Software Concepts. 2-8 Explain the process of translating and executing a Java program. How might the World Wide Web be involved?

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1 CHAPTER 2 Software Concepts Notes This chapter explores how programs are written and the process of compiling a program into machine language. Basic programs are presented and discussed. High-level software concepts are discussed, so that students have some introduction to a larger picture (software components) in which to place the low-level details in context over the next couple of chapters. Several basic operation issues should be discussed before beginning to use a Java compilation environment. Access to the environment in which the students will work should be described and practiced. Students should master the basic operation of an editor. Trouble with access or editing will only distract from the learning experience. Our experience shows students that learn a few basic operational practices early have less frustration when learning programming concepts later. The programs introduced in this chapter have a single class with a single method, usually either main or paint. Practicing with the compilation environment using the simple programs presented in the chapter will enable students to encounter and solve basic syntax and run-time errors. It is our recommendation that you never present a program that has more than one method and a single class, because that leads to procedural thinking. If a program is complicated enough to have multiple methods, it should be presented in the context of objects, which is explored in Chapter 4. Applets are introduced in the chapter along with the basic HTML necessary to make applets browseable. Several of the applets in the chapter are graphical in nature. It is important to understand that, although graphical programs are used, event-driven programming is not. The graphical nature of the program usually attracts student s interest, but introducing event-driven programming this early often confuses them. The concepts necessary to understand event-driven programs are not fully explained until after inheritance and interfaces are discussed. Exercises 2-8 Explain the process of translating and executing a Java program. How might the World Wide Web be involved? Java source code is compiled by the Java compiler. The output of the Java compiler is known as bytecode. Bytecode is used as input for the Java interpreter, which translates each line of bytecode into machine code for the computer the program is to run on, then runs that machine code. This process is repeated until the program terminates. Bytecode may also be moved across the Internet via the World-Wide Web. If an HTML document contains a link to an applet's bytecode, a Java compatible browser can download the bytecode. The browser's Java interpreter then translates and executes the program. 2-9 Give examples of the two types of Java comments. Explain the differences between them. here is an example of the first type of Java comment /* here is an example of the second 5

2 type of Java comment */ The difference between the two is that the first type of comment lasts from the "" to the end of the line. The second type of comment lasts from the "/*" to the "*/". Comments that start with "/**" can be used to generate external documentation for the program Which of the following are not valid Java identifiers? Why? a) Factorial Valid b) an_extremely_long_identifier_if_you_ask_me Valid c) 2nd_level Invalid. An identifier cannot begin with a digit. d) level2 Valid e) highest$ Valid f) hook&ladder Invalid. & is not a valid character in an identifier Why are the following valid Java identifiers not considered to be good identifiers? a) i Not a good identifier because it is not very descriptive. b) tot_val Not a good identifier because it is not clear enough. Abbreviations can make your code difficult to read or interpret. c) the_next_value_in_the_list Not a good identifier because it is long without adding much value to its descriptions Java is case-sensitive. What does that mean? Case sensitive means that upper case and lower case letters are distinct characters with distinct meanings. As a result, test, Test, and TEST are all different identifiers Categorize each of the following situations as a compile-time error, run-time error, or logical error. a) multiplying two numbers when you meant to add them Logical b) dividing by zero Run-time c) spelling a word wrong in the output 6 Chapter 2 Software Concepts

3 Logical d) producing inaccurate results Logical e) typing a { when you should have typed ( Compile-time 2-14 Why is the English language ambiguous? Give two examples of English ambiguity (other than the example used in this chapter). Why is ambiguity a problem for programming languages? English is an ambiguous language because a sentence may have two different meanings. Two examples of English ambiguity are: 1. A garbage truck has four wheels and flies. 2. He left it to him. Ambiguity is a problem for programming languages because if a computer came across an instruction that could have more than one meaning, the computer would not know which instruction to perform What is the relationship between a class and an object? Classes are descriptions or blueprints of objects, while an object is a particular entity whose properties are described by the class Categorize each of the following as either a class or an object: a) a book Class b) the book Java Software Solutions Class c) my copy of Java Software Solutions with the dog-eared pages Object d) my dog Object e) the picture of my dog biting the mailman Object f) a car Class g) a Chrysler LaBaron Class 7

4 Programming Projects 2-17 Enter, compile, and run the following application: class Test { public static void main (String [] args) { System.out.println ("Testing the Emergency Broadcast Program"); } } 2-18 Introduce the following errors, one at a time, to the program in Problem Record any error messages that the compiler produces. Fix the previous error each time before you introduce a new one. If no error messages are produced, explain why. a) Change Test to test No compile time error produced. If the user types java Test, the user would get a runtime error: "Can't find class Test" because the output of the compilation would be test.class. However, if the user types java test, the user would get no runtime error, since the Java interpreter would find test.class. Note however, that this error only occurs on operating systems that are also case sensitive. b) Change Emergency to emergency No compile time or run-time error produced. A slight logical error occurs. The program outputs the word "emergency" instead of "Emergency" c) Remove the first quotation mark in the string literal Test.java:10: ')' expected. System.out.println(Testing the Emergency Broadcast Program"); Test.java:10: String not terminated at end of line. System.out.println(Testing the Emergency Broadcast Program"); 2 errors d) Remove the last quotation mark in the string literal Test.java:10: String not terminated at end of line. System.out.println("Testing the Emergency Broadcast Program); Test.java:11: ')' expected. } 2 errors e) Change main to man No compile time error produced, however a run-time error is produced. The message was: void main(string argv[]) is undefined. 8 Chapter 2 Software Concepts

5 f) Change println to bogus Test.java:10: Method bogus(java.lang.string) not found in class java.io.printstream. System.out.bogus("Testing the Emergency Broadcast Program"); 1 error g) Change Broadcast to Brxoadxcaxst No compile time or runtime error produced. A slight logical error occurs. The program prints the word "Brxoadxcaxst" instead of "Broadcast" h) Remove the semicolon at the end of the println statement Test.java:10: ';' expected. System.out.println("Testing the Emergency Broadcast Program") 1 error i) Remove the last brace in the program Test.java:11: '}' expected. } 1 error 2-19 Write an application which prints the phrase Knowledge is Power a) on one line b) on three lines c) inside a box made up of the characters = and Not Provided 2-20 Write an applet which prints the phrase Knowledge is Power inside an oval. Not Provided 2-21 A mad-lib is a sentence with certain words left out. You ask people who don t know what the sentence is to give you words of particular parts of speech, which are then inserted into the sentence to produce weird results. For example: The professor my, and I ve never recovered. This mad-lib needs an adjective, a verb (past tense), and a noun. When the blanks are filled in, it might produce the following sentences: The wise professor graded my paper, and I ve never recovered. The purple professor ate my computer, and I ve never recovered. 9

6 Using this mad-lib, write a program which accepts three words as command-line arguments and prints the completed sentence. Run your program several times (without recompiling), providing different words. ******************************************************************* Mad_Lib.java Programming Project Application Authors: Lewis and Loftus Classes: Mad_Lib ******************************************************************* class Mad_Lib { =========================================================== Uses command line arguments to print a mad lib. =========================================================== public static void main(string[] args) { System.out.println("The " + args[0] + " professor " + args[1] + " my " + args[2] + ", and I never recovered."); } method main } class Mad_Lib 2-22 Rewrite the program of Problem 2-20, using a mad-lib that you create which requires four blanks to be filled. ******************************************************************* Mad_Lib2.java Programming Project Application Authors: Lewis and Loftus Classes: Mad_Lib2 ******************************************************************* class Mad_Lib2 { =========================================================== Prints a 4-word mad lib. 10 Chapter 2 Software Concepts

7 =========================================================== public static void main(string[] args) { System.out.println("Now is the time for all " + args[0] + " " + args[1] + " to come to the " + args[2] + " of their " + args[3] + "."); } method main } class Mad_Lib Create a personal web page using HTML (see Appendix K). Not Provided 2-24 Write an applet which writes your name using the drawstring method. Embed a link to your applet in an HTML document and view it using a web browser. Not Provided 2-25 Write an applet which draws a stick figure similar to the following: ******************************************************************* Person.java Programming Project Applet Authors: Lewis and Loftus Classes: Person ******************************************************************* import java.applet.applet; import java.awt.graphics; public class Person extends Applet { =========================================================== Draws a stick figure. =========================================================== 11

8 public void paint(graphics page) { page.drawoval(50, 10, 20, 20); the head page.drawline(60, 30, 60, 70); the trunk page.drawline(60, 70, 45, 100); the legs page.drawline(60, 70, 75, 100); page.drawline(60, 45, 40, 45); the arms page.drawline(60, 45, 80, 40); } method paint } class Person 12 Chapter 2 Software Concepts

CS 553 Compiler Construction Fall 2007 Project #1 Adding floats to MiniJava Due August 31, 2005

CS 553 Compiler Construction Fall 2007 Project #1 Adding floats to MiniJava Due August 31, 2005 In this assignment you will extend the MiniJava language and compiler to enable the float data type. The