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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 is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
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 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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