Object-Oriented Principles Worksheet

1. Can I make an instance of an Animal?

   abstract class Animal {
     ...
     public abstract void speak();
   }
   

   Will the following definition of Tiger compile without error?

   class Tiger extends Animal {
     public void scratch() {;}
     public String getName() { return "tigger"; }
   }
   

2. Considering Tiger and the Animal class, which class is the subclass and which is the superclass?
   
   
3. What is the difference between an instance of Tiger and the class Tiger? Does

     String n = Tiger.getName();
   

   make sense and compile properly?
   
   
4. Can I redefine Tiger to make the speak() method private like the following? Why or why not?

   class Tiger extends Animal {
     ...
     private void speak() {
       System.out.println("roar!");
     }
   }
   

5. Assume Dog and Cat are subclasses of Animal. Which of the following assignments will result in compile-time warnings?

     Animal a = ... ;
     Dog d = ... ;
     Cat c = ... ;
   
     a = d;
     a = (Animal)d;
     d = a;
     d = (Dog)a;
     c = d;
     d = c;
   

6. Which method is called in each of the following:

   Bag b = ...;
   b.insert("some string");
   b.insert(3);
   b.insert("3");
   

   given the following:

   class Bag {
     ...
     public void insert(String s) {...}
     public void insert(int i) {...}
     public void insert(float f) {...}
   }
   

7. Consider the following class definitions:

   class Human {
     String name;
     ...
     public String getName() {
       return name;
     }
     public String getInfo() {
       return getName();
     }
   }
   
   class Student extends Human {
     String ID;
     ...
     public String getName() {
       return name+":"+ID;
     }
   }
   ...
   Student s = new Student("Madonna","111-22-3333");
   Human h = s;
   

   1. What does s.getName() return?
   2. What does h.getName() return?
   3. What does s.getInfo() return?
   4. What does h.getInfo() return?
   
   
   
8. Which of the following assignments result in compile-time errors? Will any assignments compile properly, but eventually lead to run-time errors (ClassCastException)?

   Airplane a = ...;
   Bird b = ...;
   FlyingSquirrel s = ...;
   
   CanFly f;
   
   f = a;
   f = b;
   f = s;
   f = (CanFly)s;
   a = b;
   

   Assume the following definitions:

   interface CanFly {
     public void fly();
   }
   
   class Airplane implements CanFly {
     public void fly() {...}
   }
   
   class Bird implements CanFly {
     public void fly() {...}
   }
   
   class FlyingSquirrel {
     public void fly() {...}
   }
   

9. Can a method of an interface be private?
   
   
10. In method reset(), what is "x = 0" shorthand for? I.e., what is the fully qualified name for "x"?

    class Point {
      int x,y;
      public void reset() {
        x = 0;
        y = 0;
      }
    }
    

11. Can two individual employee objects refer to two different company names given the following class? Can they each have a different ID?

    class Employee {
      String ID;
      static String companyName;
      static public void setCompanyName(String n) {
        companyName = n;
      }
      public void getID() {
        return ID;
      }
      ...
    }
    

    Could getID() refer to method setCompanyName()? Could setCompanyName() refer to getID()? Is the following class method definition equivalent/valid to the above definition?

      static public void setCompanyName(String n) {
        this.companyName = n;
      }