8.7 Is it possible to include other classes as instance or static members?

A. Introduction

If you observe the triangle.java in 8.6 carefully, the repetition of x and y coordinates of each vertex almost makes you to ask: is it possible to include a Vertex or Point class as a member of the Triangle class?

  private int Ax;
  private int Ay;
  private double ABlength;
  private int Bx;
  private int By;
  private double BClength;
  private int Cx;
  private int Cy;
  private double CAlength;

The answer is YES. Not only as instance members, but also as static members as necessary. This is called class inclusion, as shown below:

public class Point {
  double x; 
  double y;
  public Point(){ //empty constructor     
  }
  public Point(double i, double k){ //full constructor
    this.x = i; //use i & k because the formal parameters are general and hold no importance to code
    this.y = k;
  }
   public double getX(){ //getter or accessor
       return x;
  }
   public void setX(double i){ //setter or mutator
       x = i;
    }
    public double getY(){ //getter
       return y;
  }
   public void setY(double k){ //setter
       y = k;
    }  
   public double distance (Point aPoint){
     return Math.sqrt(Math.pow(this.x-aPoint.x,2)+Math.pow(this.y-aPoint.y,2));
   }
  
   public String toStr(){ // it is NOT called by System.out.println() by default
        return "(" + x + ", " + y +")";
   }  
   public String toString(){ // it is called by System.out.println() by default
        return "(" + x + ", " + y +")";
   } 
   public boolean equals(Point p){ // to compare two Point objects
        return p.x==x&&p.y==y;
   }
  
}

class TriangleByVertex {
  Point a;
  Point b;
  Point c;
  
  public double perimeter() {
    return a.distance(b)+b.distance(c)+c.distance(a);   
  }
  public double area() {
    double s = 0.5 * perimeter();
    return Math.sqrt(s*(s-side(b,c))*(s-side(a, c))*(s-side(a, b)));   
  }
  public double side(Point x, Point y){
    return x.distance(y);
  }
  
  public double angle(Point x){
    double cos;
    if(x.equals(a)) // use equals() method of Point class to check if two points are the same
      cos=(side(a,b)*side(a,b)+side(a,c)*side(a,c)-side(b,c)*side(b,c))/(2*side(a,b)*side(a,c));
    else if(x.equals(b)) 
      cos=(side(b,c)*side(b,c)+side(a,b)*side(a,b)-side(a,c)*side(a,c))/(2*side(b,c)*side(a,b));
    else if(x.equals(c))
      cos=(side(a,c)*side(a,c)+side(b,c)*side(b,c)-side(b,a)*side(b,a))/(2*side(a,c)*side(b,c));
    else {
      System.out.println("This vertex does not below the triangle!");
      return -1;
    }
    return Math.acos(cos);
  }
}