Name: KEY CMPS 109 Quiz 5  March 9, 1999

This is a closed notes, closed book exam.

THERE ARE PROBLEMS ON BOTH SIDES OF THE PAPER

• In general, which results in a more compact representation of numeric data, a text file or a binary file? Circle the correct answer.

  binary is more compact

• True or false, the method readInt() from the class DataInputStream can be used to read integers from the keyboard by constructing a DataInputStream from System.in. Circle the correct answer.

  False. readInt() from DataInputStream reads binary data not text. It will read for bytes and then interpret those 4 bytes as a single int value.

• The Java package java.io includes four abstract classes, InputStream, OutputStream, Reader, and Writer, that are all derived directly from java.lang.Object. Which two are the superclasses of the I/O classes designed for text (character) based I/O? Circle the correct answer.

  java.io.Reader is for reading text. java.io.Writer is for writing text.

• One way to detect EOF using Java input classes is with a special method such as hasMoreElements used by tio.ReadInput. Describe the other two mechanisms for detecting EOF and explain when each is appropriate.

  One method is to return a special value. This only works when there is some distinguished value. For example, null can be returned from readLine() because null doesn't correspond to any legal string.

  Another method is to use exceptions. This is appropriate when there is no reserved value that can be used as with the previous approach. For example, when reading an int, all return values are legal, so EOF is indicated by throwing a java.io.EOFException.

• Using writeInt() from DataOutputStream, how many bytes will be written for the value 123456789 (put answer here)? How many bytes would be written if the same value was written using println() from PrintWriter (put answer here)?

  Using DataOutputStream all ints use exactly 4 bytes, regardless of the value.

  Using a PrintWriter, there will be either 1 or 2 bytes for each character. It depends whether the system supports Unicode or just ASCII. Either 9 or 18 will be accepted as correct answers.

• How does Java distinguish between exceptions that must be declared using a throws clause, and those that do not require a throws clause? It is not necessary to name specific classes in your answer. A general description of the approach is fine.

  Any exception that is derived (directly or indirectly) from RuntimeException or Error, does not need to be declared using a throws. All others require a throws. So Java distinguishes those that require a throws by looking at the super classes of the exception.

• What is printed by the following program?

  class Quiz5
  {
    public static void main(String[] args)
    {
      int x;
      try {
        x = foo(10);
      }
      catch (MyException e)
      {
        System.out.println("Caught an exception: " + e);
        x = 99;
      }
      System.out.println(x);
    }
    static int foo(int x)
    {
      System.out.println("foo started with " + x);
      int temp = bar(x);
      System.out.println("foo returning " + temp);
      return temp;
    }
    static int bar(int y)
    {
      System.out.println("bar started with " + y);
      if(y > 0)
        throw new MyException("just a test");
      System.out.println("when is this executed?");
      return y;
    }
  }
  
  class MyException extends RuntimeException 
  {
    public MyException(String message)
    {
      super(message);
    }
  }
  
  
  OUTPUT:
  
  foo started with 10
  bar started with 10
  Caught an exception: MyException: just a test
  99
  

  The exact wording of the third line is not important, so long as it includes ``just a test''.