To Notes

CSC 212 -- 2/26/08

Review Questions

1. Given the inheritance hierarchy
   Object ← A ← B ← C ← D ← E ← F

   and the code

   F x = new F;
   x.f( );
   

   how does the compiler decide which f is called?

   Ans: The compiler looks first in the class F. If an f method is defined, use it. Otherwise, search, in order, E, D, C, B, A, Object. Use the first f that is found. If no f is found, print an error message.

2. Variable trace problem: Construct the variable trace and predict the output:
   Trace 2 Example:   C.java   D.java   Main2.java

       
     +---+                +---+
   c | O------+         d | O------+
     +---+    |           +---+    |
              V                    V                
            x   y              x   y   z
          ----+----          ----+---+----
      C( ) 10   20    D(4, 5)  8   12   5
           12   23             8   16   5 
                              10   16  13 
                              10   19  13
   
            Output:   25
                      28
                      35
                      33 
                      24 
                      2913
   

3. Redo the Stack Example of Week 5 to accept Object objects. Note that we cannot insert primitive datatypes into this Stack. Use the corresponding wrapper class instead:

   Stack s = new Stack( );
   s.push(new Person("Alice", 'F', 23));
   s.push(new Integer(243));
   s.push(new String("dog"));
   s.push(new Double(3.14159265));
   System.out.println(s);
   

   Ans: Simply use the editor to replace all instances of the word Person with the word Object.

4. How can an object determine from which class it was instantiated?

   Ans: If x is the object, use x.getClass( ).getName( ).

5. Give an example that shows how the finalize method of the Object class works.

   Ans: TestFinalize.java   Person.java

Virtual Methods

• A reference variable of type A can be assigned the address of an object of type A or of any type derived from A.

• Example: If B is a derived class of A,
  

  A x = new B( );
  

• In the method call x.f( ), the method that the compiler chooses to run is determined by the object, not to the type of the reference variable x. f is said to be a virtual method.

• Java overridden methods are always virtual.

• Here is the the Virtual Example:
  Virtual.java   Person.java   Employee.java   Executive.java

Exceptions

• An exception is an event that disrupts the normal flow of a program's execution.

• Example:   This example causes an ArrayIndexOutOfBoundsException.

  public static void main(String[ ] args)
  {
      int[ ] a = {1, 2, 3, 4, 5};
      System.out.println(a[5]);
  }
  

• Here is where the ArrayIndexOutOfBoundsException fits in its inheritance hierarchy:

  Object
      Throwable
          Exception
              RuntimeException
                  IndexOutOfBoundsException
                      ArrayIndexOutOfBoundsException
  

• Sometimes an exception does not occur in the main method as in the following StackTrace Example.

• StackTrace1 Example:   StackTrace1.java

• To catch an exception
  1. Place the code that might cause an exception in a try block.

  2. Place the code to be run if an exception occurs in a catch block.

  try
  {
      // Code that might cause an exception.
  }
  catch(Exception e)
  {
      // Code to run if exception occurs.
  }
  

• StackTrace2 Example:   StackTrace2.java

• DivideByZero Example   DivideByZero.java

• To prevent a program crash if a file is not found:

  Scanner s = new Scanner(System.in);
  String fileName = s.next( );
  FileReader fr;
  try
  {
     fr = new FileReader(fileName); 
  }
  catch
  {
      System.out.println("File " + fileName + 
                         " not found.");
  }
  

• Some exceptions, such as FileNotFoundException, are required either to be caught or thrown.

• A main method that throws a FileNotFoundException:

  public static void main(String[ ] args) 
                     throws FileNotFoundException
  {
      . . .
  }
  

• Example:   LineCount.java

• Here is the inheritance hierarchy of some important exceptions for CSC212:

  Object
      Throwable
          Exception
              RuntimeException
                  ArithmeticException
                  IndexOutOfBoundsException 
                      ArrayIndexOutOfBoundsException 
                      StringIndexOutOfBoundsException 
                  IOException
                      EOFException
                      FileNotFoundException
                  IllegalArgumentException
                      NumberFormatException
                  NoSuchElementException
                      InputMismatchException
                  NullPointerException
          Error
  

• Whereas exceptions should usually be caught in production code, errors usually should not be caught. An error is a truly abnormal situation that should not happen with well-functioning hardware and software.