COMP 111. Introduction to Computer Science and Object-Oriented Programming. Week 2

Transcription

1 COMP 111 Introduction to Computer Science and Object-Oriented Programming

2 Programming A program solves a problem or provides a service Useful programs apply to many instances of a problem First use for one instance Second use for another instance 5 41 Temperature Converter Program

3 Programming in 111 You create Java source code Using an editor To create one or more files Consisting of lines of text Conforming to Java syntax rules That can be compiled by a Java compiler Then tested and run To solve a problem or provide a service

4 What Do You Need to Know? Syntax of Java statements & constructs Knowledge of the problem What Java statements do when are run Design of Java programs Create Java Source Java conventions Style guidelines Source Files And how to use the tools

5 Java Programs Source code consists of One or more classes Each class is in one file Name of class matches name of file Syntax rules dictate how a file is laid out

6 Java Program Layout public class CorrectProgram { } A required keyword // stuff goes here You choose the name But must be capitalized Matching braces required Contents of CorrectProgram.java Must match the name of file

7 Java Program Layout /** * To practice working with syntax and logic errors. * J. David Fleig 1.0 */ public class CorrectProgram { } // stuff goes here Comments are expected

8 Java Program Layout Comments are expectedpublic class CorrectProgram { } /** * The program starts running here */ public static void main(string args[]) { } // statements to do stuff A common but special method

9 Java Program Layout public class CorrectProgram { } /** * The program starts running here */ public static void main(string args[]) { } Statements System.out.println("This is a fully working example program."); System.out.println("The answer of 5-6 is " + (5-6) ); Mind the semicolons

10 Questions public class MessyProgram { public static void main (String args[]) { System.out.println ("This is a messy example." );System.out.println("The answer of is " + (5+6) );}} Is this a Java program? Will it compile? Will it run?

11 Values Programs use values Input values Store values Operate on values Output values Values have types Java programs refer to values literally indirectly

12 Values In System.out.println("The answer of 5-6 is " + (5-6) ); What values? What types? What operations? Are the values given literally?

13 Self Check What is the type of the values 0 and "0"? Answer: int and String.

14 How Useful Is It? public class CorrectProgram { } /** * The program starts running here */ public static void main(string args[]) { System.out.println("This is a fully working example program."); System.out.println("The answer of 5-6 is " + (5-6) ); } Does it ever give a different result? Does it work with different instances?

15 Variables Allow us to write more useful program Think of them as a named place for a value The value stored in the place can vary over time Java statements can indirectly reference use the name to refer to the value of a variable Declare the variable Refer to variable s value int firstnumber = 5; System.out.println("The answer " + (firstnumber-6) );

16 More on Variables Java variables have a name (fixed when declared) type (fixed when declared) value (can vary over time) firstnumber 5 int firstnumber = 5; int

17 More on Variables Java variables have a name (fixed when declared) type (fixed when declared) value (can vary over time) firstnumber int firstnumber = 5; int System.out.println("The answer " + (firstnumber-6) ); 5

18 More on Variables Java variables have a name (fixed when declared) type (fixed when declared) value (can vary over time) int firstnumber = 5; System.out.println("The answer " + (firstnumber-6) ); firstnumber = 8; firstnumber int 8

19 More on Variables Java variables have a name (fixed when declared) type (fixed when declared) value (can vary over time) int firstnumber = 5; System.out.println("The answer " + (firstnumber-6) ); firstnumber = 8; System.out.println("The answer " + (firstnumber-6) ); firstnumber 8 int

20 You Must be Able To Define (or declare) a variables Name a variable with an identifier Assign a value to a variable Reference a variable in a Java statement Select the appropriate type for a variable

21 Define a Variable typename variablename = value; or typename variablename; Example: String greeting = "Hello, Dave!"; Purpose: To define a new variable of a particular type and optionally supply an initial value.

22 Naming a Variable You create an identifier - a name of a variable, method, or class Rules for identifiers in Java: Can be made up of letters, digits, and the underscore (_) character Cannot start with a digit Cannot use other symbols such as? or % Spaces are not permitted inside identifiers You cannot use reserved words They are case sensitive

23 Conventions for Identifiers Variable names start with a lowercase letter Can use an uppercase letter within E.g. firstnumber This is called camelcase Class names start with an uppercase letter Can also use camelcase

24 Self Check Which of the following are valid identifiers? Which could be names of classes? Which could be variable names? 1. Greeting1 2. g 3. void dalmatians 5. Hello, World 6. <greeting>

25 Self Check Define a variable to hold your name and give it an initial value. Use camel case in the variable name.

26 Assign a Value to A Variable You can give an initial value in defining it You can give it a value with an assignment statement Of the form variablename = value; Not this is not a test for equality int luckynumber = 13;

27 Assignment Operator Replaces the previous value int luckynumber = 13; luckynumber = 12;

28 Uninitialized Variables Do not reference a variable that has been given no value int luckynumber; System.out.println(luckyNumber);

29 Self-Check Is 12 = 12 valid in the Java language?

30 Self-Check How do you change the value of the variable greeting to "Hello, Nina!"

31 Classes A class Is a model of an entity needed to solve a problem Examples: Employee, PrintQueue, Point, PurchaseOrder Defines the attributes (data) and the actions (methods) of the entity

32 More on Classes Note that attributes (data) of a class are nouns: firstname, message, xcoordinate actions (methods) of a class are verbs: setname, getxcoord, encryptmessage Methods are called on take an action are a sequence of instructions related to the class manipulate data of an instance of a class

33 Objects An object is a specific instance of a class Think of a class as a mold and an object as the manufactured widget A method may or may not: receive input change data produce a result The format for calling a method is: objectvariable.methodname(inputs)

34 Example String greeting = Hello World! ; Creates an object - an instance of class String Also say the object is of type String The object is referenced by the variable greeting

35 Example int n = greeting.length(); length is a method defined by the String class The method has an implicit input greeting - the object to which it is applied The method produces a result (in this case 12) The type of result is int The result (12) is assigned to the integer type variable n

36 Another View of a Class A collection of objects With the same set of behaviors And the same set of variables But different objects may have different values for variables

37 A Different Example Represents an object What s the name of the class? How do we refer to the object? What are the methods? What do we know about the data of the class?

38 Two String Objects

39 Two String Methods length: counts the number of characters in a string String greeting = "Hello, World!"; int n = greeting.length(); // sets n to 13 touppercase: creates another String object that contains the characters of the original string with lowercase letters converted to uppercase String river = "Mississippi"; String bigriver = river.touppercase(); // sets bigriver to "MISSISSIPPI"

40 Self Check Is this valid in Java? System.out.length();

41 Self Check How can you compute the length of the string "Mississippi"

42 Self Check How can you print out the uppercase version of "Hello, World!"

43 Self Check Is it legal to call river.println(); Why or why not?

44 Invoking a Method Implicit parameters method Return values Explicit parameters What it does is defined in the class

45 Example System.out.println( Hello world );

46 Example int n = greeting.length(); // return value stored in n

47 Example System.out.println(greeting.length());

48 Example How do you explain this? river.replace( issipp, our )

49 Self Check What are the implicit parameters, explicit parameters, and return values in the method call river.length()

50 Some Java Primitive Types Types Declaration Examples Typical Size Integer - 0, negative,positive - no fractions int numberrows; short sumgrades; int numcopies = 25; int short long 32 bits 16 bits 64 bits Floating point - real numbers - fractional parts float changedue; double milestraveled = 2.5; float double 32 bits 64 bits

51 Next Week Read in Big Java Sections 2.6 to 2.10 View Testing with JUnit multimedia presentation Read Intro and Key Points for Module 3

52 Next Week Submit homework from Assignment 2-3 Submit before class Into Course Drop Box Worth 15 points