COP-2800 Homework #5 Model Solution

11.1) What is the printout of running the class C in (a)?

a)
class A {
  public A() {
    System.out.println( "A's no-arg constructor is invoked" );
  }
}

class B extends A {
}

public class C {
  public static void main(String[] args) {
    B b = new B();
  }
}

-- Output: "A's no-arg constructor is invoked"

What problem arises in compiling the program in (b)?

b)
class A {
  public A(int x) {
  }
}

class B extends A {
  public B() {
  }
}

public class C {
  public static void main(String[] args) {
    B b = new B();
  }
}

-- Cannot compile since the no-arg constructor of B attempts
-- to invoke the no-arg of constructor of A, which doesn't exist.

11.2) True or false?

 1. A subclass is a subset of a superclass.
 -- False (this depends on your point of view: all subclass objects
 -- are also superclass objects, that is, all Llamas are Animals; but
 -- usually the subclass, or child or derived class, contains all the
 -- fields and methods of the superclass, plus more, so subclass'
 -- object's members are a superset of the superclass' object's members.)

 2. When invoking a constructor from a subcass, its superclass's no-arg
    constructor is always invoked.
 -- False (only if no other constructor is explicitly invoked)

 3. You can override a private method defined in a superclass.
 -- False (you can only override *accessible* instance methods.)

 4. You can override a static method defined in a superclass.
 -- False (you can only override accessible *instance* methods.)

11.3) Identify the problems in the following classes:
 1. public class Circle {
 2.   private double radius;
 3. 
 4.   public Circle(double radius) {
 5.     radius = radius;
 6.   }
 7.
 8.   public double getRadius() {
 9.     return radius;
10.   }
11.
12.   public double getArea() {
13.     return radius * radius * Math.PI;
14.   }
15. }
16.
17. class B extends Circle {
18.   private double length;
19.
20.   B(double radius, double length) {
21.     Circle(radius);
22.     length = length;
23.   }
24.
25.   /** Override getArea() */
26.   public double getArea() {
27.     return getArea() * length;
28.   }
29. }

-- Line 5 should be changed to:
--    this.radius = radius;
--
-- Line 21 and line 22 should be changed to:
--    super(radius);
--    this.length = length;
--
-- Line 27 should be changed to:
--    return super.getArea() * length;

11.11) Show the output of the following program:

 public class Test {
   public static void main(String[] args) {
     A a = new A(3);
   }
 }

 class A extends B {
   public A(int t) {
     System.out.println("A's constructor is invoked");
   }
 }

 class B {
   public B() {
     System.out.println("B's constructor is invoked");
   }
 }

-- The output is:
--    B's constructor is invoked
--    A's constructor is invoked

Is the no-arg constructor of Object invoked when new A(3) is invoked?

-- Yes, class Object's constructor is invoked before any statements
-- in class B's constructor are executed.

11.13) Suppose that Fruit, Apple, Orange, Golden Delicious Apple, and
       Macintosh Apple are declared, as shown in Figure 11.6:

                        Fruit
               -----------^-----------
 	     Apple                 Orange
        -------^----------
  GoldenDelicious    Macintosh

(Figure 11.6: GoldenDelicious and Macintosh are subclasses of Apple;
Apple and Orange are subclasses of Fruit.)

 Assume that the following declaration[s] is [are] given:
     Fruit fruit = new GoldenDelicious();
     Orange orange = new Orange;

 Answer the following questions:

1) Is fruit instanceof Fruit?
-- true
2) Is fruit instanceof Orange?
-- false
3) Is fruit instanceof Apple?
-- true
4) Is fruit instanceof GoldenDelicious?
-- true
5) Is fruit instanceof Macintosh?
-- false
6) Is orange instanceof Orange?
-- true
7) Is orange instanceof Fruit?
-- true
8) Is orange instanceof Apple?
-- false
9) Suppose the method makeApple Cider is defined in the Apple class.
   Can fruit invoke this method?
--   Yes
   Can orange invoke this method?
--   No
10) Suppose the method makeOrangeJuice is defined in the Orange class.
    Can orange invoke this method?
--   Yes
    Can fruit invoke this method?
--   No
11) Is the statement Orange p = new Apple() legal?
--   No
12) Is the statement Macintosh p = new Apple() legal?
--   No
13) Is the statement Apple p = new Macintosh() legal?
--   Yes

11.15) When overriding the equals method, a common mistake is mistyping
       its signature in the subclass.  For example, the equals method
       is incorrectly written as equals(Circle circle), as shown in (a)
       in the code below; instead, it should be equals(Object circle),
       as shown in (b).  Show the output of running class Test with
       the Circle class in (a) and in (b), respectively.

  public class Test {
    public static void main(String[] args) {
      Object circle1 = new Circle();
      Object circle2 = new Circle();
      System.out.println(circle1.equals(circle2));
    }
  }

a)
  class Circle {
    double radius;

    public boolean equals(Circle circle) {
      return this.radius == circle.radius;
    }
  }

-- The output is "false" in this case. The Circle class has two
-- overloaded methods: equals(Circle circle) defined in the Circle
-- class and equals(Object circle) defined in the Object class,
-- inherited by the Circle class.  At compilation time,
-- circle1.equals(circle2) is matched to equals(Circle circle),
-- because equals(Circle circle) is more specific than
-- equals(Object circle).

b)
  class Circle {
    double radius;

    public boolean equals(Object circle) {
      return this.radius ==
        ((Circle)circle).radius;
    }
  }

-- The output is "true" this time.  The Circle class overrides the
-- equals(Object circle) method defined in the Object class.  At
-- compilation time, circle1.equals(circle2) is matched to
-- equals(Circle circle) and at runtime, the equals(Object circle)
-- method implemented in the Circle class is invoked.

11.21) In the code below, classes A and B are in different packages.

a)
  package p1;
  public class A {
    ?????? int i;
    ?????? void m() {
      ...
    }
  }

b)
  package p2;
  public class B extends A {
    public void m1(String[] args) {
      System.out.println(i);
      m();
    }
  }

 If the question marks are replaced by blanks, can class B be compiled?
-- No (Can only be accessed from within the same package)

 If the question marks are replaced by private, can  class B be compiled?
-- No (Can only be accessed from within the same class)

 If the question marks are replaced by  protected, can class B be compiled?
-- Yes (Can be accessed from within the same package, *or by subclasses*)