Identifiers - Naming

1. ***** SWTJC STEM ***** Identifiers - Naming • Identifier, the name of a Java programming component. • Identifier naming rules . . . • Must start with letter, underscore (_) or dollar sign ($) • Can be of any length • Cannot contain “+” or “-” • Cannot be a keyword used by Java (see Appendix A of text) • Should have meaning (“radius” preferred to “r”) • Identifier naming guidelines must be followed . . . • Package Names - A collection of classes • First word lower case • Additional words lower case, separated by “.” • Example geometry.areas Chapter 2-2 cg 21-22

2. ***** SWTJC STEM ***** Identifiers - Naming cont. • Identifier naming guidelines cont. • Class Names - A collection of data and methods • First word upper case • Additional words upper case • Example ShellSort • Method Names - A collection of statements • Are verbs that represent actions • First word lower case • Additional words upper case • Example linearSearch Chapter 2-2 cg 22

3. ***** SWTJC STEM ***** Identifiers - Naming cont. • Identifier naming guidelines cont. • Variables - Used to store data • Are nouns that represent items or things • First word lower case • Additional words upper case, • Example circleArea • Constants - Used to store data that does not change • All characters upper case • Additional words separated by underscore _ • Example PI, FLOW_FACTOR Chapter 2-2 cg 22

4. ***** SWTJC STEM ***** Programs with Simple Algorithms • Our first programs will implement simple algorithms. • Example: Find the area of a circle. • Given radius = 2 and PI = 3.14156 • Find circleArea = PI * radius * radius • We will need to learn how Java handles . . . • Inputting data, variables, and constants • Calculations with expressions • Outputting to PC Monitor - System.out object Chapter 2-2 cg 23

5. Data and Variables Input ***** SWTJC STEM ***** How to Input Something? Calculate Area of Circle A = PI * r2 Radius r Circle Area Chapter 2-2 cg 23

6. ***** SWTJC STEM ***** Java handles data as primitive data types. Handling Data • Java provides for three primitive data types . . . • Numeric data - Numbers • Character data - Text • Boolean data - Logical states, true or false • Data typing provides increased . . . • Efficiency of memory use • Ease of programming • Speed of program execution Chapter 2-2 cg 23

7. Keyword Description Size/Format (integers) byte Byte-length integer 8-bit [-128 to 127] short Short integer 16-bit [-32768 to 32767] int Integer 32-bit [approx.  2*109] long Long integer 64-bit [approx.  9*1018] (real numbers) float Single-precision floating point 32-bit [ approx. 3.4*1038] double Double-precision floating point 64-bit [ approx. 1.7*10308] (other types) char A single character 16-bit Unicode character boolean A Boolean value (true or false) true or false ***** SWTJC STEM ***** Primitive Data Types Chapter 2-2 cg 23

8. ***** SWTJC STEM ***** Data Types Literals • A literal is a primitive data type appearing directly in a program. • 123.4, ‘b’,“Hello World”,false are literals. • The type of a literal is as follows: • The type of an integer literal that ends with L or l is long; the type of any other integer literal is int; e.g. 10. • The type of a floating-point literal that ends with F or f is float; the type of any other floating-point literal is double. (Floating-point literals contain a decimal point; e.g., 2.0) • The type of a character literal is char; e.g., ‘p’. • The type of a Boolean literal is Boolean; e.g., true. Chapter 2-2 cg 24

9. Literal Data Type 178 int 8864L long 37.266 double 37.266f float 37.266F float 26.77e3 double ' c ' char true boolean false boolean ***** SWTJC STEM ***** Examples: Data Types Literals Examples Chapter 2-2 cg 24

10. ***** SWTJC STEM ***** Unicode Characters • Character data holds a single character value like ‘H’ or ‘5’. • Character data is stored as unicode, a 16 bit encoding scheme. • Includes ASCII character set plus many other alphabets and symbols - See appendix B of text. • “\u0041” represents the letter “A” and is stored in memory as two hexadecimal bytes, “00” & “41”. • Examples: • \u0041 is equivalent to the character ‘A’. • \u0042 is equivalent to the character ‘B’. • \u0041 + 1 is equivalent to the character ‘B’. Chapter 2-2 cg 24

11. Description Character Sequence Unicode Backspace \b \u0008 Tab \t \u0009 New line \n \u000A Carriage return \r \u000D Backslash \\ \u005C Single quote \’ \u0027 Double quote \” \u0022 ***** SWTJC STEM ***** Escape sequence is available for convenience. Escapes Sequences • Example: • ‘\’’ is the literal for a single quote. You cannot use ‘’’ ! • ‘\u0022’ will also work. Chapter 2-2 cg 24

12. ***** SWTJC STEM ***** Boolean Data and Literals • Boolean data holds only one of two values, true or false. • Boolean representations are useful to compare two values. • Is iValue less than jValue? • The answer is either trueor false. • And, is representable as Boolean. • Boolean is also used to represent logical states. • Is the intrusion detector switch open? • It is either, true - open orfalse - closed. Chapter 2-2 cg 24

13. ***** SWTJC STEM ***** Handling Variables • Variables are used to store data. • Java provides for three basic variable types . . . • Numeric variable - Holds a numeric value. • Character variable - Holds a single character value. • Boolean variable - Holds a logical value, true or false. • Creating a Java variable involves two steps . . . • Declaringthe name and data type of the variable • Assigning the variable a value • Note: Variables must be declared before used. Chapter 2-2 cg 25

14. ***** SWTJC STEM ***** Creating a Variable - Declaring • Step 1 - To declare a variable . . . • Use the declaration statement . . .datatype varName1, varName2, . . ., varNamen; • The declaration statement consists of two parts . . . • The datatype (byte, int, float, char, etc.). • Name(s) for variable(s) separated by commas. • Meaningful noun • First word lower case • Additional words upper case, • Note that all Java statements end with a semicolon ;. Chapter 2-2 cg 25

15. ***** SWTJC STEM ***** Declaring Variables - Examples • Examples of declaring numeric variables . . . • int count;where data type is integer and name is count. • float bulletSpeed, time, distance;where data type is floating point and multiple variables names arebulletSpeed, time, and distance. • char startingLetter;where data type is character and name is startingLetter • boolean doorAjar, trunkOpen; where data type is Boolean and names are doorAjar and trunkOpen. Chapter 2-2 cg 25

16. ***** SWTJC STEM ***** Creating a Variable - Assigning • Step 2 - To assign a value to a variable, • Use the assignment statement. • variable = expression; • The assignment statement consists of three parts . . . • The variable whose value is to be assigned. • The assignment symbol, =. • An expression to be evaluated or a literal value to be assigned to the variable. Chapter 2-2 cg 26

17. ***** SWTJC STEM ***** Creating a Variable - Examples • Examples of assigning values to variables . . . • time = 4.0;assigns the value 4.0 to numeric variabletime. • distance = time * 60.0 + 15.0;evaluates the expression to 255.0 and assigns the value to numeric variable distance. • firstLetter = ‘F’;assigns the character F to the character variable firstLetter. • gateOpen = true;assigns the logical condition true to the Boolean variable gateOpen. Chapter 2-2 cg 26

18. ***** SWTJC STEM ***** Note About the “=“ Sign • Note: The = sign means assign a value, not equals. • Variable (on the left) is replaced by evaluated result of the expression (on the right). • Consider the code below.count = 4;count = count + 1; After executing, count is 5. • It may look like an equation, but it is not! Chapter 2-2 cg 26

19. ***** SWTJC STEM ***** • To assign a value to a several variables, use . . . • variable1 = variable2 = . . . = variablen = expression; • Example: iCount = jCount = kCount = 4;is equivalent to . . .iCount = 4;jCount = 4;kCount = 4; Multiple Assignments Chapter 2-2 cg 26

20. ***** SWTJC STEM ***** Combining Declare and Assign • To declare and assign a value to a variableat the same time, combine the declaration and assignments statements. • datatype variable = expression; • variable is . . . • Declared as datatype • Assigned the value of expression • Examples: • int count = 4;integercountis assigned value of 4. • double rate = 0.15, maxRate = 2.0 * rate;rate and maxRate are declared as double and assigned values 0.15 and 0.30 (calculated) respectively. Chapter 2-2 cg 26

21. ***** SWTJC STEM ***** Declaring a Constant • To declare and assign a value to a constant, use . . .final datatype CONSTANT_NAME = VALUE; • final means exactly that. Once the constant is assigned a value, it cannot change! • Examples: • final double PI = 3.14159;assigns double constantPIthe value 3.14159. • final int I_COUNT_MAX = 1000;assigns constantI_COUNT_MAXthe value 1000. • Remember that constant names are all caps with multiple names separated by underscores! Chapter 2-2 cg 27

22. Calculate Assignment Statement ***** SWTJC STEM ***** How to Calculate Something? Calculate Area of Circle A = PI * r2 Data and Variables Radius r Circle Area Chapter 2-2 cg 22-27

23. Expressions Output System.out Object ***** SWTJC STEM ***** How to Output Something? Calculate Area of Circle A = PI * r2 Data & Variables Radius r Circle Area Chapter 2-2 cg 22-27

24. ***** SWTJC STEM ***** print and println Methods • To output something with Java: • Invoke the out object of the System class (program). • Use println method to print and move to the next line • System.out.println(“Hello World”); prints HelloWorld. • By default out outputs to the PC monitor. • The println method is the action (or service) the object out performs for us. • Method print is similar to println but without next line. • The actual Java statement looks like this . . . • System.out.println(parameter); • Parameteris the piece of data we want to output. • Parameter can be a variable or literal. Chapter 2-2 cg 28

25. ***** SWTJC STEM ***** print and println Examples • Examples: • System.out.println(“Hello World”); Outputs the string literal HelloWorld to the PC monitor. • Assume iCount is 10,System.out.println(“iCount = ” + iCount);Outputs the string literal iCount = 10 to the PC monitor.Note: The plus + concatenates(ties together) the character literal and the numeric variable value. • System.out.print(“Hello ”);System.out.println(“World”);Outputs identically to example 1. Chapter 2-2 cg 28

26. Expressions System.out Object ***** SWTJC STEM ***** Putting It All Together Calculate Area of Circle A = PI * r2 Data & Variables Radius r Circle Area Chapter 2-2 cg 21-28

27. ***** SWTJC STEM ***** public class CircleArea { public static void main(String[] args) { // Declare variables double radius, circleArea; final double PI = 3.14159; // Input data radius = 2.0; // Process data circleArea = PI * radius * radius; Example Procedural Program Chapter 2-2 cg 21-28

28. ***** SWTJC STEM ***** Example Procedural Program // Output result System.out.println("Let’s begin."); System.out.print("A circle with radius = " + radius); System.out.println(" has an area = " + circleArea + "."); } } Chapter 2-2 cg 21-28