Lecture Summary for 212 (Barat) on 9/18/03

Review Questions

1. How many bytes do these datatypes have?
   byte, short, float, double, char
   Answer: 1, 2, 4, 8, 2

2. What is wrong with this Java initialization?
   long population = 6000000000;
   Answer: The constant should have the long suffix L:
   long population = 6000000000L;

3. What is the difference between short circuit operators and complete evaluation operators?
   Answer: In short circuit evaluation, if the first argument of logical and or logical or is sufficient to determine the answer, the second term is not evaluated. If complete evaluation, both terms of logical and or logical or are always evaluated.

   Here are two examples of a java statements that use short circuit evaluation:
   if (p != null && p.getGender() == 'F')

   if (i > 0 && i < a.length && a[i] == 0)

4. What is the output?
   int x = 3;
   int y = 7;
   int z;
   z = (++x) + (y++);
   System.out.println(x + " " + y + " " + z);
   Answer: 4 8 11

5. What is the output?
   int x = 3;
   int y = 7;
   System.out.println(x + " " + y + " " + (x++ > 3 && y-- > 7));
   Answer: 4 7 false

6. What happens if the && is changed to & in the preceding problem?
   Answer: The output then becomes 4 6 false

7. What is 13 in binary and hex?
   1101 and d.

8. What is 1010 in hex and decimal?
   a and 10.

9. What is f in binary and decimal?
   1111 and 15.

10. Convert the byte variable 10101101 to decimal and hex.
    Answer: It is 2³ + 2² + 2⁰ = 8 + 4 + 1 = 13 and 0d in hex.

11. Convert the short variable 0000000100010101 to decimal and hex.
    Answer: It is 2⁴ + 2² + 2⁰ = 16 + 4 + 1 = 21 in decimal and 0015 in hex.

12. Is this int variable odd or even?
    00011110011001101110010010111010
    Answer: It ends with a 0 so it is even.

13. Convert 109 to one byte binary and hex.
    Answer: 109 - 64 = 45, 45 - 32 = 13 --> 109 = 01101101 binary = 6d hex.

14. Convert 263 to two byte binary and hex.
    Answer: 263 - 256 = 7 --> 0000000100000111 binary = 0107 hex.

15. Carry out these bitwise operations:
    0x7d & 0x36     0x7d | 0x36     0x7d ^ 0x36
    Answers:
    0x7d & 0x36 = 01111101 & 00110110 = 00110100 = 0x34
    0x7d | 0x36 = 01111101 | 00110110 = 01111111 = 0x7f
    0x7d ^ 0x36 = 01111101 ^ 00110110 = 01011011 = 0x4b

16. How do you print the int variable x in hex?
    Answer: System.out.println(Integer.toHexString(x));
    
17. How do you display the message "Hello." in an input dialog?
    Answer: JOptionPane.showMessageDialog(null, "Hello.");

18. How do you find an input dialog or message dialog when it is hidden behind other windows?
    Answer: Use Alt-Tab.

19. How does a do loop differ from a while loop?
    Answer: A do loop checks the condition at the end of the loop so the loop always executes at least once.

20. How does an if statement differ from a switch statement?
    Answer: A switch statement compares an expression to a sequence of constant values. An if..else statement checks a sequence of conditions, which may be more general than the checks in the switch statement.

21. What are amicable numbers?
    Answer: n and m are amicable numbers if the sum of the factors of n is m and the sum of the factors of m is n.

Motivation for Bitwise Operations

• A bitmap is an integer variable for which the individual bits represent individual boolean values (0 == false and 1 == true).

• Examples of uses for bitmaps:
  
  • answers to yes/no questions,
    
  • status of records in a file (deleted or not deleted),
    
  • pages in an operating system's memory (changed or not changed),
    
  • characteristics of sections of a document like bold or not bold, italic or not italic, underlined or not underlined.

• A bitmap takes up less memory than an array of boolean values.

• Use bitwise operations to change the individual bits in a bitmap:
  
  • Bitwise Or can change bits to 1.
    
  • Bitwise And can change bits to 0.
    
  • Bitwise XOr can toggle bits (change 1s to 0 and 0s to 1).

• Example:   Write a hex bitmask to change bits numbered 1, 2, 5, and 8 to 1 in the byte variable x.
  The bitmask acts on x using bitwise or. Answer: 0xc9.

• Example:   Write a hex bitmask to change bits numbered 2, 3, 4, 5, 7, 8 to 0 in the byte variable x.
  The bitmask acts on x using bitwise and. Answer: 0x14.

• Example:   Write a hex bitmask to toggle the bits numbered 2, 4, 6 in the byte variable x.
  The bitmask acts on x using bitwise xor. Answer: 0xb4.

Review of Arrays

• Look at the Arrays Example in ArraysClasses.

Review of Classes

• Look at the Person Example in ArraysClasses.

• Review public, private, instance variables, constructors, methods, accessor methods, mutator methods, toString method.

UML (Unified Modeling Language) Diagrams

• A UML class diagram has three sections:
  
  • The class name,
    
  • The names of the instance variables,
    
  • The names of the methods.

• Private members are marked with -.

• Public members are marked with +.

• Here is a UML diagram for the Person class:

  

The DecimalFormat Class

• Use a DecimalFormat object to format a floating point number.

• A format string is passed to the DecimalFormat constructor. The format string determines
  
  
  • the number of digits after the decimal point
    
  • if leading zeros are displayed
    
  • if commas are used for grouping

• Here are some examples of format strings:

  Format String	Digits After Decimal Point	Leading Zero?	Commas?
  "#0.000"	3	Yes	No
  "#.00"	2	No	No
  "#,##0.0"	1	Yes	Yes
  "#,##0.0"	1	No	Yes

Arrays of Objects

• Look at the PersonArray Example in Composition.

• See how to create and use an array of objects.

• When the object p is printed using
  System.out.println(p);
  the object's toString method is invoked.

• If p == null
  System.out.println(p);
  prints null.

Collection Classes

• A collection class is a class that contains one or more objects.

• See the AllPersons Example in ArraysClasses.

The QuickPick Project

• Look at the Die Example in ArraysClasses.

• See the QuickPick Project Description.