先创建一个Point类，然后定义Triangle类。在Triangle类中定义三个Point的实体来表示一个三角形的三个顶点

package allTest;
class Point {
    double x,y;
    public Point(double x,double y){
        this.x = x;
        this.y = y;
    }
}
class Triangle {
     Point point1,point2,point3;
    private double a,b,c;
    public Triangle(Point point1,Point point2,Point point3){
        this.point1 = point1;
        this.point2 = point2;
        this.point3 = point3;
    }
    public void Getlength(){
        a = Math.sqrt(Math.pow((point1.x-point2.x),2)+Math.pow((point1.y-point2.y),2));
        b = Math.sqrt(Math.pow((point2.x-point3.x),2)+Math.pow((point2.y-point3.y),2));
        c = Math.sqrt(Math.pow((point3.x-point1.x),2)+Math.pow((point3.y-point1.y),2));
    }
    public double GetArea(){
        Getlength();
        double m = (a+b+c)/2.0;
        return Math.sqrt(m*(m-a)*(m-b)*(m-c));
    }
    public double Getall(){
        Getlength();
        return a+b+c;
    }
}
public class Test_5_2Main {
    public static void main(String[] args) {
        Point p1 = new Point(0,0);
        Point p2 = new Point(3,0);
        Point p3 = new Point(0,4);
        Triangle t1 = new Triangle(p1,p2,p3);
        System.out.println("周长："+t1.Getall());
        System.out.println("面积："+t1.GetArea());
    }
}