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Let's show the Circle class again:
import java.lang.Math;
public class Circle
{
// Accessor methods
public double getX() { return m_x; }
public double getY() { return m_y; }
public double getRadius() { return m_r; }
// Mutator methods
public void setX(double v) { m_x = v; }
public void setY(double v) { m_y = v; }
public void setRadius(double r) { m_r = r; }
// Operations/Attributes
public double getCircumference()
{ return 2 * Math.PI * m_r; }
public double getArea()
{ return Math.PI * m_r*m_r; }
double m_x, m_y; // coordinates of center
double m_r; // radius
}
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The above methods are actually instance
methods.
This means that each method 'knows' which
instance it is being called against.
For example, if we have code like:
Circle c1 = new Circle();
Circle c2 = new Circle();
double r1 = c1.getRadius();
double r2 = c2.getRadius();
the calls to each getRadius() method will return the size of the radius for
its respective instance.
The code for getRadius() looks like:
public class Circle
{
// ...
public double getRadius() { return m_r; }
// ...
}
which could have been written:
public class Circle
{
// ...
public double getRadius() { return this.m_r; }
// ...
}
Essentially, the call to the method:
double r1 = c1.getRadius();
appears as if it were being called like this:
double r1 = getRadius(c1);
and as if the method were defined:
public double getRadius(Circle this) { return this.m_r; }
In other words, it is as if there is a hidden parameter
being implicitly passed to the function.
this:
- is an implicitly declared variable
- is local to each instance method
- references the instance against which
the method was invoked
- is normally implicit; however, there are times when you need to
use this explicitly (see
later).
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