'class' Object

The class is the foundation upon which the entire Java language is built. class defines the nature of an object and forms the basis for object-oriented programming in Java.

'class' Fundamentals

class created using keyword class is a template that defines the form of an object. It specifies both the data and the code that will operate on that data.

Objects are instances of a class. Thus, a class is essentially a set of plans that specify how to build an object. A class is a logical abstraction. It is not until an object of that class has been created that a physical representation of that class exists in memory.

Methods and Variables that constitute a class are called members of the class. The data members are also referred to as instance variables.

class classname {
	// declare instance variables
	type var1;
	type var2;
	// ...
	type varN;
	
	// declare methods
	type method1(parameters) {
		// body of method
	}
	
	type method2(parameters) {
		// body of method
	}
	
	// ...
	type methodN(parameters) {
		// body of method
	}
}

IMPORTANT

A well-designed class should define one and only one logical entity. a well-designed class groups logically connected information.

General form of a class does not specify a main( ) method which is required only if that class is the starting point for your program. Also, some types of Java applications don’t require a main( ).

Defining a Class

A data-only class.

class Vehicle
{
	int passengers;
	int fuelcap;
	int kpl;
}

// To actually create a Vehicle object
Vehicle minivan = new Vehicle(); 

// create a Vehicle object called minivan

Every Vehicle object will contain its own copies of the instance variables passengers, fuelcap, and kpl. dot (.) operator object.member is used to access these variables.

The dot operator links the name of an object with the name of a member. minivan.fuelcap = 16;

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class Vehicle
{
	int passengers;
	int fuelcap;
	int kpl;
}

class VehicleDemo
{
	public static void main(String[] args)
	{
		Vehicle minivan = new Vehicle();
		int range;
		minivan.passengers = 7;
		minivan.fuelcap = 16;
		minivan.kpl = 21;
		
		range = minivan.fuelcap * minivan.kpl;
		System.out.println("Minivan can carry " + minivan.passengers + " with a range of " + range);
	}
}

Both the Vehicle and VehicleDemo classes can be in the same source filecalling it VehicleDemo.java. This name makes sense because the main() method is in the class called VehicleDemo.

The Java compiler automatically puts each class into its own .class file when it is compiled using javac. Two .class files will be created, one for Vehicle and one for VehicleDemo.

NOTE

It is not necessary for both the Vehicle and the VehicleDemo class to be in the same source file. You could put each class in its own file, called Vehicle.java and VehicleDemo.java, respectively. If you do this, you can still compile the program by compiling VehicleDemo.java.

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How objects are created

The new operator dynamically allocates (at run time) memory for an object and returns a reference to it. This reference is the address in memory of the object allocated by new. This reference is then stored in a variable.

Thus, in Java, all class objects must be dynamically allocated.

Vehicle minivan; // declare reference to object

minivan = new Vehicle(); // allocate a Vehicle object

• First line declares minivan as a reference to an object of type Vehicle. Thus, minivan is a variable that can refer to an object, but it is not an object itself.

• Next line creates a new Vehicle object and assigns its reference to minivan. Now, minivan is linked with an object.

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Reference variables and assignment

In an assignment operation, object reference variables act differently than do variables of a primitive type, such as int.

Assigning one primitive-type variable to another, variable on the left receives a copy of the value of the variable on the right.

Assigning one object reference variable to another, changes the object that the reference variable refers to.

Vehicle car1 = new Vehicle();

vehicle car2 = car1;

car1.mpg = 26;

System.out.println(car1.mpg);

System.out.println(car2.mpg);

// 26.

car1 and car2 will both refer to the same object. Thus, the object can be acted upon by either car1 or car2.

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