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Humans classify things all the time.
It helps us from drowning in details:
"What vehicle ran
over you, madam?"
"A blue Buick
four-door sedan."
rather than:
"What vehicle ran
over you, madam?"
"Why, officer,
it had a blue body, four wheels, four doors, six windows,
a front, back and two sides of some kind of metal. There
were four people sitting inside it, and one of them was
driving. Each door had a window, a handle, and
pinstripes, was about five feet high and four feet wide.
In front, there was a shiny metal grille, and ..."
For example:
- A Dog is a distillation
of the common attributes and behaviors of dogs of all
sizes, types, and ferocities.
- A Car is a similar
distillation applied to cars of all sizes, colors,
speeds, styles, makes, etc.
- A Nut describes common
attributes of Filberts, Brazils, Almonds, Macadamias,
etc. Or it describes the attributes of the kind of Nut that attaches
to a Bolt (or perhaps even yours truly...)
- A Road describes the
common attributes of dirt tracks, town streets,
county roads and superhighways.
- A Student encompasses
those of types young and old, single and married,
male and female, tall and short, etc.
- A College describes
Technical Colleges, Liberal Arts Colleges, Colleges
of Art and Design, Universities, etc.
To perform this kind of
classification, we need to distill out:
- Attributes or Properties
(has fur, four legs, a tail, sharp teeth, etc.)
- Behaviors or Operations
(barks, walks, runs, wags tail, etc.)
Here's an example of a Java class:
public class Dog
{
public void StartBarking() { /* ... */ }
public void StopBarking() { /* ... */ }
public int getAge() { return m_age; }
public int getColor() { return m_color; }
public int getTemperament() { return m_temperament; }
public static final int DOCILE = 1;
public static final int GOOD_WITH_CHILDREN = 2;
public static final int VICIOUS = 3;
int m_age;
int m_color;
int m_temperament;
}
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Here's another example:
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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Note: Think of a class as a template, or a recipe
from which to make instances of that class.
Note: It's hard to
tell an abstract concept "Dog" to go fetch!
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