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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.
Example:
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.
// Line-oriented - comment goes to end of the current line.
/*
block-orientedcan span several lines.
*/
[…] 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.
A field list consists of zero or more field declarations of the form
[static] [final][public | private | protected] field-type field-name [assignment];
A constructor list consists of zero or more constructor definitions of the form
[public | private | protected] class-name ([parameter-list]){
[statement-list;]}
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:
for example,
public double getArea() {
// code ...}
type1 param1, type2 param2, ... , typeN paramN
for example,
private void doSomethingWith(int n, double x, Pizza p) {
// code ...}
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
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