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An introduction to Java syntax covering simple Java expressions and the declaration of concrete classes and methods.


A Java program consists of one or more classes one of them must be public and must have a method with the following signature:

public static void main (String[] args)

Basically, the main() method will instantiate appropriate objects and send them "messages" (by calling their methods) to perform the desired tasks.  The main() method should not contain any complicated program logic nor program flow control. In the example shown below, PizzaClient is the main class with the main() method.

UML Class Diagram of a Java program


public class PizzaClient { /*** Prints the answer to the problem stated in the above.. */public void run() { Pizza round = new Pizza (3.99, new Circle (2.5));Pizza rect = new Pizza (4.99, new Rectangle (6, 4)); PizzaDeal pd = new PizzaDeal();System.out.println(round + " is a better deal than " + rect + ": " + pd.betterDeal(round, rect));} /*** Main entry to the program to find the better deal. * Instantiates an instance of PizzaClient and tells it to run.* This is what all main() should do: instantiates a bunch of objects and * "turn them loose"!* There should be no complicated logic and/or control in main(). * @param nu not used*/ public static void main (String[]nu) { new PizzaClient().run();} }public class Rectangle implements IShape { private double _width;private double _height; /*** Initializes this Rectangle width a given width and a given height * @param w width of this Rectangle,>= 0. * @param h height of this Rectangle,>= 0. */public Rectangle(double w, double h) { _height = h;_width = w; }/** * @returns this Rectangle's area.*/ public double getArea() {return _height * _width; }/** * Overrides the inherited method from class Object.* @returns a String describing a Rectangle with its width and height. */public String toString() { return "Rectangle(h = " + _height + ", w = " + _width + ")";} }

Notes on the toString() method:

toString() is a method that is inherited all the way from the base class, Object. It is the method that the Java system calls by default whenever a string representation of the class is needed. For instance, "This is "+ myObject is equivalent to "This is " + myObject.toString() . DrJava will call an object's toString() method if you type the object's name in the interaction window, without terminating the line with a semicolon. The return value of toString() is what prints out on the next line.

Comments syntax:

// Line-oriented - comment goes to end of the current line.

/* block-orientedcan span several lines. */

Class definition syntax:

[…] means optional .

[public] class class-name [inheritance-specification]{ [field-list;][constructor-list;] [method-list;]


Inheritance Specification looks like

[extends SomeClass] [implements Interface1,..., InterfaceN]

We will discuss the meaning of inheritance in due time.

Java statement syntax:

Each Java statement must terminate with a semi-colon. 

Field list syntax:

A field list consists of zero or more field declarations of the form

[static] [final][public | private | protected] field-type field-name [assignment];


Constructor list syntax:

A constructor list consists of zero or more constructor definitions of the form

[public | private | protected] class-name ([parameter-list]){ [statement-list;]}
The constructor's name must the same as the class name. Constructors are used for initialization of the object during the object's instantiation only.

Method list syntax:

A method list consists of zero or more method definitions of the form

[static] [final][public | private | protected] return-type method-name ([param-list]) { [statement-list;]}

A return type void means the method does not return any value.

param-list can be:

  • empty;

for example,

public double getArea() { // code ...}
  • of a fixed number of parameters

type1 param1, type2 param2, ... , typeN paramN

for example,

private void doSomethingWith(int n, double x, Pizza p) { // code ...}
  • of a variable number of parameters ( this is a new feature for Java 5.0 and upward )
type1 param1, type2 param2, .., typeN... params

In the above, typeN... params is called a variable argument list.  It must appear at the end of the whole parameter list.

for example,

private void doSomethingWith(int n, double x, Pizza... p) { // code ...}

The variable argument list can have zero or more arguments.  Here is a simple example:  use DrJava to define the following class in the definition pane.

public class VarArgExample { /*** returns the number of arguments in the variable argument list. */int argNum(int... nums) { return nums.length; // the var arg list is viewed as an array} }

Compile the above class and in the interactions pane, type the following:

VarArgExample vae = new VarArgExample(); // creates a VarArgExample object vae.argNum() // returns 0vae.argNum(42) // returns 1 vae.argNum(34, -99) // returns 2

Questions & Answers

what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
preparation of nanomaterial
Victor Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Principles of object-oriented programming. OpenStax CNX. May 10, 2013 Download for free at http://legacy.cnx.org/content/col10213/1.37
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