1. John Hurley Cal State LA
CS 201 Lecture 3:
John Hurley
Cal State LA

2. Reserved Words
Reserved words or keywords are words that have a specific meaning to the compiler and cannot be used for other purposes in the program.
For example, when the compiler sees the word class, it understands that the word after class is the name for the class.
Some other reserved words include public, private, static, and void. Their use will be introduced later. Don’t use them as names for variables or classes.

3. Assignments
radius = 1 is an assignment that sets radius to the value 1
To the computer, this means “go find the memory you set aside to keep track of radius and replace the value there with 1”
radius == 1 is a test that is true if radius is equal to 1, otherwise false.
Usually used in a expression like
if(radius == 1)
System.out.println(“radius is equal to 1!”);

4. More Fun With Data Types
boolean
Value that is either true or false
inequalities and tests in loop statements evaluate to boolean values
1 < 2 is true, so
2 < 1 is false
2 == 2 is true
int i = 2;
System.out.println(i + " < " " ? " + (i < 5));
prints "2 < 5 ? true"

5. More Fun With Data Types
package demos; public class BooleanDemo { public static void main(String[] args) { int i = 1; System.out.println(i + " < " "? " + (i < 5)); System.out.println(i + " == " "? " + (i == 5)); System.out.println(i + " > " "? " + (i > 5) + "\n"); i = 5; i = 6; System.out.println("(1 == 1) == (2 == 1)? " + ((1 == 1) == (2 == 1))); System.out.println("(1 < 2) == (2 < 3)? " + ((1 < 2) == (2 < 3))); }

6. More Fun With Data Types
char
One character
Set value using single quotes:
char myChar = 'A';
‘A’ is not the same as ‘a’

7. More Fun With Data Types
String
Sequence of zero or more characters.
Note the capital S
String is a class; we will learn what that means later.
Set values using double quotes:
String myString = “Get your own String, This one is mine.”
“” is called a null String or empty String.

8. More Fun With Data Types
public class CharStringDemo{
public static void main(String[] args){
char myChar = 'a';
String myString = "John Hurley";
System.out.println(myChar);
System.out.println(myString); }

9. Math Operations Java's math operations are easy to understand
x = 1; // sets x to 1
x = x + 1; // adds 1 to x
x = x * y; // multiplies x by y
x = 100.0/9.2; // x should be a floating point // type, not an integer
x = y + 100;

10. Math Operations: Modulo
Modulo may be unfamiliar. It performs integer division and yields the remainder. The modulo operator is the symbol %
x = y % 3; means "divide y by 3, ignore the quotient, and assign the remainder as the new value of x." y should be an integer.
Modulo has many uses. The simplest one is that it tells us whether an integer is evenly divisible by another integer. If, for example a % b == 0, then a is evenly divisible by b.
9 % 3 is 0
10 % 3 is 1

11. Math Operations and Data Type
What will the output from this code be?
public class Mathematician{
public static void main(String[] args){
int total = 5;
int divisor = 4;
int result = total / divisor;
System.out.println( total + "/" +divisor + " = " + result); }

12. Math Operations and Data Type
What about this one?
public class Mathematician{
public static void main(String[] args){
int total = 5;
int divisor = 3;
int result = total / divisor;
System.out.println( total + "/" +divisor + " = " + result); }

13. Math Operations and Data Type
This one still doesn’t work!
public class Mathematician{
public static void main(String[] args){
int total = 5;
int divisor = 4;
double result = total / divisor;
System.out.println( total + "/" +divisor + " = " + result); }

14. Math Operations and Data Type
To get a floating point result from division, you must make at least one of the operands a floating point type:
public class Mathematician{
public static void main(String[] args){
int total = 5;
double divisor = 4;
double result = total / divisor;
System.out.println( total + "/" +divisor + " = " + result); }

15. Conversion Rules
When performing a binary* operation involving two operands of different types, Java automatically converts the operand based on the following rules: 
1.    If one of the operands is double, the other is converted into double.
2.    Otherwise, if one of the operands is float, the other is converted into float.
3.    Otherwise, if one of the operands is long, the other is converted into long.
4.    Otherwise, both operands are converted into int.
* In this case binary means “involving two operands,” not “ in base 2”

16. Casting Casting converts data types
You should rarely have to do this at this point
For primitive types, the syntax looks like this:
int x = (int) 5.1;
Casting a floating point type to an integer truncates, doesn’t round!
int x = (int) 1.6;
sets x to 1, not 2!

17. Casting
Casting the value back to the original type does not restore lost data, so if you need the original value, go back to the original variable
public class Caster{
public static void main(String[] args){
double x = 1.6;
int y = (int) x;
System.out.println(x);
System.out.println(y);
System.out.println((double) y); }
outputs:
1.6 1
1.0

18. Constants Imagine you are writing aprogram which can calculate
the circumference of a circle: 2πr
the area of a circle: πr2
the volume of a cylinder: πr2h
You could type in a value of pi each time you will use it
double radius = 5.1;
double height = 3; …
double circumference = 2 * 3.14 * radius;
double area = 3.14 * radius * radius;
double volume = 3.14 * radius * radius * height;

19. Constants Better idea: define a constant and set its value once
Less risk of errors or inconsistencies
Clearer code; it’s easier to recognize the constant name than the value
Easier to change precision
Used 3.14 before for simplicity, but results were not accurate enough. Now you want to use

20. Constants Use all caps for constant names Use final keyword
final double PI = ;
… code omitted…
double circumference = 2 * PI * radius;

21. Constants

22. Combining Operations
Just as in arithmetic and algebra, you can combine simple operations into complex calculations of any length
public class Converter{
public static void main(String[] args){
double celsius;
double fahrenheit = 212;
celsius = (fahrenheit -32) * 5.0/9;
System.out.println(fahrenheit + " degrees F = " celsius + " degrees C"); }

23. Operator Precedence

24. Operator Associativity
Binary operators are those that take two operands.
Example: a - b
All binary operators except assignment operators are left-associative.
a – b + c – d is equivalent to  ((a – b) + c) – d
Assignment operators are right-associative. Therefore, the expression
a = b += c = 5 is equivalent to a = (b += (c = 5))
x = 1
x = 10 * b;

25. Formatting Output Use the printf statement.
System.out.printf(format, items);
Where format is a string that may consist of substrings and format specifiers. A format specifier specifies how an item should be displayed. An item may be a numeric value, character, boolean value, or a string. Each specifier begins with a percent sign.

26. Frequently-Used Specifiers
Specifier Output Example
%b a boolean value true or false
%c a character 'a'
%d a decimal integer
%f a floating-point number
%e a number in standard scientific notation e+01
%s a string "Java is cool"

27. Printf
public class PrintfDemo{ public static void main(String[] args){ int intVar = 1; double doubleVar = ; double moonRadius = ; // in meters char charVar = 'a'; String stringVar = "Hi, Mom"; boolean boolVar = true; System.out.printf("\nintVar: %d; \ndoubleVar: %f; \ndoubleVar to 4 places: %7.4f; \nmoonRadius: %e; \ncharVar: %c; \nstringVar: %s", intVar, doubleVar, doubleVar, moonRadius, charVar, stringVar); } // end main() } // end class

28. Naming Conventions Choose meaningful and descriptive names.
Variables and method names:
Use lowercase. If the name consists of several words, concatenate all in one, use lowercase for the first word, and capitalize the first letter of each subsequent word in the name. For example, the variables radius and area, and the method computeAreaInSqCm().

29. Naming Conventions, cont.
Class names:
Capitalize the first letter of each word in the name. For example, the class name ComputeArea.
Constants:
Capitalize all letters in constants. Use underscores to connect words or just run words together. For example, the constant PI and MAX_VALUE or MAXVALUE

30. What to Expect
Programming is frustrating, but you will not experience anything that every other programmer who ever lived has not also experienced.
You will never get anything right on the first try, and rarely on the tenth.
This is the real origin of the term “hacking”
The problem is not that you are not smart enough, it is that programming requires closer attention to detail than is normal for a human being.
Rough guess: you will spend 5% of your time writing the frist draft of your code and 95% of your time figuring out why it doesn’t work and fixing it
Debugging = the process of finding and correcting defects in a piece of software (definition adapted from Wikipedia)
Professional software development organizations spend more time testing than programming, and programming itself consists mostly of debugging.

31. What to Expect
Programmers’ proverb: “A computer is like an idiot with a perfect memory who never gets tired”
it will do exactly what you say, but it is very hard for you to figure out how to say what you want
it does not understand your intentions or any kind of subtext. It will do neither more nor less than what you tell it to do
you will never be able to think out your programs well enough to account for all possibilities
Assume everything that goes wrong is your fault. At this stage in your programming career, problems will almost always be caused by your failure to think things out. You will be tempted to blame hardware failures, problems with Java, or bugs in Windows/OSX/Linux; you’ll almost always be wrong.

32. What to Expect
This joke is from Philogelos, the oldest known book of jokes, probably compiled around 300 AD:
A poor student went swimming and almost drowned. Now he swears he will never go in the water again until he learns to swim