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Advanced Java Programming

https://edu.gcfglobal.org/en/computerbasics/about-this-tutorial/content/

Introduction


Class
The class is at the core of Java. It is the logical construct upon which the entire Java language is built because it defines the shape and nature of an object. As such,the class forms the basis for object-oriented programming in Java. Any concept you wish to implement in a Java program must be encapsulated within a class.
Perhaps the most important thing to understand about a class is that it defines a new data type. Once defined, this new type can be used to create objects of that type. Thus, a class is a template for an object, and an object is an instance of a class. Because an object is an instance of a class, you will often see the two words object and instance used interchangeably.

The general form of class
class classname {
type instance-variable1;
type instance-variable2;
// ...
type instance-variableN
; type methodname1(parameter-list) {
// body of method1
}
type methodname2(parameter-list) {
// body of method2
}
// ...
type methodnameN(parameter-list) {
// body of methodN
}
}

The data, or variables, defined within a class are called instance variables. The code is contained within methods. Collectively, the methods and variables defined within a class are called members of the class.

Example of Class
class Box {
double width;
double height;
double depth;
// compute and return volume
double volume() {
return width * height * depth;
}
// sets dimensions of box
void setDim(double w, double h, double d) {
width = w;
height = h;
depth = d;
}
}
class BoxDemo {
public static void main(String args[]) {
Box mybox1 = new Box();
Box mybox2 = new Box();
double vol;
// initialize each box
mybox1.setDim(10, 20, 15);
mybox2.setDim(3, 6, 9);
// get volume of first box
vol = mybox1.volume();
System.out.println("Volume is " + vol);
// get volume of second box
vol = mybox2.volume();
System.out.println("Volume is " + vol);
}
}

Creating Objects
When you create a class, you are creating a new data type. You can use this type to declare objects of that type. However, obtaining objects of a class is a two-step process. First, you must declare a variable of the class type. This variable does not define an object. Instead, it is simply a variable that can refer to an object. Second, you must acquire an actual, physical copy of the object and assign it to that variable. You can do this using the new operator. The new operator dynamically allocates (that is, allocates at run time) memory for an object and returns a reference to it. This reference is, more or less, the address in memory of the object allocated by new.This reference is then stored in the variable. Thus, in Java, all class objects must be dynamically allocated. Box mybox = new Box();
This statement combines the two steps just described. It can be rewritten like this to show each step more clearly:
Box mybox; // declare reference to object
mybox = new Box(); // allocate a Box object

Constructors
Java allows objects to initialize themselves when they are created. This automatic initialization is performed through the use of a constructor. A constructor initializes an object immediately upon creation. It has the same name as the class in which it resides and is syntactically similar to a method. Once defined, the constructor is automatically called immediately after the object is created, before the new operator completes.

/* Here, Box uses a constructor to initialize the dimensions of a box.*/
class Box {
double width;
double height;
double depth;
// This is the constructor for Box.
Box() {
System.out.println("Constructing Box");
width = 10;
height = 10;
depth = 10;
}
// compute and return volume
double volume() {
return width * height * depth;
}
}
class BoxDemo6 {
public static void main(String args[]) {
// declare, allocate, and initialize Box objects
Box mybox1 = new Box();
Box mybox2 = new Box();
double vol;
// get volume of first box
vol = mybox1.volume();
System.out.println("Volume is " + vol);
// get volume of second box
vol = mybox2.volume();
System.out.println("Volume is " + vol);
}
}

When this program is run, it generates the following results:
Constructing Box
Constructing Box
Volume is 1000.0
Volume is 1000.0


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