Download L7_Intro to Java

The Java Programming Language
import java.util.*;
public class Greeter1
{
public static void main (String [] args)
{
Date currentDate = new Date();
String today = currentDate.toString();
Screen theScreen = new Screen();
theScreen.println ("Welcome! today, " +
today + ", you begin to study Java!");
}
}
1
Introduction to Java Application
Programs
• Java is an object oriented programming language
 Uses objects to carry out the tasks
 Sends messages to the objects to perform the
tasks
 Objects interact with each other to do the tasks
 An actual object is called an instance of a class
• The class is the declaration of or blueprint
for the object
• Object oriented programs:
 A collection of object interactions that solve a
problem
1
Java Program Structure
• In the Java programming language:
 A program is made up of one or more
classes
 A class contains one or more methods
 A method contains program statements
1
Lincoln.java
//********************************************************************
// Lincoln.java
// Demonstrates the basic structure of a Java application.
//********************************************************************
public class Lincoln
{
//----------------------------------------------------------------// Prints a presidential quote.
//----------------------------------------------------------------public static void main (String[] args)
{
System.out.println ("A quote by Abraham Lincoln:");
System.out.println ("Whatever you are, be a good one.");
}
}
1
The Class
• A class is the basic building block of an objectoriented language
• Is a template that describes the data and behavior
associated with instances of that class
• Each class represented by a single file
• When you instantiate a class you create an object
that looks and feels like other instances of the
same class
• The data associated with a class or object is
stored in variables
• The behavior associated with a class or object is
implemented with methods
1
Java Program Structure
//
comments about the class
public class MyProgram
{
class header
class body
Comments can be placed almost anywhere
}
1
Class Definition Block
• In the Java language, the simplest form of a class
definition is
• public class name { . . . } The keyword class begins
the class definition for a class named name. The
variables and methods of the class are embraced
by the curly brackets that begin and end the class
definition block.
public class MyProgram
{
}
1
The Method
• Java lets a programmer break down complex tasks
into simpler units called methods
• A method definition must occur inside a class
• A method can occur anywhere inside the class
although custom places all the other methods of a
class before the main method.
• The method body consists of one or more method
variables and program statements
1
Java Program Structure (methods)
// comments about the class
public class lotteryodds
{
//
comments about the method
method header
public static void long lotteryodds (int high, int number )
{
long r = 1;
int i;
for (i = 1; i <= number; i++)
{ r = r * high/i;
method body
high--;
}
}
}
return r;
1
signature or method signature
• What variable types and in what order they are passed to a
method.
The return type is not a part of the signature.
In the following method "String, double" is the signature:
public int returnAnInt (String stringIn, double doubleIn)
{
int intToReturn = 1;
if (stringIn.equals(String.valueOf(doubleIn))
{
intToReturn = 2;
} return intToReturn;
}
1
Main Method
• Every Java application must contain a main
method whose signature looks like this
 public static void main(String[] args)
• The method signature for the main method
contains three modifiers:
 public indicates that the main method can be called by
any object.
 static indicates that the main method is a class method.
 void indicates that the main method doesn't return any
value.
1
Java Program Structure (methods)
//
comments about the class
public class MyProgram
{
//
comments about the method
public static void main (String[] args)
{
method body
method header
}
}
1
How main Gets Called
• When the Java interpreter executes an application
(by being invoked upon the application's
controlling class), it starts by calling the class's
main method. The main method then calls all the
other methods required to run your application. If
you try to invoke the Java interpreter on a class
that does not have a main method, the interpreter
refuses to run your program and displays an error
message
1
Comments
• Comments in a program are called inline
documentation
• They should be included to explain the purpose
of the program and describe processing steps
• They do not affect how a program works
• Java comments can take three forms:
// this comment runs to the end of the line
/*
this comment runs to the terminating
symbol, even across line breaks
/** this is a javadoc comment
*/
*/
1
Identifiers
• Identifiers are the words a programmer uses in a
program
• An identifier can be made up of letters, digits, the
underscore character ( _ ), and the dollar sign
• Identifiers cannot begin with a digit
• Java is case sensitive - Total, total, and
TOTAL are different identifiers
• By convention, programmers use different case
styles for different types of identifiers, such as
 title case for class names - Lincoln
 upper case for constants - MAXIMUM
1
Identifiers
• Sometimes we choose identifiers ourselves when
writing a program (such as Lincoln)
• Sometimes we are using another programmer's
code, so we use the identifiers that they chose
(such as println)
• Often we use special identifiers called reserved
words that already have a predefined meaning in
the language
• A reserved word cannot be used in any other way
1
Reserved Words
• The Java reserved words:
abstract
boolean
break
byte
case
catch
char
class
const
continue
default
do
double
else
enum
extends
false
final
finally
float
for
goto
if
implements
import
instanceof
int
interface
long
native
new
null
package
private
protected
public
return
short
static
strictfp
super
switch
synchronized
this
throw
throws
transient
true
try
void
volatile
while
1
White Space
• Spaces, blank lines, and tabs are called white
space
• White space is used to separate words and
symbols in a program
• Extra white space is ignored
• A valid Java program can be formatted many ways
• Programs should be formatted to enhance
readability, using consistent indentation
1
Syntax and Semantics
• The syntax rules of a language define how we can
put together symbols, reserved words, and
identifiers to make a valid program
• The semantics of a program statement define what
that statement means (its purpose or role in a
program)
• A program that is syntactically correct is not
necessarily logically (semantically) correct
• A program will always do what we tell it to do, not
what we meant to tell it to do
1
Errors
• A program can have three types of errors
• The compiler will find syntax errors and other
basic problems (compile-time errors)
 If compile-time errors exist, an executable version of the
program is not created
• A problem can occur during program execution,
such as trying to divide by zero, which causes a
program to terminate abnormally (run-time errors)
• A program may run, but produce incorrect results,
perhaps using an incorrect formula (logical errors)
1
Basic Program Development
Edit and
save program
errors
errors
Compile program
Execute program and
evaluate results
1