![Where to declare variables
Where to declare variables
Where to declare variables
Where to declare variables
• Declare a variable
• Just before it is used or
• At the beginning of the section of your program that is
enclosed in {}.
public static void main(String[] args)
{ /* declare variables here */
. . .
}
28](https://image.slidesharecdn.com/csc111-chap02-221101052111-28965a72/85/CSC111-Chap_02-pdf-28-320.jpg)
CSC111-Chap_02.pdf
- 1. Computer Programming I - CSC111 Chapter 2 – Basic Computation Dr. Mejdl Safran [email protected] 1
- 2. Outline Outline Outline Outline • Java program structure • Hello program • Saving, compiling and running Java programs • Comments • What is a variable? • What is a data type? • Basic data types • Variables identifiers • How to select proper types for numerical data • Assignment statement • Simple keyboard input/output 2
- 3. Java Program Structure Java Program Structure Java Program Structure Java Program Structure 3
- 4. Java Program Structure Java Program Structure Java Program Structure Java Program Structure 4
- 5. Hello Program Hello Program Hello Program Hello Program 5
- 6. Saving, compiling, and running Java program Saving, compiling, and running Java program Saving, compiling, and running Java program Saving, compiling, and running Java program • Saving a Java program ‒ A file having a name same as the class name should be used to save the program. The extension of this file is “.java” •Compiling a Java program ‒ Call the Java compiler javac: ‒ javac HelloProgram.java ‒ generates a file called “HelloProgram.class” (the bytecode) •Running a Java program ‒ Call JVM java: ‒ java HelloProgram 6
- 7. Comments Comments Comments Comments • The best programs are self-documenting • Clean style • Well-chosen names •Comments are written into a program as needed to explain the program •They are useful for programmer, but they are ignored by the compiler 7
- 8. Comments Comments Comments Comments • A comment can begin with // • Everything after these symbols and to the end of the line is treated as a comment 8
- 9. Comments Comments Comments Comments • A comment can begin with /* and end with */ • Everything between these symbols is treated as a comment 9
- 10. Hello Program Hello Program Hello Program Hello Program 10
- 11. Comments Comments Comments Comments • A javadoc comment, begins with /** and end with */ • It can be extracted automatically from Java software 11
- 12. When to use comments When to use comments When to use comments When to use comments • Begin each program file with an explanatory comment ‒What the program does ‒The name of the author ‒Contact information for the author ‒Date of the last modification • Provide only those comments which the expected reader of the program file will need in order to understand it. 12
- 13. Sum program Sum program Sum program Sum program 13
- 14. Variables Variables Variables Variables •Variables are used to store (represent) data such as numbers and letters. • Think of them as places to store data. • They are implemented as memory locations. •The data stored by a variable is called its value. • The value is stored in the memory location. •Its value can be changed. 14
- 15. Variables and values Variables and values Variables and values Variables and values • Variables: numberOfStudents studentGPA age • Assigning values: numberOfStudents = 100 ; numberOfStudents = numberOfStudents - 1 ; 15
- 16. Naming and declaring variables Naming and declaring variables Naming and declaring variables Naming and declaring variables • Choose names that are helpful such as count or speed, but not c or s. • When you declare a variable, you provide its name and type • without initial value int count, sum; double avg; • with initial value int count = 5, sum = 200; double avg = sum / count; • A variable's type determines what kinds of values it can hold (int, double, char, etc.). • A variable must be declared before it is used. 16
- 17. Syntax and examples Syntax and examples Syntax and examples Syntax and examples • Syntax type variable_1, variable_2, …; •Examples int styleChoice, numberOfChecks; double balance, interestRate; char jointOrIndividual; 17
- 18. Data types in Java Data types in Java Data types in Java Data types in Java • In Java, you have to specify a data type of variables that tells what kind of data to be stored in it. • Two kinds of data types: 1) Primitive data types: for single number or single letter, e.g. int 2) Class types: for objects of a class, e.g. class type String 18
- 19. Primitive types Primitive types Primitive types Primitive types 19 • Four integer types (byte, short, int, and long) • int is most common • Two floating-point types (float and double) • double is more common • One character type (char) • One Boolean type (Boolean)
- 20. Primitive data types Primitive data types Primitive data types Primitive data types 20
- 21. Primitive data types Primitive data types Primitive data types Primitive data types 21 Primitive data types never allow us to store multiple values of same type! (one value at a time) int a; // valid a = 10; // valid a = 10, 20, 30; // invalid
- 22. Examples of primitive values Examples of primitive values Examples of primitive values Examples of primitive values 22 • Integer types 0 -1 365 12000 • Floating-point types 0.99 -22.8 3.14159 5.0 • Character type 'a' 'A' '#' ' ' • Boolean type true false
- 23. Java identifiers Java identifiers Java identifiers Java identifiers • The name of something in a java program such as a variable, class, or method, is called an identifier. • Identifiers may contain only • Letters • Digits (0 through 9) • The underscore character (_) • And the dollar sign symbol ($) which has a special meaning (avoid it) • The first character cannot be a digit. 23
- 24. Java identifiers Java identifiers Java identifiers Java identifiers • Identifiers may not contain any spaces, dots (.), asterisks (*), or other characters: 7-11 netscape.com util.* (not allowed) • Identifiers can be arbitrarily long. • Since Java is case sensitive, stuff, Stuff, and STUFF are different identifiers. 24
- 25. Keyword or reserved words Keyword or reserved words Keyword or reserved words Keyword or reserved words • Words such as if are called keywords or reserved words and have special, predefined meanings. • Cannot be used as identifiers. 25
- 26. Constant declaration Constant declaration Constant declaration Constant declaration • A constant is a variable whose value cannot change once it has been assigned. • Syntax final type constIdentifer = literal | expression; • Examples final double PI = 3.14159; final int MONTH_IN_YEAR = 12; final int MAX = 1024; final int MIN = 128; final double AVG = (MAX + MIN)/2; 26
- 27. Naming conventions Naming conventions Naming conventions Naming conventions • Class types begin with an uppercase letter (e.g. String). • Primitive types begin with a lowercase letter (e.g. int). • Variables: • All lowercase letters, capitalizing the first letter of each word in a multiword identifier, expect for the first word (e.g. myName, myBalance). •Constants: • All uppercase letters, separating words within multiword identifier with the underscore symbol (e.g., IP, MONTH_IN_YEAR). 27
- 28. Where to declare variables Where to declare variables Where to declare variables Where to declare variables • Declare a variable • Just before it is used or • At the beginning of the section of your program that is enclosed in {}. public static void main(String[] args) { /* declare variables here */ . . . } 28
- 29. Select Select Select Select proper types for numerical data proper types for numerical data proper types for numerical data proper types for numerical data • Define the following variables for a phone application: ‒ Number of users ‒ Number of photos ‒ Number of comments ‒ Average number of active users in a week ‒ Is the app available? ‒ Version ‒ User’s gender ‒ Account balance ‒ Max number of photos allowed 29 • Define the following variables for a phone application: ‒ Number of users (int) ‒ Number of photos (int) ‒ Number of comments (int) ‒ Average number of active users in a week (double) ‒ Is the app available? (boolean) ‒ Version (float or double) ‒ User’s gender (char) ‒ Account balance (double) ‒ Max number of photos allowed (final int)
- 30. Assignment Assignment Assignment Assignment statements statements statements statements • An assignment statement is used to assign a value to a variable. answer = 42; •The "equal sign" is called the assignment operator. •We say, "The variable named answer is assigned a value of 42," or more simply, "answer is assigned 42." 30
- 31. Assignment Assignment Assignment Assignment statements statements statements statements • Syntax variable = expression where expression can be another variable, a literal or constant (such as a number), or something more complicated which combines variables and literals using operators (such as + and -) 31
- 32. Assignment Assignment Assignment Assignment examples examples examples examples amount = 3.99; firstInitial = 'W'; score = numberOfCards + handicap; eggsPerBasket = eggsPerBasket - 2; 32
- 33. Assignment Assignment Assignment Assignment evaluation evaluation evaluation evaluation • The expression on the right-hand side of the assignment operator (=) is evaluated first. • The result is used to set the value of the variable on the left-hand side of the assignment operator. avg = sum / count; counter = counter - 2; 33
- 34. Initializing v Initializing v Initializing v Initializing variables ariables ariables ariables • A variable that has been declared, but no yet given a value is said to be uninitialized. • Uninitialized class variables have the value null. •Uninitialized primitive variables may have a default value. •It's good practice not to rely on a default value. 34
- 35. Initializing v Initializing v Initializing v Initializing variables ariables ariables ariables • To protect against an uninitialized variable (and to keep the compiler happy), assign a value at the time the variable is declared. •Examples: int count = 0; char grade = 'A'; 35
- 36. Initializing v Initializing v Initializing v Initializing variables ariables ariables ariables • syntax type variable_1 = expression_1, variable_2 = expression_2, …; 36
- 37. Simple input Simple input Simple input Simple input • Sometimes the data needed for a computation are obtained from the user at run time. • Keyboard input requires: import java.util.Scanner at the beginning of the file 37
- 38. Simple input Simple input Simple input Simple input • Data can be entered from the keyboard using Scanner keyboard = new Scanner(System.in); followed by num1 = keyboard.nextInt(); which read one int value from the keyboard and assign it to num1 38
- 39. Simple input Simple input Simple input Simple input 39
- 40. Simple screen output Simple screen output Simple screen output Simple screen output 40
- 41. Common Scanner methods Common Scanner methods Common Scanner methods Common Scanner methods 41 • Last two methods will be discussed and used when we study the class String