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Thread: DrJava Accel/Velocity
09-23-2012, 05:29 AM #1
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- Sep 2012
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Hello sorry if this is a stupid question, but I am a beginner programmer learning the basics im taking intro to comp sci. One of our assignements is to make a ball in a window go up to the top of the frame and then down to the bottom right.
The only part of my code I need to change is adjusting the velocity and acceleration of the ball. The values he gives us to start(which are wrong on purpose) were 0.001 for accel and -.5 for velocity. You don't have to do my homework but I would appreciate getting a little hint on how to start to find the values. It seems as if it should be simple but I dont know the way of easily getting those values corrected.
Here is the code if interested in looking(I made the part I need to change in bold/underlined:
import java.util.List; // resolves problem with java.awt.List and java.util.List
* A class that represents a picture. This class inherits from
* SimplePicture and allows the student to add functionality to
* the Picture class.
* Copyright Georgia Institute of Technology 2004-2005
* @author Barbara Ericson email@example.com
public class Picture extends SimplePicture
///////////////////// constructors //////////////////////////////////
* Constructor that takes no arguments
public Picture ()
/* not needed but use it to show students the implicit call to super()
* child constructors always call a parent constructor
* Constructor that takes a file name and creates the picture
* @param fileName the name of the file to create the picture from
public Picture(String fileName)
// let the parent class handle this fileName
* Constructor that takes the width and height
* @param width the width of the desired picture
* @param height the height of the desired picture
public Picture(int width, int height)
// let the parent class handle this width and height
* Constructor that takes a picture and creates a
* copy of that picture
public Picture(Picture copyPicture)
// let the parent class do the copy
* Constructor that takes a buffered image
* @param image the buffered image to use
public Picture(BufferedImage image)
////////////////////// methods ///////////////////////////////////////
* Method to return a string with information about this picture.
* @return a string with information about the picture such as fileName,
* height and width.
public String toString()
String output = "Picture, filename " + getFileName() + " height " + getHeight() + " width " + getWidth();
public static void main(String args)
//String fileName = FileChooser.pickAFile();
//Picture pictObj = new Picture(fileName);
Picture ballFlight = new Picture(800,600);
//Purpose: Make a new Picture to draw the ball flight upon, and store
//a reference to that picture in a new variable ballFlight, so Picture
//methods may be called on that Picture in the future. Also, specify
//the new Picture's width and height in Pixel units.
double yInitVel;//Purpose: Make a variable for the initial velocity.
double yInitPos;//(1)Purpose: Make a variable for the initial position.
double yAccel;//Purpose: Make a variable for the downward acceleration.
yAccel = .001; //Purpose:Specify the value of acceleration to use.
yInitVel = -.5;//Purpose:Specify the value of velocity in the down direction.
yInitPos = 300;//Purpose:Specify the initial position in the down direction.
double time;//Purpose:Make a variable to keep track of time.
time = 0.0;//Purpose:Program that time starts at zero.
while(time < 800)//purpose:Plot images of the ball as long as time is
//less than the simulation period.
double xPos;//Purpose:Make a variable for calculating current x position.
double yPos;//Purpose2)Make a variable for calcuating the current y position.
xPos = 1.0*time;
//Purpose3)Makes the current xPos variable equal to the current time.
yPos = (0.5*yAccel*time*time) + yInitVel*time + yInitPos;
//Purpose4)Makes the current y position equal to half the acceleration multiplied by the
//current time squared added to the sum of initial velocity multiplied by time and
//the initial position value
System.out.println("xPos=" + xPos + "yPos=" + yPos);
//Purpose:Enable you to see the current positions in numeric form.
Pixel pix = ballFlight.getPixel((int)xPos, (int) yPos);
//Purpose:Retrieve a Pixel near the current position, and make a variable
//you can use to refer to it, so you can call that Pixel's methods.
//Purpose:Set that Pixel's red intensity to zero.
//Purpose:Set that Pixel's green intensity to zero.
//Purpose: Set that Pixel's blue intensity to zero.
//(5)The purpose of the above statements is to make
//the pixels color equal to black since black.
time = time + 1.0;
//Purpose:AFTER plotting the ball's position at the current time, calculate
//and write in the time variable
//(6)Calculate and write in the time variable AFTER plotting the ball's position at the current time.
//Purpose7)Look up the file "ballFlight"
} // this } is the end of class Picture, put all new methods before this