Lecture 1: Comments, Variables, Assignment

Definitions The formal (human-readable) instructions that we give to the computer is called source code The compiler translates our source code into a format that the computer can understand

Comments Comments are places in your source code where you write stuff intended only for humans to read The compiler always totally ignores comments, as if they weren't even there There are two types of comments in Java Single-line comments – Everything including the ``//'' and after on that line is ignored. – System.out.println("Hello World!"); //Greet the user – //This is a comment, too. – ``Commenting-out'' code is a quick way to make a line of code do nothing temporarily. // System.out.println("Hello World!");

Multi-line comments – Start with ``/*'' and end with ``*/'' – /* They can be just one line, too */ Here is a Java program that illustrates different ways to use comments:

/////////////////////////////////////////////////////////////////////////////// // ALL STUDENTS COMPLETE THESE SECTIONS // Title: HelloWorld2 // // Author: Michael Kowalczyk // CS Login: mkowalcz // Lecturer's Name: // Lab Section: // public class HelloWorld2 { public static void main(String[] args) { //This is a single line comment /* This is a multi-line comment */ //Note that the following System.out.println statement is //commented out, so it won't do anything. //System.out.println("Hello, World!"); System.out.println("Hi!"); //Prints a greeting to the user. } }

Identifiers Identifiers are the names that we give to different things (variables, methods, classes) in Java. – Rules for making identifiers: Identifiers can be made up of a-z, A-Z, 0-9, as well as _ and $ Identifiers cannot start with a number Can't be a ``reserved word'' (public, void, class, int, etc.) Don't try to memorize all the reserved words; just remember that they are words with special meaning in Javaall the reserved words Case sensitive, so myIdentifier is different than MyIdentifier or MYIDENTIFIER. Identifiers in this code fragment have been underlined.this code fragment Some examples of good/bad identifiers?

Data Types There are precisely two categories of data types in Java: primitives and classes. There are only 8 primitive types and they are all listed herehere – The most important ones in this course are int, double, boolean, and char A ton of classes are already given in the Java libraries, and we can make new ones, too.

Variables Every variable has a name, type, and contents – The name must be an identifier – The type is fixed, and is either one of the 8 primitives or some class – The contents must match the type, and can be changed over time (unlike typical variables in math) In general, you can declare (that is, define) a variable by using a statement of one of the following 2 forms: – type identifier; – type identifier = value; You have to declare a variable before using it and you can only declare it once. If you want to change the contents of a variable once declared, you just use an assignment statement, of this form: – identifier = value; Let's write up an example to make this all clear

/* * This application class demonstrates how to declare, assign, and use * variables with primitive types. */ public class Primitives { public static void main(String[] args) { //Declare a variable called letter that can hold a char char letter; //Declare a variable called myInteger that can hold an int, and make it 3 int myInteger = 3; //Store the character "a" in the variable called letter letter = 'a'; System.out.println(letter); //prints out the letter a //Declare j (which can hold an int) and make it equal 5 int j = 5; j = j - myInteger; //evaluates 5-3 and stores the result in j System.out.println(j); //prints 2 double d = 3.7; //myInteger = d; //causes compile-time error (can't convert) d = myInteger; //But this conversion is ok. System.out.println(myInteger); //prints 3 System.out.println(d); //prints 3.0 } }

We'll now start our first lab Mind-Reading Number Trick 1) Think of a number, any positive integer (but keep it small so you can do computations in your head). 2) Square it. 3) Add the result to your original number. 4) Divide by your original number. 5) Add, oh I don't know, say 17. 6) Subtract your original number. 7) Divide by 6. 8) The number you are thinking of now is 3!