Writing Classes and Instance Methods

Instance methods allow us to define the behavior of an object. Lets say you are wanting to get the area of a specific rectangle object you created using the Rectangle class. To do this, you would need to define an instance method that will return the area of our rectangle object. It is important to remember that instance methods belong to the specific instance of your object.

Creating Instance Methods

The general form of an instance method is as follows:

[visability] [returnType] name(parameters)
  public       double    getArea()
  private       int      truncateArea()

The visibility of your method determines if it can be used outside your object's class. This is usually public or private.

The returnType is the type of our return value. If there is no return value for the method we use void.

The name is the name of our method.

The parameters is the list of our parameters, or the information we give to the method.

Here is what our Rectangle class looks like:

public class Rectangle
{
    // Instance variables
    private double width;
    private double height;

    public Rectangle(double rWidth, double rHeight)
    {
        width = rWidth;
        height = rHeight;
    }

    // Our instance method
    public double getArea()
    {
        return width * height;
    }
}

Using Instance Methods

To call instance methods we use the following format: objectName.methodName(parameters); The objectName is the name of the object we are calling the method on. The methodName is the name of the instance method we are calling. The parameters are the inputs we give the method (if any).

In practice, this looks like:

Rectangle rect = new Rectangle(7, 10);

// Get the area of our rectangle by calling instance method `area();`
double rectArea = rect.getArea();