In-Class Quiz 2 Topics

The quiz will cover:

• Sections 5.1 and 5.3 of the book.
• The readings: multiple objects, object interactions, constructors.
• Sections 7.3, 7.4 and 7.7 of the book.

Sample quiz questions (and answers):

1. Write a single Java statement that defines the named constant "size" with the value 1002.

2. What is the scope of a parameter variable?

3. What is the scope of a private instance variable?

4. When is a constructor called?

5. How do objects interact?

6. Write a boolean expression that is true if and only if the value of the variable "tonage" is between 1000 and 10000 (inclusive).

7. Write a boolean expression that is true if and only if the values of the variables "x", "y" and "z" are all different.

8. What does the following program draw? Note that there are two classes (Quiz1 and Thing).

   import animator.*;
   import java.awt.*;
   
   public class Quiz1 extends Animated {
   
       public void startup() {
   
   	Thing th = new Thing(55);
   	animator.include(th);
       } // end of startup method
   
   } // end of Quiz1 class
   
   import animator.*;
   import java.awt.*;
   
   public class Thing extends Animated {
   
       private int x;
   
       public Thing(int xx) {
   
   	x = xx;
   	} // end of constructor
   
       public void draw(Graphics g) {
   
   	g.setColor(Color.yellow);
   	g.fillOval(0,0,x,x);
   	} // end of draw method
   
       } // end of Thing class
   

9. What does the following program draw? Note that there are three classes (Quiz2, This and That).

   import animator.*;
   import java.awt.*;
   
   public class Quiz2 extends Animated {
   
       public void startup() {
   
   	This th1 = new This();
   	That th2 = new That(th1);
   	animator.include(th1);
   	animator.include(th2);
       } // end of startup method
   
   } // end of Quiz2 class
   
   import animator.*;
   import java.awt.*;
   
   public class This extends Animated {
   
       private int here;
   
       public void draw(int x, int y, Graphics g) {
   
   	g.setColor(Color.red);
   	g.fillOval(x-10,animator.getSceneHeight()-20,20,20);
   	here = x;
       } // end of draw method
   
       public int get() {
   
   	return here;
       } // end of get method
   
   } // end of This class
   
   import animator.*;
   import java.awt.*;
   
   public class That extends Animated {
   
       private This it;
   
       public That(This is) {
   
   	it = is;
       } // end of constructor
   
       public void draw(Graphics g) {
   
   	g.setColor(Color.black);
   	int where = it.get();
   	int width = animator.getSceneWidth();
   	int height = animator.getSceneHeight();
   	if (where < width/2) {
   	    g.fillOval(0,height-20,20,20);
   	}
   	else {
   	    g.fillOval(width-20,height-20,20,20);
   	}
       } // end of draw method
   
   } // end of That class
   

Solutions:

1. private final int size = 1002;

2. A parameter variable is known (accessible) anywhere inside the method it is a parameter of.

3. A private instance variable is known (accessible) anywhere inside the class it is defined in.

4. A constructor (method) is called when an object is created.

5. Objects interact by calling each other's methods.

6. 1000 < tonage && tonage < 10000

7. x != y && x != z && y != z

8. It draws a diameter 55 yellow circle in the upper left corner of the animation window.

9. It draws a diameter 20 red circle that rests on the bottom and moves left/right according to the arrow keys. It also draws a diameter 20 black circle that rests on the bottom. This black circle is drawn at the left edge of the window if the center of the red circle is left of the center of the window, at the right edge of the window if the center of the red circle is right of the center of the window.