created 06/23/00

Programming Exercises

Exercise 1—Finally

Rewrite the ReadIntEOF program to use a finally block to ensure that a file that has been opened is told to close. Of course, the close() might fail.

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Exercise 2—Testing the File Copy Program

Copy the file copy program to Notepad, compile it, and run it. Use the program to copy its own source file to a temporary file. Look at the copy to see if it is OK. Rename the copy to the name of the original file (thus deleting the original). Compile the copy to see if there are problems.

Now use the program to copy its own bytecode file (the *.class file) to a temporary file. Rename the copy to the name of the original bytecode file. Run the new bytcode file. It should work. (On Unix systems you may have to use the chmod command to make the copy executable.)

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Exercise 3—Data Compressor

Write a program that reads in a binary file of 4-byte int data. All the data is expected to be in the range -32768 to 32767, which will fit into a 2-byte short. The program writes a binary file that contains the same integers as the input file (in the same order), but each integer is now a short. Write an error message whenever an input value is outside the expected range, and write a 0 to output.

For data in the expected format, this program will compress the file to 50 percent of its original size. Of course, any file not in this format will be turned into complete garbage. It might be interesting to try.

The Data Translator program from the previous chapter can be used to create data to test this program.

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Exercise 4—Input File with Header

Say that you have several files of integer data using various formats: byte, short, and int. Each file starts out with a header like this:

First four bytes:    an int that says how many bytes there are per
                     integer in this file: 1, 2, or 4.

The data in the designated format immediately follow the header.

Write a program that uses a file like that as input. The program checks the header to determine how to read in the rest of the data. Then it reads in each integer and writes it (in character form) on the monitor screen.

Modify the Data Translator program from the previous chapter so that it creates files suitable for testing this program. It is OK to make several versions of the Data Translator program, one for each type of testing file.

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Exercise 5—Encryption and Decryption Program

Modify the file copy program so that the copy is encrypted (encoded). Each byte of the source file is altered by reversing each bit. For example,

input byteoutput byte

This operation is sometimes called a bit-wise complement. "Bit-wise" means that each bit is treated independently of all the others. "Complement" is just another word for reversal.

All the bits in an integer can be reversed as follows:

int value;

value = ~value;

The "~" (tilde) is the bit-wise complement operator. Visually it looks like a reversal of up and down. It reverses all the bits in value, even though you may be interested in only the low-order byte. But since the operation is bit-wise, the result for the low-order byte is the same no matter how many others are affected.

Encrypt a text file using your program. For amusement purposes, look at the result with Notepad. Now use the program again to encrypt the encrypted file. Look at the result with Notepad. Will you need to write a program to decrypt (decode) files?

This is not a very secure method of encryping a file. Java has other bit-wise operators that can be used to scramble bit patterns in other ways.

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Exercise 6—Hex Dump

Write a program that reads in any file byte-by-byte and writes each byte to the monitor using two characters per byte. The eight bits of each byte are divided into two 4-bit groups, for example:

01011101 ==> 0101 1101

Each 4-bit group is represented with one character according to the following table:

pattern characterpattern character
00000 10008
00011 10019
00102 1010A
00113 1011B
01004 1100C
01015 1101D
01106 1110E
01117 1111F

Even though you are only interested in bytes, it is convenient to do all your bit manipulation using int variables. To make all bits in the int zero except for the low-order 4 bits, do this:

int data, lowFour;

lowFour = data & 0X0000000F ;

The "&" is the bit-wise AND operator. Now lowFour will contain one of the 16 possible patterns in its low-order 4 bits. To make all bits in the int zero except for the other 4 bits of the low-order byte, do this:

int data, highFour;

highFour = data & 0X000000F0 ;

To shift those four bits right so that they occupy the low-order 4 bit positions, do this:

highFour = highFour>>>4 ;

Display each byte of the input file using two characters followed by a space. Display 16 bytes per line (fewer on the last line). Here is an example of the program working:

06/23/00  10:52p                 1,859 HexDump.class
06/23/00  10:52p                 2,855
06/23/00  08:23p                    60 ints.dat

C:\Programs>java HexDump ints.dat
00 00 00 0C 00 00 03 FF FF FF FF C8 00 01 4B 2D
00 00 00 00 FF FF DB 65 00 00 00 22 00 00 02 8E
FF FF FF E9 FF FF FE DA 00 00 00 12 FF FF FF E7
00 00 00 AD 00 00 00 2D FF FF FF FE

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End of Exercises.