Objects

Java is Object-oriented

Java is object-oriented, which allows overall program design to focus on what is being dealt with rather than on how the program operates. For example, a car race game application would be designed using Car objects – where each Car object has specific attribute values and actions which it can perform. A banking program would be designed using Account object, where each Account object would have attributes (perhaps a name, account number and balance), and be able to perform actions upon itself (perhaps deposit, withdraw, check balance).

An object-oriented design is most useful for programming in sophisticated projects because one can break things down into understandable pieces. Additionally, these pieces can easily be reused in another project, reducing the cost of software development.

What is an Object?

By definition, an object is an entity with attributes and behavior. That means that an object has specifics by which it can be described (attributes) and can perform actions (behavior). The world is comprised of objects.

For example, many people have a car. Cars are objects. A car has attributes by which it can be described: it’s color, make, model, quantity of gas it holds. A car also has actions it can perform: drive forward, drive backward, fill gas, display gas level.

Joe and Mary both have cars, which are described in the object diagrams below. Note that both objects have the same attributes, but each object has different values for those attributes. Both cars can perform the same actions.

Joe’s car	Mary’s car
color = blue make = Honda model = Accord gas = 20.5	color = red make = Ford model = Taurus gas = 35.1

Joe’s car and Mary’s car are objects of the same type. That is, they have the same attributes and the same behavior (as opposed to, say, a boat). Joe’s car and Mary’s car are objects of the same class.

Another example would be a dog. I have a dog, my neighbor has a dog. All dog’s have

a breed, a weight, a name. These attributes of a dog have values which describe the dog. Dog’s can perform actions such a run, sleep, eat. My dog and my neighbor’s dog are both object of the same type, or class.

What is a Class?

A class is a template, or blue print, which describes an object. It specifies the attributes and actions which are associated with a particular type of object. In Jave, once a class has been defined, one or more objects of that class type can be created.

Let’s suppose that we were writing a program which was going to manipulate cars. Our program would need to be able to create and manipulate Car objects. The program would need to define a Car class. Then objects of this type (say Joe’s car and Mary’s car) could be created.

The following code creates a simple Car class. Don’t worry if you don’t understand everything in this example, just get the general idea:

public class Car {

// class variables (attributes)

private String color;

private String make;

private String model;

private double gas;

// class methods (behaviors)

public Car (String col, String mk, String mod, double galofgas){

color = col;

make = mk;

model = mod;

gas = galofgas;

}

public void moveForward(double miles){

gas = gas - miles/18.0;

}

public void moveBackward(double miles){

gas = gas - miles/18.0;

}

public void fillGas(double gallons) {

gas = gas + gallons;

}

public double howMuchGas () {

return gas;

}

} //end class Car

The Car class above indicates that each object of type car:

· has a specific color, make, model, and quantity of gas

· can be created if the above attribute values are provided

· can move forward

· can move backward

· can be filled with more gas

· can report the quantity of gas it has

The Car class above is not a Car object, but only describes what variables (attributes) and methods (actions) are associated with a Car object, if one is created.

To actually create a Car object (an instance of the Car class) and use it’s behavior we need to declare a variable of class Car, create the Car and then call upon it’s methods:

//create marys car object

Car marys;

marys = new Car( “red”, “Ford”, “Taurus”, 35.1 );

// create joes car object

Car joes;

joes = new Car( “blue”, “Honda”, “Accord”, 20.1 );

//these objects now perform actions

marys.moveForward(17);

joes.moveBackward(.002);

joes.fillGas(15.0);

System.out.println(“Mary’s gas level is” + marys.howMuchGas() );