Today…
• “Hello World” ritual.
• Brief History of Java & How Java Works.
• Introduction to Java class structure.
• But first, next slide shows Java is No. 1
programming language for 2015. (From
Tiobe.com).
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Over Time:
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A Very Brief History of Java
• The language was first developed by James
Gosling at Sun Microsystems in 1991.
– He was designing a language, called “Oak”, for the
“Green Project”.
– The Green Project envisaged the centralized control of
many processor-based devices in the home.
− “Oak” was designed to be a
robust, efficient language with
maximum portability to different
processors.
− The Green Project flopped…
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Java History, Cont.
• What else happened in the early 90’s?
• Internet use started to blossom in the early 90’s.
Web pages had to do more than just display
static text and graphics.
• Needed dynamic and interactive content.
• But, web pages are viewed on a wide variety of
platforms, from Mac’s to Unix to IBM-PC’s.
• So any page-embedded language would need to
run on all these platforms.
• Needed a robust, compact, multiplatform
language, so let’s dust off Oak and call it
something racy like “Java”!
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Java History, Cont.
• In 1994, Sun demonstrated the use of Java in
small bundles of code embedded in a web page called applets.
• Netscape browsers started supporting applets in
1995, starting Java’s rise to fame.
• Sun programmers continued to develop a code
base for the language, adding many libraries.
• They showed that Java could be used for more
than just applets, and that full-blown applications
could be written in this high-level language.
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How Java Works
• The Java language standard (the syntax) is
identical for all platforms.
• A compiler ( part of the “JDK”, or “Java
Development Kit” – sometimes called javac.exe)
which is designed to run on your development
platform, compiles your source code (*.java file) to
a byte code file (*.class file).
• The byte code file is platform-independent, and is
the thing you attach to your web page as an
applet.
• Every browser written for every platform and OS,
can have an embedded code processor called a
JVM, or “Java Virtual Machine”, built-in.
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How Java Works, Cont.
• The JVM takes the byte code and executes it by
generating the machine code that will be
recognized by the platform that is running the
browser.
Local Client
Remote File Server
Browser
HTML File
Internet
Applet
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Applet
Byte code files
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JVM
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How Java Works, Cont.
• Of course it did not take long before people took
the JVM out of the browser so that they could run
stand-alone Java applications. This is the JRE or
“Java Runtime Engine” (java.exe).
• The concept of write once, run anywhere is very
appealing! “Save your $$!”
• And, Oracle distributes the JDK’s for free, making
development on many platforms inexpensive.
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How Java Works, Cont.
• However, all IDE’s, including Eclipse, must use
the appropriate JDK in the background.
• Two components of the JDK are the programs
“javac.exe” and “java.exe”.
• javac.exe is the byte code compiler, and java.exe
is the JRE which executes the byte code file.
• “Compilation” is the process of converting the
*.java file to a *.class file (the byte code file). This
is done by calling javac.exe in the background,
and supplying that program with all the required
command line parameters.
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How Java Works, Cont.
• The java.exe program:
–
–
–
–
–
accepts the byte code file,
links in any required libraries,
creates executable code in memory
converts it to machine language
and sends it to the CPU.
• The java.exe program must know the right
machine language commands for just the type of
CPU it is designed for!
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OOP in Java
• A class definition or “object definition”, consists of the
definition of instance variables and/or methods.
• By convention, instance variables are all declared before
the methods:
public class ShowStructure {
// instance variables or “attributes” here
// methods here
} // end class ShowStructure
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OOP in Java – Cont.
• In Java, a class is an Object, and an Object is a
class (rather Zen is it not!)
• A variable can be used to address an instance of
a class, which is also an Object.
• Code and attributes cannot be defined outside of
a class.
• The only code that can exist outside a method are
attributes or other (“inner”) class definitions.
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Attributes
• Also called “class variables” or “instance
variables” or “fields”.
• Declared within a class at the same level as the
method declarations.
• These variables are known to all methods within a
class (their “scope”).
• You can control their privacy and the way they are
stored in memory (using
public/private/protected and static).
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Attribute Declaration
• Syntax:
[private|public] [static] [final] type
attributeName [= literalValue];
• Note that the type part is not optional – this is
why java is a “declarative” language.
• And, a variable cannot change its type later
(“static typing”).
• You cannot use a variable unless you have
declared it first.
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Variable Declaration
• Declaring a variable inside a method gives that
variable the scope of just inside the method, not
outside the method.
• Generally a variable is only available inside the
block ({…}) in which it is declared.
• The syntax for declaration inside a method is the
same except you don’t need the
[private|public] [static] [final] parts.
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Method Declaration (a “Header”)
• The syntax for simple method declaration:
[private|public] [static] [final] returnType
methodName ([parameterList]) {…}
• If main invokes methods or uses attributes in the
same class as itself then those attributes and
methods must also be declared static.
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Method Declaration - Cont.
• A method must have a returnType.
• The returnType can be any single Object or a
primitive type. (For example: an int, double,
String, an array (like int[], or any other predefined Object.)
• If the method does not return anything, then the
keyword void is used instead.
• The main method does not return any value, so
that’s why it is declared as in:
public static void main (String[] args) {…}
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Aside - The main Method
• For the JVM to run an application, it must know
where to start.
• By design, the starting point is always the
execution of the main method.
• The JVM expects the main method to be
declared exactly as shown – the only thing you
can change is the name of the String array, called
args above.
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Method Declaration - Cont.
• parameterList provides a means of passing
items, or parameters, into a method.
• It is optional.
• It can consist of one or many parameters,
separated by commas.
• Each parameter type must be declared in the
parameter list, as in type variableName,
type variableName, …
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