//-----------------------------------------------------------------------------
// 2/24/200 TYan and DIS
// bouncing balls
// Use methods
//-----------------------------------------------------------------------------
class Ball {
    
    // Instance Variables
        char name;
        double pos;
        double vel;

    // THOUGHT1: How to set values of instance variables
    // when instantiating the objects? See constructors.

    // Instance Method

    void updatemotion(double dt, double accel) {

	    pos += vel*dt + accel*dt*dt/2.0;
	    vel += accel*dt;
	    if (pos <= 0.0)
		vel = -vel;

	} // method updatemotion


    // THOUGHT2: how to make accel and dt available to all objects?
    // See static and final for ideas.

} // class Ball



//-----------------------------------------------------------------------------
// balls --not Ball!-- is the target class for the main method
//-----------------------------------------------------------------------------
public class balls_methods {
    public static void main(String[] args) {
        // Allocate and assign 3 ball objects a, b, c
            Ball a = new Ball();
            Ball b = new Ball();
            Ball c = new Ball();

        // Initial Values
            double accel = -0.2;
            a.name = '*'; a.pos = 15.0; a.vel =  4.0;   
            b.name = '+'; b.pos = 28.0; b.vel =   .5; 
            c.name = '#'; c.pos =  3.0; c.vel = -2.0;  
           
        // Repeatedly move and draw balls for limit time units
        // Each loop is a time step lasting dt time units

           double limit = 100.0; // total time for simulation
           double time  =   0.0; // elapsed time
           double dt    =   1.0; // length of each time step

           while (time < limit) {

               // Sort Balls a.pos <= b.pos <= c.pos
                   Ball tmp;
                   if (a.pos > b.pos) { tmp = a; a = b; b = tmp; }
                   if (b.pos > c.pos) { tmp = b; b = c; c = tmp; }
                   if (a.pos > b.pos) { tmp = a; a = b; b = tmp; }
                  
               // Print balls
                   int i; // next column to draw in current row
                   for (i = 0; i < (int) a.pos; i++) System.out.print(" ");
                   if (0 <= (int) a.pos) 
                        { System.out.print(a.name); i++; }
                   for ( ; i < (int) b.pos; i++) System.out.print(" ");
                   if ((int) a.pos < (int) b.pos) 
                        { System.out.print(b.name); i++; }
                   for ( ; i < c.pos; i++) System.out.print(" "); 
                   if ((int) b.pos < (int) c.pos) 
                        { System.out.print(c.name); i++; }
                   System.out.println(); // advance to next line
                   for (i = 0; i<1e6; i++) ; // delay

               // Move balls -- bounce elastically with the ground
		   a.updatemotion(dt, accel);
		   b.updatemotion(dt, accel);
		   c.updatemotion(dt, accel);
               
               // Update elapsed time
                   time += dt;
           }
    } // method main
} // class balls_methods