Big Java

1
Big Java

• Chapters 3-6

2
Object-Oriented Concepts (review)

• Encapsulation hiding unimportant details
• Black box something that magically does its
  thing
• Abstraction taking away inessential features
• Example car
• if engine control module fails, replace it
• mechanic can provide inputs, test outputs
• doesnt need to know how it works
• OO challenge finding the right abstractions
  (what black boxes do we need to solve the
  problem at hand?)

3
Object-Oriented Concepts (review)

• How would you represent
• a bank account?
• a book?
• a library?
• a customer?
• an oil field?
• a mutual fund?
• an employee?

4
javadoc comments

• Remember the API had a standard format.
  User-defined classes can easily create html
  documentation in that same format, by inserting
  comments that meet certain specifications
• Start with /
• First sentence describes purpose
• _at_ tags specify information (e.g., _at_param,
  _at_return, _at_throws, _at_author, _at_version)
• run javadoc from command line or Eclipse to
  generate html pages

5
BankAccount

• See Eclipse notes for example of creating class
  with comments
• As in C, constructors have name of class, can
  be overloaded
• Instance fields should be made private, accessors
  and modifiers (getters/setters) are used
• Additional methods are defined as needed

6
More on Variables

• Instance variables
• each object has its own copy (as in C)
• objects are automatically garbage collected
  (unlike C!)
• fields are initialized with a default value
  (e.g., 0, null) if not explicitly set in
  constructor. Still good to do your own
  initialization.
• NullPointerException if you forget to call new
• Local variables
• must be explicitly initialized
• only exist inside method (as in C)

7
Reading Assignment

• Read Random Fact 3.1 (Electronic Voting Machines)
  on page 110 and be prepared to discuss
• Read Shape Classes (3.9) and be prepared to
  discuss

8

• On to Chapter 4
• Fundamental Data Types

9
Numeric Data Types (review)

• Integer values can be represented exactly, but
  numeric operations may result in overflow
• Floating point values may not be exact, so
  rounding errors may occur (shouldnt use with
  floating point values, use tolerance)
• double is therefore not appropriate for financial
  calculations
• java.math has BigInteger and BigDecimal classes
  which are slow but have better size/precision.
  Must use add, subtract and multiply (no operator
  overloading in Java)
• READ Random Fact 4.1, Pentium Floating-Point Bug
  on page 139

10
Constant values

• preceded by keyword final (vs const in C)
• naming convention is all uppercase
• e.g.,
• final double QUARTER_VALUE 0.25
• if used in a class, often use keyword static,
  meaning constant belongs to the class
• public static final double DIME_VALUE 0.1
• Math class has some useful constants, e.g.,
• double circumference Math.PI diameter
• (vs MathPI in C)

11
Numeric Operations

• , -, , /, - same precedence as C
• Math class has a number of static functions
  (sqrt, pow, sin, cos, exp, log, round, max, min,
  etc.)
• / of two integers yields an integer result (same
  as C)
• Quality Tips
• put space after every Java keyword, but not
  between a method name and parentheses
• put space around all binary operators
• factor out common code
• example
• x1(-bMath.sqrt (bb-4ac))/(2a)
• x2(-b-Math.sqrt (bb-4ac))/(2a)
• becomes
• double root Math.sqrt(b b 4 a c)
• x1 (-b root) / (2 a)
• x2 (-b root) / (2 a)

12
Numeric Operations, continued

• Remember that you may need to round floating
  point values
• double f 4.35
• int n (int) (100 f)
• System.out.println(n) // prints 434!
• Replace with
• int n (int) Math.round(100 f)
• Read How To 4.1, good review of how to design
  programs containing numeric computations

13
Unicode

• C and other languages used ASCII to encode
  characters
• Java uses a 16-bit encoding known as Unicode
• First 128 characters (i.e., English alphabet) are
  the same as ASCII
• Also includes German umlauts, about 21000 Chinese
  ideographs and others
• Can encode as escape sequence, e.g. \u00E9 is e

14
Reading Input and Formatting Numbers

• // required for input from keyboard
• import java.util.Scanner
• // required for formatting
• import java.text.NumberFormat
• public class SalesTax
• public static void main(String args)
• final double TAX_RATE 0.06
• // 6 sales tax
• double subtotal, tax, totalCost
• // Calculated values
• // Set up Scanner for keyboard
• Scanner scan new Scanner
  (System.in)
• System.out.print("Enter the price
  ")
• double price scan.nextDouble()
• System.out.print("Enter the quantity
  ")

• // Create desired formatting objects
• NumberFormat fmt1 NumberFormat.getCurrencyInstan
  ce()
• NumberFormat fmt2 NumberFormat.getPercentInstanc
  e()
• // Print messages using formatting objects
• System.out.println ("Subtotal "
  fmt1.format(subtotal))
• System.out.println ("Tax " fmt1.format(tax)
  " at fmt2.format(TAX_RATE))
• System.out.println ("Total "
  fmt1.format(totalCost))
• // Can also use System.out.printf for formatted
• // output similar to C printf
• // System.out.printf(Total 5.2f, totalCost)

15
Simple Dialog Boxes

• /   MaxDialog.java/import
  javax.swing.public class MaxDialog      
  public static void main(String args)   
   String numStr    int max0    int num,
  more    do           numStr
  JOptionPane.showInputDialog("Enter an
  integer")      num Integer.parseInt(numStr) 
  // must extract integer from string      if
  (num gt max)          max num        //
  parentComponent is null, uses default frame     
  JOptionPane.showMessageDialog(null, "Max so far
  " max)      more JOptionPane.showConfirmDia
  log (null, "Check Another?")     while (more
  JOptionPane.YES_OPTION) 

16

• On to Chapter 5
• Decisions

17
Decisions

• if, if-else and nested if-else just like C
• use of braces for compound statements just like
  C
• Relational operators (gt gt lt lt !) just like
  C
• switch is just like C (remember break!)
• dangling else is a problem in Java, just as in
  C
• logical operators (, , !) just like C
• short-circuit evaluation applies

18
Comparing Strings

• To compare contents of strings, use equals
• if (string1.equals(string2)) . . .
• May prefer to ignore case
• if (string1.equalsIgnoreCase(string2)) . . .
• Can use compareTo to find out the relationship
  (lt0 if first is less, 0 if same, gt0 if first is
  greater)
• if (string1.compareTo(string2)) lt 0) . . .
• Order numbers lt uppercase lt lowercase
• shorter strings lt longer strings (e.g., car lt
  cargo

19
String subtlety

• String nickname Rob
• // Creates a literal string Rob
• if (nickname Rob) // succeeds
• But
• String name Robert
• String nickname name.substring(0, 3)
• if (nickname Rob) // fails

20
Enumerated Types

• public enum FilingStatus SINGLE,MARRIED
• FilingStatus status FilingStatus.SINGLE
• if (status FilingStatus.SINGLE) . . .
• Often declared inside a class
• if (status TaxReturn.FilingStatus.SINGLE) . .
  .
• An enumerated type variable can be null

21
De Morgans Law

• !(A B) !A !B
• !(A B) !A !B
• Example
• !(0 lt amount amount lt 1000)
• !(0 lt amount) !(amount lt 1000)
• (0 gt amount) (amount gt 1000)
• NOTE opposite of lt is gt, not gt

22
Reading/Homework Assignment

• Read
• Section 5.5 Test Coverage on page 212
• 5.3 Calculate Sample Data Manually on page 213,
  and
• 5.4 Prepare Test Cases Ahead of Time on page 214
• Questions to answer
• What is black-box testing?
• What is white-box testing?
• What are boundary tests?
• Design a set of test cases for the TaxCalculator
  program. That is, list the exact inputs you
  would test along with the expected outputs.
• Turn in this assignment in a .txt file on
  Blackboard.

23

• On to Chapter 6
• Iteration

24
Loops in Java

• while loops same as C
• same common errors infinite loops, off-by-one
• do loops same as C
• for loops same as C (but with another useful
  syntax for collections)
• same common errors forget semicolon if need
  empty body, or include semicolon on for statement
• Quality tip for loops are best for counting
  loops. Use a while loop for other types (see 6.1
  on page 241)

25
Loops in Java (continued)

• nested Loops same as in C
• Quality tip dont use ! to test the end of a
  range, better to use lt, lt etc.
• sentinel loops same as in C

26
Reading Assignment 1

• How to 6.1 Implementing Loops, page 250
• Quality Tip 6.3 Symmetric and Asymmetric Bounds
• Quality Tip 6.4 Count Iterations
• Advanced Topic 6.3 Loop and a Half Problem
• Advanced Topic 6.4 break and continue
• Reflect on your own coding style as you read, and
  be ready to discuss next time.

27
Reading Assignment 2

• Advanced Topic 6.5 Loop Invariants, page 261
• Random Fact 6.2 Correctness Proofs, page 263
• Thought question what loops have you
  seen/written where this technique might be
  applicable?

28
Exercise

• Bring your books to class!
• Download Word.java and SyllableCounter.java from
  Blackboard
• Work through 6.7 Sample Debugging Session