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individual bits in java

Goals

1)  Be able to work with individual bits in java.

2)  Understand the serializable interface.

3)  Understand the comparable interface.

4)  Answer questions about a general-purpose class to be developed.

5)  Understand the use of a driver program for ‘glass box’ debugging.

6)  Develop a program that can grade true/false tests.

 

 

Description

1)  The first step is to develop a general-purpose class that will be able to perform operations on strings of bits. The class API follows:

 

public class BitMap implements Comparable, Serializable

{   public static final int  BITSIZE = 64;

private long bitString;

 

public  BitMap()       // Three constructors.

public  BitMap(String s)

throws IndexOutOfBoundsException,ArithmeticException

public  BitMap(boolean[] bits)

throws IndexOutOfBoundsException

 

private long    bitMask(int b) // Other class methods.

public  void    setBit(int b)

public  void    clearBit(int b)

public  boolean checkBit(int b)

public  int     countTrue()

public  void    clearAll()

public  void    setAll()

 

public  int     compareTo(Object bm) //For Comparable.

public  boolean equals(BitMap bm)

public  String  toString()

}

 

Notes:

  1. The only instance variable that is needed is bitString.
  2. Use BITSIZE for the maximum index value of your loops.

 

The above looks like a lot of methods, but the whole class requires about a page of code when the methods are filled in. Some methods can use others instead of duplicating code. For example, the first constructor can just call the method, clearAll(). The method bitMask() can be used by four or five other methods whenever a bit operation is called for. We’ll discuss that in class.

 

The operations to be performed by each method are briefly described below:

  1. a) Constructors

(i)   The first constructor just needs to set all bits to false (or zero). The method clearAll() does the same thing.

(ii)  The second constructor takes a character string as input. Each character of the string is either a t (for true) or f (for false) value. The bits with a t character are to be set on in the bit map; bits with an f character should be set off in the bit map. Throw an ArithmeticException if the input string has characters other than ‘t’, ‘T’, ‘f’, or ‘F’ appear.  Throw IndexOutOfBoundsException if the string is too long.

(iii) The third constructor works just like the second except the input is a boolean array. The bits corresponding to the array elements that have a true value should be set to a value of one; the elements having a false value should be set to a value of zero.

 

  1. b)  Primary Methods

The methods, setBit(int), clearBit(int), and checkBit(int) respectively set a given bit on, off or check a bits current value. The method, countTrue() returns the total number of bits that are set. It can be used by the compareTo() method. The method setAll() turns on all the bits; clearAll() clears all bits. Both setAll() and clearAll() methods can be written with one instruction.

 

  1. c)  Comparable interface

The compareTo() and equals() methods should be provided to conform to the standard way that java programs compare objects. One BitMap object is considered less than another if it contains less true bits. In effect, this method of comparison can be used to determine if one BitMap object has more bits on than another. The equals() method can compare whether two BitMaps have all of their on and off bits in the same positions.

 

  1. d)  The toString() method should return an appropriate string of ‘t’ and ‘f’ values. The System.out.print methods use this method.

 

2)  The next step is to debug the class that was created in the above step. I provide the program driver.java for this purpose; its code is at the bottom of this document. Don’t modify this program in any way; use it to test your class.  It contains a menu that has options to test every option. Once the testing is complete, BitMap, could be used as a general tool for working with bits and could be used in many programs.

 

3)  Use notepad to create a file of true/false questions.

 

4)  Now write a program (CreateTest.java) that constructs a true/false test. This program reads the file created in step 3 to ask a series of true false questions and record the resulting answers in a bit map object. Be sure to use a fileChooser to ask the user for the file name. Make sure you catch all exceptions (programs should never crash).

 

You can use your TextReader from the previous lab as the starting point for this program. Better yet, just instantiate a BufferedReader and read lines from the text file till you encounter a null line.

 

Make sure to have at least 25 questions in your test. When the test is completed, the program should use an ObjectOutputStream to write one record (the BitMap object) to a sequential binary file. That is why the BitMap class must have ‘implements serializable’ on its signature line. Name the disk file ans.bin. Hopefully, you’ll know the answers to your questions so the answer file will represent a perfect score.

 

5)  Finally, create a new application (Test.java). You can copy and paste the program that you just wrote (and save it as Test.java), and then modify it appropriately. This program should read the answer file (using an ObjectInputStream) and compare the answers given by someone taking the test to ans.bin. Display the score earned by the test taker.

 

6)  Answer the synthesis questions in an rtf or doc file (answers.rtf or answers.doc). Type your name and the lab number on this file and include the questions with the answers.

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