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COMP 14 Introduction to Programming Adrian Ilie June 28, 2005 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Today • Variables and expressions • Input/Output • Writing a whole program 2 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Variables • Associated with data ♦ Input data ♦ Output data ♦ Intermediate data • We need to define: ♦ Data type ♦ Identifier • Values will be assigned in expressions 3 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Variables - Steps 1. Identify all data from the algorithm 2. Determine data types (based on the range and nature of the values) 3. Find meaningful names • 4 Example: Ch. 1, Exercise 10. Compute average score. Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Declaration of Variables dataType identifier; • Must be declared before it can be used • Can be (but doesn't have to be) initialized when declared • Identifier should start in lowercase, indicate separate words with uppercase (good style) • Example: ♦ number of students in class int numStudents; • Multiple variables (of the same data type) can be declared on a single line int numStudents, numGrades, total; 5 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Assignment variable = expresssion; • Assignment Operator (=) • expression can be a value (3) or a mathematical expression (2 + 1) • The expression must evaluate to the same data type as the variable was declared 6 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Assignment • The assignment operator has a lower precedence than the arithmetic operators First the expression on the right hand side of the = operator is evaluated answer = 4 sum / 4 + MAX * lowest; 2 3 1 Then the result is stored in the variable on the left hand side 7 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Assignment • The right and left hand sides of an assignment statement can contain the same variable First, one is added to the original value of count count = count + 1; Then the result is stored back into count (overwriting the original value) 8 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Example (average score) • Write assignments and expressions • Declaration of variables (good style) a) At the beginning b) Before being used 9 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Named Constant static final dataType IDENTIFIER = value; • Declared by using the reserved word final • Always initialized when it is declared • Identifier should be in ALL CAPS, separate words with underscore (_) (good style) • Example: ♦ 1 inch is always 2.54 centimeters final double CM_PER_INCH = 2.54; 10 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Questions What is stored in the memory location referred to by the identifier num after each of the following statements is executed in order? int num num num num num; = 3; = 5 + 4 - 2; = num * 2; = 3.4 + 5; ______ unknown 3 ______ 7 ______ 14 ______ error ______ would give an error since 3.4 + 5 would not result in an int 11 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Questions Which of the following are valid Java assignment statements? Assume that i, x, and percent have been declared as double variables and properly initialized. 1. 2. 3. 4. 12 i = i + x + 2 = x = 2.5 percent Adrian Ilie 5; x; * x; = 10%; valid ______ invalid ______ valid ______ invalid ______ The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Input 1. Standard input 2. Dialog windows (tomorrow) 3. File (tomorrow) 13 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Reading Keyboard Input • We can let the user assign values to variables through keyboard input • In Java, input is accomplished using objects that represent streams of data • A stream is an ordered sequence of bytes • System.in is the standard input stream object (by default, the keyboard) 14 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Reading Keyboard Input • The idea is that, with System.in, we have access to a stream of bytes coming from the keyboard. • In other words, we have access to the keys pressed on the keyboard. • But how do we actually get the values of the keys that are pressed? 15 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Reading Keyboard Input The input stream is made up of multiple objects: InputStreamReader charReader = new 1 character InputStreamReader (System.in); at a time BufferedReader keyboard = new BufferedReader (charReader); Whole line at once OR, all in one statement: BufferedReader keyboard = new BufferedReader (new InputStreamReader (System.in)); 16 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Reading Keyboard Input • The readLine method of the BufferedReader class reads an entire line of input as a String String line = keyboard.readLine(); 17 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Example (average score) 18 • Input scores • Input weights Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL String to Numbers • String to int Integer.parseInt(strExpression) Integer.parseInt("6723") 6723 • String to float Float.parseFloat(strExpression) Float.parseFloat("345.76") 345.76 • String to double Double.parseDouble(strExpression) Double.parseDouble("1234.56") 1234.56 19 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Example (average score) 20 • Convert scores to int • Convert weights to double Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Output • Standard output device is usually the monitor • Access the monitor using the standard output object ♦ System.out • Two methods to output a string: 1. print 2. println 21 Adrian Ilie Puts the cursor on the next line at the end The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Examples System.out.println ("Hi"); System.out.println ("There"); Hi There System.out.print ("Hi"); System.out.println ("There"); HiThere int num = 5+3; System.out.println (num); 8 int num = 5; System.out.println ("The sum is " + (num + 3)); The sum is 8 22 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL String Concatenation • A string cannot be split between two lines X String greeting = "How are you doing today"; • Concatenation (+) - produces one string where the second string has been appended to the first string String greeting = “How are you doing” + “ today?”; greeting How are you doing today? 23 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL String Concatenation • Operator + can be used to concatenate two strings or a string and a numeric value or character • Precedence rules still apply • Example: String str; int num1 = 12, num2 = 26; str = "The sum = " + num1 + num2; str 24 Adrian Ilie The sum = 1226 The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Example (average score) 25 • Output result • Formatting numeric strings Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Writing a Whole Program • Class - used to group a set of related operations (methods), allows users to create their own data types • Method - set of instructions designed to accomplish a specific task • Package - collection of related classes • Library - collection of packages 26 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Class Libraries • A collection of classes that we can use when developing programs • The Java standard class library is part of any Java development environment • The System class and the String class are part of the Java standard class library 27 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Packages • The classes of the Java standard class library are organized into packages. • Some of the packages in the standard class library are: 28 Package Purpose java.lang java.applet java.awt javax.swing java.net java.util javax.xml.parsers General support Creating applets for the web Graphics and graphical user interfaces Additional graphics capabilities and components Network communication Utilities XML document processing Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Using Packages We need to import some of the packages we want to use ♦ java.io for BufferedReader import packageName; ♦ import java.io.*; • imports all of the classes in the java.io package ♦ import java.io.BufferedReader; • imports only the BufferedReader class from the java.io package 29 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Using Predefined Classes and Methods • To use a method you must know: ♦ Name of class containing method (Math) ♦ Name of package containing class (java.lang) ♦ Name of method (round), its parameters (double a), what it returns (long), and function (rounds a to the nearest integer) • See Appendix E for more Java predefined classes 30 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Using Predefined Classes and Methods • Example method call: int num = (int) Math.round (4.6); • (Dot) . Operator: used to access the method in the class 31 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Creating a Java Program • Java application program - collection of one or more classes ♦ every application must have at least one class • Class ♦ basic unit of a Java program ♦ collection of methods and data members • Method - set of instructions designed to accomplish a specific task ♦ print, readLine 32 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Programming in Java • Java programming language ♦ object-oriented approach to problem solving • In the Java programming language: ♦ a program is made up of one or more classes ♦ a class contains one or more methods ♦ a method contains program statements 33 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Creating a Java Program • All Java application programs must have a method called main ♦ there can be only one main method in any Java application program • Most of the time, our programs will have only one class • Name of source file must be ClassNameWithMain.java 34 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Anatomy of a Java Program • Syntax of class public class ClassName { classMembers } • Syntax of main method public static void main (String[] args) throws clause { statement1 ... statementn } 35 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Throws Clause • throws clause - exceptions thrown by the main method • exception - occurrence of an undesirable situation that can be detected during program execution ♦ can either be handled or thrown • readLine throws the exception IOException • We won't handle the exception, we'll just throw it 36 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Throws • If we're allowing user input to the program, the heading of the main method should look like: public static void main (String[] args) throws IOException 37 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Import Statements • Tell the compiler which packages are used in the program • Import statements and program statements constitute the source code • Source code saved in a file with the extension .java • Source code file must have the same name as the class with the main method 38 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL The main method • Heading public static void main (String[] args) throws IOException • Body ♦ statements enclosed by { } ♦ declaration statements • used to declare things such as variables ♦ executable statements • perform calculations, manipulate data, create output, accept input, etc. 39 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Skeleton import statements if any public class ClassName { declare named constants and/or stream objects public static void main (String[] args) throws IOException { variable declarations executable statements } } 40 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL static • Heading of the main method has the reserved word static • Statements to declare named constants and input stream objects are outside the main method • These must also be declared with the static reserved word 41 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Style • Syntax ♦ beware! a syntax error in one place might lead to syntax errors in several other places • Use of semicolons, braces, commas ♦ all Java statements end with semicolon ♦ braces {} enclose the body of a method and set it off from other parts of the program (also have other uses) ♦ commas separate list items 42 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Style • Semantics ♦ set of rules that gives meaning to a language ♦ beware! the compiler will not be able to tell you about semantic errors (example: missing parentheses in mathematical expression) • Documentation ♦ comments ♦ naming rules • use meaningful identifiers ♦ prompt lines • let the user know what type of input is expected 43 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Style and White Space • White space ♦ ♦ ♦ ♦ blanks, tabs, blank lines used to separate words and symbols extra space is ignored by computer blank line between variable declaration and rest of code • Programs should be formatted to enhance readability, using consistent indentation 44 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Comments • Not used by the computer ♦ only for human consumption • Used to help others understand code ♦ explain and show steps in algorithm ♦ comments are essential! • Should be well-written and clear • Comment while coding • Also called inline documentation 45 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Java Comments // this is a one-line comment ♦ “comments out” the rest of the line after marker // /* this is a multi-line comment */ ♦ “comments out” everything between markers /* and */ 46 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL To do • Practice input. Ch. 2 (pp.45-54) • Practice output. Ch. 2 (pp.55-65) • Ch. 2 examples: ♦ Convert Length ♦ Make Change • Homework 2 due Thursday night. • Read Ch. 3 47 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL