Transcript java basics - Brookwood High School

JAVA BASICS:
Variables and
References
SYNTAX, ERRORS, AND
DEBUGGING
Vocabulary & Terms
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Computer programming
Machine language
Machine code
Compiler
Source code
Integrated Development Environment (IDE)
Statement
Bytecode
Java virtual machine (JVM)
Class
Object
Reference
Command
Method
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College Board
Computer Science A Topics Covered
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Program Design - Read and understand a
problem's description, purpose, and goals.
Program Implementation - Console output
(System.out.print/println)
Program Analysis - Identify and correct
errors.
Computing Context - Major hardware
components; Language
translators/compilers ; Virtual machines
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How Java Works!
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You create a source document using an established protocol (in
our case, the Java language).
Then your program is run through a source code compiler. The
source code compiler checks for errors and won’t let you compile
until it’s satisfied that everything will run correctly.
The compiler creates a new document,coded into Java bytecode.
Any device capable of running Java will be able to
interpret/translate this file into something it can run. The compiled
bytecode is platform independent.
The Java Virtual Machine (JVM) translates the bytecode into
something the underlying platform understands, and runs your
program.
The next slide shows an illustration of this sequence.
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How Java Works!
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What is Computer Programming?
Before we jump into Java, let’s talk a bit about
exactly what computer programming is.
 Computer programming is the art of creating a
set of instructions, called a computer program,
for a computer.
 Computers have no judgment, so instructions
must be detailed and unambiguous!
Unfortunately, creating complex sets of
unambiguous instructions is not easy. It requires
both training and practice.
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public class BareBones
{
}
// end class BareBones
All Java programs start with a class.
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public class CompSci
{
public static void main( String args [] )
{
System.out.println(“Java Rules!");
}
OUTPUT
}
Java Rules!
Type this program in Dr. Java and run it. Then change String args [] to
String [] args and run the program again. You should notice no change!
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public class CompSci
{
public static void main( String args [] )
{
System.out.println(“Java Rules!");
}
}
Every method and every class must have an
opening ( { ) brace and a closing ( } ) brace.
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public class CompSci
{
public static void main(String args[])
{
System.out.println(“Java Rules!");
}
}
Every program statement is terminated with
a semi-colon ( ; ).
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Never put a ;
before an open { brace
;{ //illegal
}; //legal
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public class CompSci
{
public static void main(String args[])
{
System.out.println(“Java Rules!");
}
}
Indent all code 3 spaces to make it easier to read.
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What is a Convention?
In Java, a convention is an accepted
practice, not necessarily a rule.
 Indenting 3 spaces is a Java convention
that you will see in many textbooks. In
reality, the compiler doesn’t really care
about spacing!
 Spacing is for the human readers, not the
computer.
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Conventions For Use In This Class
You should download and print out a copy
of the document Conventions, which is the
next link.
 You must follow the conventions listed on
this document in every program that you
turn in for a grade!
 You will lose points on your programs if
you do not follow these conventions!
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A reference variable refers to a location in
memory that stores an object.
Monster fred = new Monster();
Monster sally = new Monster();
In the above example Monster is a class.
fred and sally are references to Monster objects.
new is a keyword which is used to create an object. It is followed by
the constructor method, which is the class name, and parenthesis.
The first part: Monster fred – declares a reference variable to hold a
Monster object.
The second part: = new Monster(); - creates the Monster object, and
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assigns fred to the location where this object is in memory.
Monster fred = new Monster();
fred
0xF5
0xF5
Monster
Monster fred creates a reference.
new Monster(); creates the Monster at location
0xF5.
The assignment operator (=) gives fred the
address 0xF5 to store
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What is a variable?
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A variable is a storage location for some type of
value. Type the statements below into the
interactions pane. Type the name of the variable to
see what is stored in the variable. What is stored in
yesOrNo?
int days = 102;
double taxRate = 7.75;
boolean yesOrNo;
days
taxRate
102
7.75
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yesOrNo
int days = 102;
days
102
int days – declares days as a variable that
will store integer values.
= 102 assigns days to store the integer
value 102.
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How do you name
variables when
defining them?
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When naming a variable follow these rules and conventions:
• Make the variable name meaningful. That means that L is not a
meaningful variable name but length is meaningful. When reading
your program, I shouldn’t be guessing what the variable is meant to
hold.
• Start all variable names with a lower-case letter.
• Variable names may also contain letters, numbers and underscores.
• If a variable name is more than one word long, capitalize each of the
other words without adding any spaces.
Below are some examples of valid variable names:
number sum32 testAverage area_Of_Trapezoid
and below are some examples of invalid variable names:
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Science A
2ndValue test Average
question#2
Which of these would be legal
variable identifiers?
int 1stYear;
double jump Up;
double feet2Inches;
IDENTIFIER is a fancy
word for variable.
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Java is case sensitive – it interprets
upper and lower case letters as
different letters.
Brandon does not equal brandon.
Brandon != brandon
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Keywords are reserved words that the
language uses for a specific purpose.
You cannot use keywords as identifier names.
Some that you’ve seen are:
int double return void main public
static long break continue class
You can find the complete list of keywords here:
http://java.sun.com/docs/books/tutorial/java/nutsandbolts/_keywords.html
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