《JAVA编程思想》学习备忘（第237页：Reusing Classes）-3

续《JAVA编程思想》学习备忘（第237页：Reusing Classes）-2

Combining composition and inheritance

It is very common to use composition and inheritance together.The following example shows the creation of a more complex class,using both inheritance and composition,along with the necessary constructor initialization:

//:reusing/PlaceSetting.java

//Combining composition & inheritance.

import static net.mindview.util.Print.*;

class Plate{

    Plate(int i){

        print("Plate constructor");

    }

}

class DinnerPlate extends Plate{

    DinnerPlate(int i){

        super(i);

        print("DinnerPlate constructor");

    }

}

class Utenstil{

    Utensil(int i){

        print("Utensil constructor");

    }

}

class Spoon extends Utensil{

    Spoon(int i){

        super(i);

        print("Spoon constructor");

    }

}

class Fork extends Utensil{

    Fork(int i){

        super(i);

        print("Fork constructor");

    }

}

class knife extends Utensil{

    knife(int i){

        super(i);

        print("knife constructor");

    }

}

//A cultural way of doing something:

class Custom{

    Custom(int i){

        print("Custom constructor");

    }

}

public class PlaceSetting extends Custom{

    private Spoon sp;

    private Fork frk;

    private knife kn;

    private DinnerPlate pl;

    public PlaceSetting(int i){

        super(i + 1);

        sp = new Spoon(i + 2);

        frk = new Fork(i + 3);

        kn = new knife(i + 4);

        pl = new DinnerPlate(i + 5);

        print("PlaceSetting constructor");

    }

    public static void main(String[] args){

        PlaceSetting x = new PlaceSetting(9);

    }

}

输出结果：

Custom constructor

Utensil constructor

Spoon constructor

Utensil constructor

Fork constructor

Utensil constructor

kinfe constructor

Plate constructor

DinnerPlate constructor

PlaceSetting constructor

It's rather amazing how cleanly the classes are separated.You don't even need the source code for the methods in order to reuse the code.At most,you just import a package.

 

Guaranteeing proper cleanup

Consider an example of a computer-aided design system that draws pictures on the screen:

//:reusing/CADSystem.java

//Ensuring proper cleanup.

package reusing;

import static net.mindview.util.Print.*;

class Shape{

    Shape(int i){print("Shape constructor");}

    void dispose(){print("Shape dispose");}

}

class Circle extends Shape{

    Circle(int i){

        super(i);

        print("Drawing Circle");

    }

    void dispose(){

        print("Erasing Circle");

        super.dispose();

    }

}

class Triangle extends Shape{

    Triangle(int i){

        super(i);

        print("Drawing Triangle");

    }

    void dispose(){

        print("Erasing Triangle");

        super.dispose();

    }

}

class Line extends Shape{

    private int start,end;

    Line(int start,int end){

        super(start);

        this.start = start;

        this.end = end;

        print("Drawing Line: " + start + ", " + end);

    }

    void dispose(){

        print("Erasing Line: " + start + ", " + end);

        super.dispose();

    }

}

public class CADSystem extends Shape{

    private Circle c;

    private Triangle t;

    private Line[] lines = new Line[3];

    public CADSystem(int i){

        super(i + 1);

        for(int j = 0; j < lines.length; j++)

            lines[j] = new Line(j,j*j);

        c = new Circle(1);

        t = new Triangle(1);

        print("Combined constructor");

    }

    public void dispose(){

        print("CADSystem.dispose()");

        // The order of cleanup is the reverse

        // of the order of initialization:

        t.dispose();

        c.dispose();

        for(int i = lines.length - 1; i >= 0; i--)

            lines[i].dispose();

        super.dispose();

    }

    public static void main(String[] args){

        CADSystem x = new CADSystem(47);

        try{

            // Code and exception handling...

        }finally{

            x.dispose();

        }

    }

}

输出结果：

Shape constructor

Shape constructor

Drawing Line: 0, 0

Shape constructor

Drawing Line: 1, 1

Shape constructor 

Drawing Line: 2, 4

Shape constructor

Drawing Circle

Shape constructor

Drawing Triangle

Combined constructor

CADSystem.dispose()

Erasing Triangle

Shape dispose

Erasing Circle

Shape dispose

Erasing Line: 2, 4

Shape dispose

Erasing Line: 1, 1

Shape dispose

Erasing Line: 0, 0

Shape dispose

Shape dispose

(代续)