References. Date Due: Monday, October 5 th, 5:00pm

Transcription

1 New Concepts Covered References Date Due: Monday, October 5 th, 5:00pm Keeping track of references Garbage collection copy() methods When the parameter passed into a method changes Assignment Specifications The following questions will involve understanding the BallFactory and Ball classes. Ball Class Public Interface Name: Ball Purpose: This class models a ball that can change its color and can make copies of itself. Methods: public Ball( ) The constructor for the ball. A ball is created with the color black. public Ball copy( ) Returns a new instance of a Ball which has all the same values as the current ball. (i.e. the same color, position, size) public void setcolor( java.awt.color newcolor) Changes the color of the ball to the color passed in as a parameter. BallFactory For this class you will have to understand what the code is doing. This class creates new balls identical to the one passed in (as the parameter) except for the color of the ball. The methods which accomplish this are called copyballasred(...), copyballasgreen(...), and copyballasblue(...). The last method in the BallFactory class, createballs(), uses the copyballascolor methods.

2 Warning: These methods are not examples of good code or design, and may not do what the programmer intended. They are written to illustrate how parameters work. When reading the code, pay close attention to what happens to the parameter in the copyballascolor methods. It might be helpful to draw out a picture of what is happening. public class BallFactory { public BallFactory(){ public Ball copyballasyellow(ball balltocopy) { Ball newball = balltocopy.copy(); balltocopy = newball; newball.setcolor(java.awt.color.yellow); return balltocopy; public Ball copyballasred(ball balltocopy) { balltocopy.setcolor(java.awt.color.red); return balltocopy.copy(); public Ball copyballasgreen(ball balltocopy) { Ball newball = balltocopy; newball.setcolor(java.awt.color.green); return newball; public Ball copyballasblue(ball balltocopy) { Ball newball = balltocopy.copy(); newball.setcolor(java.awt.color.blue); return newball; public void createballs() { Ball ballone = new Ball(); Ball balltwo = this.copyballasyellow(ballone); Ball ballthree = new Ball(); Ball ballfour = this.copyballasgreen(ballthree); Ball ballfive = new Ball(); Ball ballsix = this.copyballasblue(ballfive); Ball ballseven = new Ball(); Ball balleight = this.copyballasred(ballseven); Example question: At the end of BallFactory.createBalls() what color are ballone and balltwo? Do they refer to the same instance?

3 Answer: (Thought: first, go through the steps manually, drawing pictures with each one.) Ball ballone = new Ball(); - The ball starts as the color black so it is black at this point. Ball balltwo = this.copyballasyellow(ballone); - we are sending in our black ball to the CopyBallAsYellow method and will return a reference to a ball to be stored as balltwo. public Ball copyballasyellow(ball balltocopy) { - our ballone is now named balltocopy for the scope of the copyballasyellow method. Thus, both the variables ballone and balltocopy refer to the same ball. Ball newball = balltocopy.copy(); - We are creating another ball, called newball which is a copy of my first ball. Thus, newball has the color black since balltocopy was black.

4 balltocopy = newball; - the variable balltocopy stops pointing to the first ball (ballone still points to it). balltocopy now points to the second ball. newball.setcolor(java.awt.color.yellow); - Now we are setting the newball to the color yellow. Since this is a different ball than the ballone, we have not changed the original ball but we have changed the color of the ball for the variable balltocopy. return balltocopy; - A third variable, balltwo now also points to the second ball. This happens because balltocopy (which points to the yellow ball on the right) is returned and the value of that variable (the actual ball) is now set to be the value of the new variable, balltwo So.. ballone is black, balltwo is yellow and they do not refer to the same instance.

5 Questions 1. At the end of BallFactory.createBalls() what color are each of the balls? (State the color of ballthree, ballfour, ballfive, ballsix, ballseven and balleight.) 2. Do any of these ball variables (ballthree balleight) refer to the same instance of Ball at the end of BallFactory.createBalls()? If yes, which ones? Hint: If two ball instances referred to the same Ball object, changing the color of one would also change the color of the other because they refer to the same object. 3. Of the three methods: copyballasred(...), copyballasgreen(...), and copyballasblue(...), which one is the best? You should determine the best method by considering which one accomplishes the task described in an intuitive and clean way. In other words, which way would you write a method to copy a ball with a different color? Explain your answer. Handin Info References is due at 5:00PM sharp on Monday, October 5 th in the wooden CS15 handin bin next to the Fishbowl (CIT Room 271). Please write your name and your account name at the top of every piece of paper you hand in and staple all of them together; we cannot give you credit for a homework we do not know you did. We will be taking off points if you don t write your login on your homework! Remember: this assignment will not be accepted if turned in late.