Java: VD về lớp trừu tượng (cả giao diện)

1. Class HINH:
public abstract class HINH
{
    public abstract double dienTich();
    public abstract double chuvi();
}

2. Class HinhCau:
public class HinhCau extends HinhTron implements IHinh3D{

    public HinhCau(double b)
    {
        super(b);
    }

    @Override
    public double dienTich()
    {
        return bk*bk*4*PI;
    }

    @Override
    public double chuvi()
    {
        return dienTich();
    }

    @Override
    public double theTich()
    {
        return 4/3*PI*bk*bk*bk;
    }

}

3. Class HinhCN:
public class HinhCN extends HINH {
    double dai,rong;

    public HinhCN(double d, double r)
    {
        dai = d; rong = r;
    }

    @Override
    public double dienTich()
    {
            return dai*rong;
    }

    @Override
    public double chuvi()
    {
            return 2*(dai+rong);
    }
}

4. Class TamGiac:
public class TamGiac extends HINH {
    double c1,c2,c3;

    public TamGiac(double a, double b, double c)
    {
        c1 = a; c2 = b; c3 = c;
    }

    @Override
    public double dienTich()
    {
        double p = (c1+c2+c3)/2;
        return Math.sqrt(p*(p-c1)*(p-c2)*(p-c3));
    }

    @Override
    public double chuvi()
    {
        return (c1+c2+c3);
    }
}

5. Class HinhTron:
public class HinhTron extends HINH implements ITron
{
    protected double bk;

    public HinhTron(double b)
    {
        bk = b;
    }

    @Override
    public double dienTich()
    {
        return bk*bk*PI;
    }

    @Override
    public double chuvi()
    {
        return 2*bk*PI;
    }
}

6. Interface ITron:
public interface ITron {
    public static final double PI = 3.14;
}

7. Interface Hinh3D:
public interface IHinh3D {
    public double theTich();
}

8. Main class:
public class TINHDTICH {
    public static void main(String args[]){

        HINH ds[] = new HINH[5];
        ds[0] = new HinhTron(1.3);
        ds[1] = new TamGiac(3,4,5);
        ds[2] = new HinhCN(2,5);
        ds[3] = new HinhTron(3.0);
        ds[4] = new HinhCN(4,3);

        //Tính tổng diện tích các hình
        double tongDT=0;

        for(int i=0; i<5; i++)
            tongDT += ds[i].dienTich();
        System.out.println("\nTong dien tich: " + tongDT);
    }

}