword int is embedded, but the identifier is not identical to a keyword. 2008Taxrate No. The identifier starts with a digit

8. Program Building Blocks The Statement • Performs some action • Terminates with a semicolon (;) • Can span multiple lines Example: System.out.println( "Programming is " + "not a spectator sport" );

9. Building Blocks - The Block The Block • 0, 1, or more statements • Begins and ends with curly braces { } • Can be used anywhere a statement is allowed. Example: public static void main( String [] args ) { System.out.println( "Hello" ); }

10. Building Blocks - White Space • Space, tab, newline are white space characters • At least one white space character is required between a keyword and identifier • Any number of white space characters are permitted between identifiers, keywords, operators, and literals Examples: int a 1 + 2 public static void main( String [] args )

11. SOFTWARE ENGINEERING TIP To increase readability of your code, surround operators and operands with white space and skip lines between logical sections of the program

12. Building Blocks - Comments • Comments explain the program to yourself and others • Block comments • Can span several lines • Begin with /* • End with */ • Compiler ignores all text between /* and */ Example: /* This program will print a message Anderson, Franceschi */

13. SOFTWARE ENGINEERING TIP Include a block comment at the beginning of each source file. It should • identify the author of the program • briefly describe the function of the program

14. Building Blocks - Comments Line comments • Start with // • Compiler ignores all text from // to the end of the line Example System.out.println( "Hello" ); // output Hello

15. A Sample Program • This program calculates the area of a circle See Example 2.2 AreaOfCircle.java • Lines 1 – 3are a block comment identifying the function and authors of the program 1 /* Calculate the area of a circle 2 Anderson, Franceschi 3 */

16. The Outer Structure • Lines 5-8 and 24-25 are the same as the skeleton application, except that the name of the class is AreaOfCircle, so we save this file as AreaOfCircle.java • Lines 9 – 23 do the work of the program 5 public class AreaOfCircle 6 { 7 public static void main( String [] args ) 8 { … 24 } 25 }

17. Identify the Data • First, we identify the data we know: • final double PI = 3.14159; • We identify other data we will need: • double radius; • double area; • We give radius a value 17 radius = 3.5; • To calculate the area of a different circle, we change this value, recompile and re-run the program.

18. Perform the Calculation • The area of a circle is calculated using the formula: πr2 • We translate this formula into Java: 20 area = PI * radius * radius; • And output the result: 23 System.out.println( "The area is " + area );

19. Data Types, Variables, and Constants • Declaring Variables • Primitive Data Types • Initial Values and Literals • String Literals and Escape Sequences • Constants

20. Data Types • For all data, assign a name (identifier) and a data type • Data type tells the compiler: • How much memory to allocate • How to store the data • The types of operations you will perform on the data • Compiler monitors use of data • Java is a strongly typed language • Java has eight primitive data types byte, short, int, long, float, double, char, boolean

21. Declaring Variables • Variables hold one value at a time, but that value can change Syntax: dataType identifier; or dataType identifier1, identifier2, …; • Naming convention for variable names: • first letter is lowercase • embedded words begin with uppercase letter

22. SOFTWARE ENGINEERING TIP Names of variables should be meaningful and reflect the data they will store. • This makes the logic of the program clearer Don't skimp on characters, but avoid extremely long names. Avoid names similar to Java keywords.

23. Integer Types - Whole Numbers TypeSize Minimum Value Maximum Value in Bytes byte 1 -128 127 short 2 -32,768 32,767 int 4 -2, 147, 483, 648 2, 147, 483, 647 long 8 -9,223,372,036,854,775,808 9,223,372,036,854,775,807 Example declarations: int testGrade; int numPlayers, highScore, diceRoll; short xCoordinate, yCoordinate; byte ageInYears; long cityPopulation;

24. Floating-Point Data Types • Numbers with fractional parts TypeSize Minimum Positive Maximum Value in Bytes Nonzero Value float 4 1.4E-45 3.4028235E38 double 8 4.9E-324 1.7976931348623157E308 Example declarations: float salesTax; double interestRate; double paycheck, sumSalaries;

25. char Data Type • One Unicode character (16 bits - 2 bytes) TypeSize Minimum Value Maximum Value in Bytes char 2 character character encoded as 0 encoded as FFFF Example declarations: char finalGrade; char newline, tab, doubleQuotes;

26. boolean Data Type • Two values only: true false • Used for decision making or as "flag" variables Example declarations: boolean isEmpty; boolean passed, failed;