CS 110 Fundamentals of Computing - Exam 1
Professor Bolker (day section)

Tuesday, March 2, 2004
Closed book and notes

ANSWERS

Attached you will find the source code for class TextLine. Use it to answer some of the questions that follow. You need not turn in that source code, so feel free to pull it loose so that it’s easier to refer to. Please keep the rest of the pages stapled together.

1)      (20%) In this fragment of TextLine.java (with line numbers) 

    33   public void paintOn( Screen s, int x, int y )
    34   {
    35       for (int i = 0; i < text.length(); i++) {
    36           s.paintAt( text.charAt(i), x+i, y );
    37       }
    38   }

a)      How many tokens are there on line 35?   19        Carefully circle them.

++ is one token

b)      What are the java keywords on lines 33-38? 

public, void, int, for
 

c)      Find all the places on line 36 where a message is sent to an object. In each case identify the name and type of the object receiving the message, the file where you would you expect to find the code for the method that will respond to the message, and write the declaration for that method, showing its signature.

A charAt message is sent to object text of type String. The code for the  charAt method should be in file String.java, with declaration                                                         public char charAt(int spot) (Of course there is no way to know what name the programmer chose for the integer parameter.)                                                         

A paintAt message is sent to object s of type Screen. The code for the  paintAt method should be in file Screen.java, with declaration                                                         public void paintAt(char c, int x, int y)

 

 

2)      (15%) Use the grid attached to this exam (ruled so that character positions are easy to identify) to show the exact, complete output from the command shown. There are two copies of the grid so you can fix mistakes without erasing. Circle the one that’s your final answer

Note: the grid is not a picture of a Screen. It’s the window on your monitor where the program is running. It is larger than what you will need to show what’s printed by the program.

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3)      (7%) Find an example of delegation in TextLine.java. Explain what is happening.

On line 59      return text.length();  asks a String object how long it is, in order to return the answer to whatever client sent the getLength() message.

One can also view line 48 as this TextLine object delegating work to itself.

4)      (10%) Explain the use of this on line 22 of TextLine.java and find a place where an optional this has been omitted but could be used.

The left hand side of the assignment statement on line 22 needs this so that it refers to the field text declared on line 12 instead of the parameter text declared on line 20.

Line 48 could read this.paintOn( s, 0, 0 );

Line 36 could read s.paintAt(this.text.charAt(i), x+i, y);

5)      (10%) Describe how the author of the TextLine class used two important Java coding conventions.

Variables have been named using the startLowerInternalCaps convention. Indenting (prettyprinting) shows program structure. Public methods have javadoc comments. Braces around blocks are vertically aligned appropriately. Class name begins with a capital letter.

6)       (15%) Draw box-and-arrow pictures showing variable t in the main method of   TextLine.java  and its value

o        just after the declaration on line 72

″world!″
 
 


TextLine
 
 

 

o        just after line 73 has executed

 

 

 

 

 

 

 

 

 

 

 

 

 

7)      (15%) Write code at the //method body comment that will make this method do what its javadoc says it should do: 

  /** 
    * Print out a range of numbers, one to a line. 
    *
    * Example: the message printRange(2,4) should print
    *
    *    2
    *    3
    *    4
    * 
    * and return the value 3.
    *
    * @param start the start of the range.
    * @param end   the end of the range.
    * 
    * @return the number of numbers printed.
    */
    public int printRange( int start, int end ) 
    {
        // method body
        for (int i = start; i <= end; i++ ) {
             System.out.println( i );
        }
        return end-start+1;
    }
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

8)      (10%) How would you test the method you wrote in the previous problem? (You may assume that what you wrote will compile.) You may write either pseudocode or real Java. If you choose to write the Java it must be correct. If you choose to write pseudocode it must be clear, and easy to turn into code. Use the other side of this page for your answer.

This is probably the hardest question on the exam. Testing is really hard. Here is some pseudocode:

Declare public class PrintRange in  which printRange is a method

In main() in class PrintRange

        Create a Terminal t

        Create a PrintRange instance pr

        // a self documenting test

        Use Terminal t to print out the example in the javadoc, as it appears there

        t.println( “printRange  should return 3, did return ” + pr.printRange(2,4));

        // allow the user to enter interactive tests

        loop while user says she wants to continue

                        prompt for start and end values

                        warn that if end < start weird things may happen!

                        send printRange message

                        report value returned by printRange