To Notes

CSC 212 -- 1/8/08

Review Questions

1. What is wrong with these Java variable names?
   number-of-employees Ans: No dashes allowed.

   number of employees Ans: No spaces allowed.

   number_of_employees Ans: Underlines are allowed but are not conformant to the Java camel casing style.

   odds&ends Ans: No & allowed.

   anExtremelyLongVariableIfYouAskMe Ans: Legal, but too long.

2. What do the terms camel casing and Pascal casing mean?

   Ans: Camel casing means that the variable name starts with a lower case letter and that each new word starts with an upper case letter. Pascal casing is like camel casing, except that the first letter is also upper case.

3. What do each of the following mean in Java?
   %d Ans: Print integer variable as a decimal integer.

   %f Ans: Print floating point variable as a decimal floating ponit number.

   %s Ans: Print as a string.

   %d Ans: Print as a character.

4. Rewrite using System.out.printf:
   int a = 2, b = 3;
   System.out.println(
       a + " + " + b + " = " + (a + b));

   Ans:,p> int a = 2, b = 3;
   System.out.printf("%d + %d == %d\n", a, b, a + b);

5. What is wrong with these Java initialization?
   short population = 100000; Ans: 100000 must be cast to a short.

   long population = 6000000000; Ans: 6000000000 needs the L suffix to make it a long constant.

6. Convert the hex number 7B to binary and decimal.

   Ans: 7B hex = 01111011 bin = 64 + 32 + 16 + 8 + 2 + 1 = 123 dec.

7. Convert the decimal number 98 to binary and hex.

   Ans: 98 dec = 64 + 32 + 2 = 01100010 bin = 62 hex.

8. How many bits are in one byte? Ans: 8.

9. How many bytes are in this hex number?
   005B

   Ans: 4 bytes. Each hex digit represents a half of a byte.

10. What are these bitwise operators?
    &   |   ^

    Ans: Bitwise and, bitwise or and bitwise exclusive or, respectively.

    See this table of operators to see how the bitwise operators fit into the operator precedence hierarchy.

11. Apply the bitwise operators in question 10 to these binary numbers:
    11011100   01101001

    Ans:

      11011100    11011100    11011100
    & 01101001  | 01101001  ^ 01101001
      --------    --------    --------
      01001000    11111101    10110101
    

Why Bitmaps?

• A bitmap is a sequence of bits.

• Here are some uses for bitmaps.

Hex in Java

• Use the prefix 0x to specify a Java hex constant.

• Example:   What is the output?
  int x = 0x2D;
  System.out.println(
      "Ox2D == " + x + " dec");

  Ans: 0x2D == 45 dec

• Use the method Integer.toHexString to display an integer variable as a hex string.

• Example:   What is the output?
  int x = 33;
  System.out.println(
      "33 dec == " + Integer.toHexString(x) + " hex");

  Ans: 33 dec == 21 hex

Negative Binary Numbers

• To represent negative binary numbers the twos complement system is used.

• In Java, the byte, short, int and long datatypes are signed.

• For nonnegative signed integers, the leftmost bit is 0.

• For negative signed integers, the leftmost bit is 1.

• Here is a table of the twos complement representation of one byte integers.

• For nonnegative signed integers, the leftmost bit is 0.

• For negative signed integers, the leftmost bit is 1.

• Here are algorithms for converting negative decimal to binary and vice versa.

Another Bitwise Operator

• The bitwise complement operator is ~.

• ~ toggles each binary bit (changes each 0 to 1 and each 1 to 0).

• Go over the Bitwise Example and Project Proj1.

Java Structured Programming Constructs

• if..else while for do switch

• See the document Java Structured Programming Constructions for details.

• See the Structured Example in Structured.java

Perfect Numbers

• A number is perfect if the sum of its factors equals the number. (Don't count the value of the number itself as a factor.)

• Example:   The factors of 6 are 1, 2 and 3. 1 + 2 + 3 = 6, so 6 is a perfect number.

• Example:   The factors of 8 are 1, 2, and 4. 1 + 2 + 4 = 7 != 8, so 8 is not a perfect number.

• The Perfect Example in Perfect.java searches for perfect numbers.

• Here is a list of the known perfect numbers.

Amicable Numbers

• Amicable numbers are numbers such that the sum of the factors of one of them equals the other.

• Example:   220 and 284 are amicable numbers.

  The factors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55. They sum to 284.

  The factors of 284 are 1, 2, 4, 74, 142. They sum to 220.

• The Proj2 Project is a modification of the Perfect Example that seaches for amicable numbers.

A Debugging Checklist

• Use this debugging checklist for Lab Exercise Ex1.