1. Data types and variables

2. today
My First program
Variables
Data types

3. Java-Why is it special? Java is an object-oriented language.
Programming methodology that views a program as consisting of objects that interact with one another by means of actions (called methods)
Java programs are portable between machines
Java is accompanied by a rich set of libraries that allow you to create graphics, interact with databases, communicate overnetworks, etc.

4. Byte-Code and the Java Virtual Machine
The compilers for most programming languages translate high-level programs directly into the machine language for a particular computer
Since different computers have different machine languages, a different compiler is needed for each one
In contrast, the Java compiler translates Java programs into byte-code, a machine language for a fictitious computer called the Java Virtual Machine
Once compiled to byte-code, a Java program can be used on any computer, making it very portable

5. Byte-Code and the Java Virtual Machine
Interpreter: The program that translates a program in Java byte-code into the machine language for a particular computer when a Java program is executed. The interpreter is also known as the Java Virtual Machine (JVM)
The interpreter translates and immediately executes each byte-code instruction, one after another
Translating byte-code into machine code is relatively easy compared to the initial compilation step

6. Platform independent
Java compiler translates a Java program into Java byte-code
Java bytecode is not tied to any particular processor type
Java interpreter executes the Java byte-code.
A Java interpreter needed for each processor type.

7. Translate Java code to Machine Code
Compiler
Java code  Java byte-code
Interpreter
Java byte-code --- Machine code

8. Java concepts
Method : group of programming statements that is given a name.
Class: named group of related methods.

9. My First Program public class MyFirstClass {
public static void main ( String args[] ) {
System.out.println( “Hi there” );
System.out.println( “Enjoy the show!” ); }

10. main method public static void main ( String args[] ) {
System.out.println( “Hi there” );
System.out.println( “Enjoy the show!” ); }

11. How can a program perform output?
System.out.println( “Hi there” );

12. System.out.println
Java programs work by having things called objects perform actions
System.out: an object used for sending output to the screen
The actions performed by an object are called methods
println: the method or action that the System.out object performs to print specified character string to the screen

13. System.out.println
Invoking or calling a method: When an object performs an action using a method
Also called sending a message to the object
Method invocation syntax (in order): an object, a dot (period), the method name, and a pair of parentheses
Arguments: Zero or more pieces of information needed by the method that are placed inside the parentheses
System.out.println("This is an argument");

14. blocks { System.out.println( “Welcome!” );
System.out.println( “Enjoy the show.” ); }

15. Comments Annotation (notes) about the code.
Computer only executes code.
Comments are for human readers of our code.
How/what/why about the code that follows (so comments appear before the code that they describe).

16. Comments
We need a way to tell the compiler that this is a comment and not code (so the compiler can ignore our comment).
// This is a one line comment. /*
This is a block comment.
It may span multiple lines. */
/* Or it may be just a single line. */

17. My First Program /* My first Java program */
public class MyFirstClass {
public static void main ( String args[] ) {
System.out.println( “Hi there” );
System.out.println( “Enjoy the show!” ); }

18. Error Messages Bug: A mistake in a program
The process of eliminating bugs is called debugging
Syntax error: A grammatical mistake in a program
The compiler can detect these errors, and will output an error message saying what it thinks the error is, and where it thinks the error is. E.g. a missing semicolon!

19. Tip: Error Messages
Run-time error: An error that is not detected until a program is run
The compiler cannot detect these errors: an error message is not generated after compilation, but after execution
Logic error: A mistake in the underlying algorithm for a program
The compiler cannot detect these errors, and no error message is generated after compilation or execution, but the program does not do what it is supposed to do

20. Variables and Data types
In math we may say, “Let x be an integer.” Or, “Let a be a real number.”
How can we do this in our code?

21. What is a variable?
The name of some location in memory used to a hold a data value.

22. Data types Data may be of different types Some data types: More later!
Different types of data require different amounts of memory
Some data types:
int (an integer)
double (a real number)
String (a string of characters)
char (a single character)
More later!

23. Variable declaration So how do we say, “Let x be an integer?” int x;
This is a variable declaration.
Instructs the compiler to reserve a portion of memory space large enough to hold a particular type of value
Indicates the name by which we refer to that location
x is the name of the variable. We made up this name.
int is a keyword or reserved word. It is part of the Java language. We can’t use int for anything else (like variable names).
We may (and need) only declare x once.

24. Variable declarations
Every variable in a Java program must be declared before it is used
Variables are typically declared just before they are used or at the start of a block (indicated by an opening brace { )

25. Variable declaration “Let x and y be integers.” int x; int y; Or
int x, y;

26. Variable declaration
“Let x be an integer and height be a real number.”
int x;
double height;
What value does x have?

27. Variable declaration and assignment (=)
//declare vars
int x;
String name; //note uppercase ‘S’
//assign initial values
x = 1;
name = “fred”;

28. Variable declaration and assignment
//declare vars
char ch;
//assign initial values
ch = ‘x’;

29. Variable declaration and assignment
//declare vars
int x;
//assign initial values
x = 12;

30. Variable declaration and assignment in one
int x = 12; //two steps in one

31. Tip: Initialize Variables
A variable that has been declared but that has not yet been given a value by some means is said to be uninitialized
In certain cases an uninitialized variable is given a default value
It is best not to rely on this
Explicitly initialized variables have the added benefit of improving program clarity

32. Tip: Initialize Variables
The declaration of a variable can be combined with its initialization via an assignment statement
int count = 0;
double distance = 55.5;
char grade = 'A';
Note that some variables can be initialized and others can remain uninitialized in the same declaration
int initialCount = 50, finalCount;

33. Primitive Data Types Basic types in java are called primitive types
Integer types:
int: most common
short, byte: for small integers
long: for huge values
Floating point types:
float: roughly 7 digits of precision
double: rouhly 15 digits of precision
char: a single character
boolean: (true or false)

34. Primitive Types

35. Valid variable names
Starts with: a letter (a-z or A-Z), dollar sign($), or underscore(_)
Followed by: zero or more letters, dollar signs, underscores, or digits(0-9)
Case-sensitive! Uppercase and lowercase are different. Rate, rate, and RATE are the names of three different variable
Cannot be any of the reserved names
Can theoretically be of any length

36. Variable names (valid or invalid?)
$$1_1
numberOfBeans
public (reserved word)
1day
peanutbutter&jelly
aZ_b
INT

37. Good variable names Do not use $
Avoid names that are identical other than differences in case
Use meaningful names
Avoid excessive length

38. FirstProgram MyClass String
Naming Conventions
Start the names of variables, methods, and objects with a lowercase letter, indicate "word" boundaries with an uppercase letter, and restrict the remaining characters to digits and lowercase letters
topSpeed bankRate1 timeOfArrival
Start the names of classes with an uppercase letter and, otherwise, adhere to the rules above
FirstProgram MyClass String

39. Identifiers
Identifier: The name of a variable or other item (class, method, object, etc.) defined in a program
A Java identifier must not start with a digit, and all the characters must be letters, digits, the underscore symbol, or the dollar sign

40. public class void static int double if for private
Identifiers
Keywords and Reserved words: Identifiers that have a predefined meaning in Java
Do not use them to name anything else. Examples:
public class void static
int double if for private
Predefined identifiers: Identifiers that are defined in libraries required by the Java language standard
Although they can be redefined, this could be confusing and dangerous if doing so would change their standard meaning
System String println

41. Next time:
Assignment and Arithmetic expressions