Understanding of Abstract Programming

What is abstract oriented programming , I didn't pay much attention to it the first time , Feeling is not very important , But when I look closely , Abstract oriented functions are so powerful , I should sum it up again .

Abstract oriented programming , seeing the name of a thing one thinks of its function , Is for abstract classes （abstract） Programming for a series of operations . in other words   When designing some important class , Don't make the class specific , It's for abstract classes , namely   Important data in design class yes   Objects of abstract classes , Not an object declared by a concrete class .

Just to make it simple , Is to define a abstract class   and   One abstract  Method , Define one or more abstract Subclasses of classes , Override... In a subclass abstract Class abstract Method （ Methods overridden in subclasses are no longer abstract Method ）, Then you can declare only one abstract class , recycling On the transformation object   Call the method it overrides . In this way, a declared variable can cope with many different changes . 

Let's take a look at the application of an example ：（ Find the volume of the cylinder ）

 abstract Classes and subclasses ：

public abstract class Geometry {
    public abstract double getArea();// Just a simple structure abstract Method , Don't add anything .
}

class Circle extends Geometry{
    double r;
    Circle(double r){// Constructors
        this.r = r;
    }
    public double getArea() {   / / Method rewrite
        return r * r * 3.14;
    }
}
class Rectangle extends Geometry{
    double a,b;
    Rectangle(double a,double b){
        this.a = a;
        this.b = b;
    }
    public double getArea() {
        return a*b;
    }
}

Get the class of column volume , No longer rely on concrete classes , But to Geometry class ：

public class Pillar {

    Geometry bottom;// Statement Geometry object （ Abstract objects ）
    double hight;
    Pillar(Geometry bottom,double h){// Constructors , Get value
        this.bottom = bottom;
        this.hight = h;
    }
    
    public double getVolume(){// Return volume value
        if(bottom == null){
            System.out.println(" No bottom , Unable to calculate area ");
            return 0;
        }
        return bottom.getArea()*hight;// The call here is getArea() The function is based on bottom To determine the transformation object .
    }
}

Create the upper transformation object , Determine its calling function , Output its value

public class Application {

    public static void main(String[] args) {
        Pillar pillar;
        Geometry bottom;
        
        bottom = null;
        pillar = new Pillar(bottom,5);
        System.out.println(" Volume ：" + pillar.getVolume());
        
        bottom = new Circle(2);// On the transformation object
        pillar = new Pillar(bottom,1);
        System.out.println(" Volume ：" + pillar.getVolume());// It's called Circle Inside getVolume().
        
        bottom = new Rectangle(5,2);// On the transformation object
        pillar = new Pillar(bottom,1);
        System.out.println(" Volume ：" + pillar.getVolume());// It's called Rectangle Inside getVolume().
    }
}

The purpose of abstract oriented programming is to cope with the changes of user needs , Separate the code in a class that often needs to be changed due to changes in requirements .

The core of abstract oriented programming is to assign each possible change in the class to a subclass of the abstract class , So that the designer of the class no longer cares about the specific implementation .