1. 1 CSE 142 Lecture Notes Introduction These lecture notes are copyright (C) Marty Stepp 2005. May not be rehosted, copied, sold, or modified without Marty Stepp's expressed written permission. All rights reserved.

2. 2 Lecture outline Introduce instructor, TAs Discuss course syllabus and resources Examine some simple programs to learn about the Java programming language How to use the TextPad editor Decomposing problems and eliminating redundancy using static methods

3. 3 Computer programs program: A set of instructions that are to be carried out by a computer. program execution: The act of carrying out the instructions contained in a program. programming language: A systematic set of rules used to describe computations, generally in a format that is editable by humans. Java: One of many modern programming languages; created by Sun Microsystems. The language we will use in this course!

4. 4 A simple Java program public class Hello { public static void main(String[] args) { System.out.println("Hello, world!"); } code, source code: The sequence of instructions in a particular program. The code in this program instructs the computer to print a message of Hello, world! on the screen. output: The messages printed to the user by a program. console: The black box onto which output is printed.

5. 5 Compiling, running a program compiler: A program that translates a computer program written in one language into an equivalent program in another language (often, but not always, translating into machine language). interpreter: A program that dynamically executes programs written in a particular programming language. Java Virtual Machine: A theoretical computer whose machine language is the set of Java byte codes. source code (Hello.java) compile byte code (Hello.class) execute output

6. 6 TextPad To run a Java program in TextPad: Open a.java file (or type in your own) Click Tools, Compile Java, or press Ctrl-1 (the window should go white and then return after a pause, while the program is compiling) Click Tools, Run Java, or press Ctrl-2 (a black console window should appear with the program's output)

7. 7 Another Java program public class Hello2 { public static void main(String[] args) { System.out.println("Hello, world!"); System.out.println(); System.out.println("This program produces"); System.out.println("four lines of output"); } The code in this program instructs the computer to print four messages on the screen.

8. 8 Structure of Java programs public class { public static void main(String[] args) { ; } Every executable Java program consists of a class... that contains a method named main... that contains the statements to be executed The previous program is a class named Hello, whose main method executes one statement named System.out.println

9. 9 Java terminology class: (a) A module that can contain executable code. (b) A category or type of object. (seen later) statement: An executable piece of code that represents a complete command to the computer. every basic Java statement ends with a semicolon ; method: A named sequence of statements that can be executed together to perform a particular action or computation.

10. 10 Syntax and syntax errors syntax: The set of legal structures and commands that can be used in a particular programming language. syntax error or compiler error: A problem in the structure of a program that causes the compiler to fail. If you type your Java program incorrectly, you may violate Java's syntax and see a syntax error. public class Hello { pooblic static void main(String[] args) { System.owt.println("Hello, world!")_ } H:\summer\Hello.java:2: expected pooblic static void main(String[] args) { ^ H:\summer\Hello.java:5: ';' expected } ^ 2 errors Tool completed with exit code 1 compiler output:

11. 11 Fixing syntax errors Notice how the error messages are cryptic and do not always help us understand what is wrong: H:\summer\Hello.java:2: expected pooblic static void main(String[] args) { ^ We'd have preferred a friendly message such as, "You misspelled 'public'" The compiler does tell us the line number on which it found the error, which helps us find the place to fix the code. Jump to the line by double-clicking the error in TextPad Java has a fairly rigid syntax. Several languages are friendlier than Java, but we must pay this price in order to learn an industrial- strength language that is used in real jobs

12. 12 In TextPad, it is easy to accidentally save your Java program with a.txt extension Text files are the default; hence the name, "TextPad" If you make this mistake, you'll see a strange error message like this: javac: invalid flag: H:\summer\Hello.java.txt Usage: javac where possible options include: -g Generate all debugging info -g:none Generate no debugging info... Common TextPad pitfalls

13. 13 Fixing TextPad issues to avoid saving as.txt: in TextPad click Configure, Preferences..., File Name Filters, then move the Java (*.java) item to the top Windows Explorer doesn't show files' real extensions, which makes it harder to find this problem to change this setting, in Windows Explorer (My Computer), click Tools, Options..., View, then uncheck the Hide extensions for known file types box

14. 14 System.out.println Java programs use a statement called System.out.println to instruct the computer to print a line of output on the console pronounced "print-linn"; sometimes called a println statement for short Two ways to use System.out.println : 1. System.out.println(" Message "); Prints the given message as a line of text on the console. 2. System.out.println(); Prints a blank line on the console.

15. 15 Strings and string literals string: A sequence of text characters that can be printed or manipulated in a program. literal: a representation of a value of a particular type String literals in Java start and end with quotation mark characters "This is a string"

16. 16 Details about Strings A string may not span across multiple lines. "This is not a legal String." A string may not contain a " character. ' is OK "This is not a "legal" String either." "This is 'okay' though." A string can represent certain special characters by preceding them with a backslash \ (this is called an escape sequence). \t tab character \n new line character \" quotation mark character \\ backslash character

17. 17 Questions What sequence of println statements will generate the following output? This program prints a quote from the Gettysburg Address. "Four score and seven years ago, our 'fore fathers' brought forth on this continent a new nation." What sequence of println statements will generate the following output? A "quoted" String is 'much' better if you learn the rules of "escape sequences." Also, "" represents an empty String. Don't forget to use \" instead of " ! '' is not the same as "

18. 18 Another example question What sequence of println statements will generate the following output? _____ / \ / \ \ / \_____/ _____ / \ / \ | | | | | | \ / \_____/ _____ / \ / \ +-------+ _____ / \ / \ What observations can we make about the output that is generated? It has a noticeable structure. (draw first figure, draw second figure, draw third figure,...) The output contains redundancy.

19. 19 Structured algorithms How does one bake sugar cookies? Mix the dry ingredients. Cream the butter and sugar. Beat in the eggs. Stir in the dry ingredients. Set the oven for the appropriate temperature. Set the timer. Place the cookies into the oven. Allow the cookies to bake. Mix the ingredients for the frosting. Spread frosting and sprinkles onto the cookies.... Can we express this process in a more structured way?

20. 20 A structured algorithm structured algorithm: A list of steps for solving a problem, which is broken down into cohesive tasks. A structured algorithm for baking sugar cookies: 1. Make the cookie batter. Mix the dry ingredients. Cream the butter and sugar. Beat in the eggs. Stir in the dry ingredients. 2. Bake the cookies. Set the oven for the appropriate temperature. Set the timer. Place the cookies into the oven. Allow the cookies to bake. 3. Add frosting and sprinkles. Mix the ingredients for the frosting. Spread frosting and sprinkles onto the cookies....

21. 21 Redundancy in algorithms How would we express the steps to bake a double batch of sugar cookies? Unstructured: Mix the dry ingredients. Cream the butter and sugar. Beat in the eggs. Stir in the dry ingredients. Set the oven... Set the timer. Place the first batch of cookies into the oven. Allow the cookies to bake. Set the oven... Set the timer. Place the second batch of cookies into the oven. Allow the cookies to bake. Mix the ingredients for the frosting. Structured: 1. Make the cookie batter. 2a. Bake the first batch of cookies. 2b. Bake the second batch of cookies. 3. Add frosting and sprinkles. Observation: A structured algorithm not only presents the problem in a hierarchical way that is easier to understand, but it also provides higher-level operations which help eliminate redundancy in the algorithm.

22. 22 Static methods static method: A group of statements that is given a name so that it can be executed in our program. Breaking down a problem into static methods is also called "procedural decomposition." Static methods are useful for: denoting the structure of a larger program in smaller, more understandable pieces eliminating redundancy through reuse

23. 23 Static method syntax The structure of a static method: Inside your program's class: public class { public static void () { ; } } Example: public static void printWarning() { System.out.println("This product is known to cause"); System.out.println("cancer in lab rats and humans."); }

24. 24 Static methods example public class TwoMessages { public static void main(String[] args) { displayMessage(); System.out.println(); displayMessage(); } public static void displayMessage() { System.out.println("Now this is the story all about how"); System.out.println("My life got flipped turned upside-down"); } Program's output: Now this is the story all about how My life got flipped turned upside-down Now this is the story all about how My life got flipped turned upside-down

25. 25 Identifiers identifier: A name that we give to a piece of data or part of a program. Identifiers are useful because they allow us to refer to that data or code later in the program. The name you give a static method is an example of an identifier. (What is another example identifier we've seen?) Java identifier names: first character must a letter or _ or $ following characters can be any of those characters or a number Example Java identifiers: legal: susan second_place TheCure ANSWER_IS_42 illegal: me+u 49er side-swipe hi there

26. 26 Keywords keyword: An identifier that you cannot use because it already has a reserved meaning in the language. Complete list of Java keywords: abstract default if private this boolean do implements protected throw break double import public throws byte else instanceof return transient case extends int short try catch final interface static void char finally long strictfp volatile class float native super while const for new switch continue goto package synchronized You may not name a method or a class char or while or this ; Java uses those words to mean other things.

27. 27 Comments comment: A note written in the source code by the programmer, to help make the program easier to understand. Comments are not executed when your program runs. TextPad and other editors turn your comments a special color to make it easier to identify them. Comment syntax: /* A comment goes here. */ /* It can even span multiple lines. */ // This is a one-line comment.

28. 28 Using comments In this course, it is considered proper programming style to put a "comment header" at the top of each program we write, explaining what the program does. We will also place a comment at the start of every static method, describing the method's behavior. Lastly, we can use comments inside methods to explain particular pieces of code.

29. 29 Comments example /* Suzy Student CSE 142 Summer 2005 This program prints lyrics from my favorite song! */ public class TwoMessages { /* Runs the overall program to print the song. */ public static void main(String[] args) { displayMessage(); // Separate the two verses with a blank line System.out.println(); displayMessage(); } /* Displays the first verse of the theme song. */ public static void displayMessage() { System.out.println("Now this is the story all about how"); System.out.println("My life got flipped turned upside-down"); }

30. 30 Suggested reading / tasks Chapter 1 Simple programs with println statements: read 1.1 - 1.7 (skip 1.7.1), 1.7.2 - 1.8, 1.10 self-checks 5-7, 12 Static methods, comments, identifiers: read 1.7.1, 1.8 - 1.9 self-checks 1-4, 8-11, 13 - 15 Make sure that you can log in to the school's labs using your UW NetID. Also, try compiling and executing a simple Java program (such as Hello) for next time.

31. 31 Working from home Download and install Java 2 Software Development Kit ("J2SE SDK") from: http://java.sun.com/j2se/1.4.2/download.html http://java.sun.com/j2se/1.4.2/download.html Download and install TextPad from: http://www.textpad.com/ (be sure to install it after installing Java) http://www.textpad.com/ Download (Save to Disk) or type in a sample program to test, such as: http://www.cs.washington.edu/education/ courses/cse142/05su/Hello.java http://www.cs.washington.edu/education/ courses/cse142/05su/Hello.java

32. 32 Working from home, cont'd To test that your home setup works: Open your Test.java program in TextPad Click Tools, Compile Java, or press Ctrl-1 (the window should go white and then return after a pause, while the program is compiling) Click Tools, Run Java, or press Ctrl-2 (a black console window should appear that says, "Hello, world!")