Big Java

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Big Java

• Chapter 7-8

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• Chapter 7
• Arrays and ArrayLists

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Array Basics

• Similar to C, but always allocated
• double data new double10
• Can also have arrays of objects
• BankAccount accounts new BankAccountMAX
• Access is similar to C, but if code goes beyond
  array bounds an exception is thrown
• for (int i0 ilt10 i)
• System.out.println(datai)
• // ArrayIndexOutOfBoundsException on data10
• Arrays have built-in field named length
• for (int i0 i lt data.length i)
• System.out.println(datai)

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Array Initialization

• Common to forget to allocate space
• double data
• data0 29.5
• Can initialize in declaration, like C
• int primes 2, 3, 5, 7, 11
• Can use an anonymous array as parameter
• new int 2, 3, 5, 7, 11

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ArrayLists

• Array lists can grow and shrink as needed
• ArrayList is a generic class (similar to C
  template)
• ArrayList has variety of methods for common
  tasks, for example
• ArrayListltBankAccountgt accounts new
  ArrayListltBankAccountgt()
• accounts.add(new BankAccount(1001))
• accounts.add(new BankAccount(2010))
• BankAccount anAccount accounts.get(1)
• int count accounts.size() // 0 to size-1 are
  valid
• BankAccount account2 new BankAccount(3000)
• accounts.set(1, account2) // overwrites
• accounts.add(0, new BankAccount(1200)) // index
  lt size
• accounts.remove(1)
• for (int i0 iltaccounts.size() i)
• System.out.println(accounts.get(i).getBalance())
  

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ArrayList (continued)

• With an unparameterized ArrayList, any type of
  object may be stored, but the object must be cast
  when retrieved
• ArrayList accounts new ArrayList()
• accounts.add(new BankAccount(1500))
• BankAccount acct (BankAccount) accounts.get(0)
• Exception will occur if cast is incorrect. Much
  better practice to use generic class (warning in
  Eclipse if use unparameterized)

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Wrappers and Auto-boxing

• Not in C (C evolved from C, Java designed
  from scratch as OO)
• Numbers are not objects, cant place directly in
  ArrayLists
• All primitive types have wrapper classes
• Wrapper contains one value of corresponding
  primitive type
• Also have useful static methods

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Wrappers and Auto-boxing (continued)

• Starting with Java 5.0, conversion between
  primitive types and wrapper classes is automatic,
  called auto-boxing (auto-wrapping would be more
  consistent!)
• Examples
• Double d 29.95
• // Double d new Double(29.95)
• double x 5
• // auto-unbox, same as double x
  d.doubleValue()
• Double e d 1
• // auto-unbox, add 1, auto-box
• ArrayListltDoublegt data new ArrayListltDoublegt()
• data.add(32.14)
• double x data.get(0)

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Efficiency Note

• Even though you dont have to write the code, if
  you have a long sequence of primitive types
  (numbers or chars), use an array rather than an
  ArrayList for efficiency

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Enhanced for loop

• Convenient new syntax for iterating an array or
  ArrayList
• double data . . .
• double sum 0
• for (double e data)
• sum sum e
• With ArrayLists
• ArrayListltBankAccountgt accounts
• . . . add, create sum
• for (BankAccount a accounts)
• sum a.getBalance()

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2D Arrays

• Similar to C, but must always allocate memory
• final int ROWS 3
• final int COLS 3
• String board new StringROWSCOLS

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More on Arrays etc.

• First choice will normally be ArrayList, unless
  storing large collection of primitives OR need 2D
• Use for each loop pattern when processing all
  elements
• To make a copy, use the clone method
• double data new double10
• . . .
• double prices (double ) data.clone()
• Or use System.arraycopy if not copying the entire
  array (see book or API for details)

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Reading Assignment 1

• Random Fact 7.1 An Early Internet Worm, page 318
• Random Fact 7.2 The Therac-25 Incidents, page 322
• Be ready to discuss in class.

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Reading Assignment 2

• Read 7.8 Regression Testing
• Questions
• What is a test suite?
• What is regression testing?
• What is cycling?
• Continued in Chapter 8

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• On to Chapter 8
• Designing Classes

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Choosing Classes

• Common class names are nouns, methods names are
  verbs
• class BankAccount, methods deposit, withdraw etc.
• class CashRegister, methods enterPayment,
  recordPurchase, etc.
• Classes may also be actors that do some type of
  work for you
• Scanner scans stream for numbers and strings
• Random generates random numbers
• Utility class may have no objects, but exists to
  provide static methods and constants
• Math class
• Some classes just exist to start a program
  (degenerate classes)

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Choosing Classes (continued)

• Which of the following are good classes?
• ComputePaycheck
• PaycheckProgram
• Paycheck
• ChessBoard
• MoveChessPiece
• A class should represent a single concept

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Cohesion and Coupling

• Cohesion All public methods (i.e., the public
  interface) should be closely related to the
  single concept the class represents
• Coupling Number of dependencies between classes

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UML Dependency Relationship
Unified Modeling Language Booch, Jacobson,
Rumbaugh
In a class diagram, dependency is a dashed line
with a shaped open arrow that points to dependent
class. (i.e., CashRegister depends on Coin)
CashRegister
Coin
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Coupling
low coupling
high coupling
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Static Methods

• Also known as class method
• Does not operate on an object, so it has only
  explicit parameters
• Example
• public class Financial
• public static double percent of(double p, double
  a)
• return (p/100) a
• double tax Financial.percentOf(taxRate, total)

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Static Fields

• Used to store values outside any particular
  object
• public class BankAccount
• private double balance
• print int lastAssignedNumber 1000 // No
• should be
• print static int lastAssignedNumber 1000

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Static Field Initialization

• 3 options
• Do nothing. Get default values.
• Use explicit initializer (like example).
  Initialization executed when class is loaded.
• Use static initialization block (rarely used)
• public class BankAccount
• . . .
• private static int lastAssignedNumber
• static
• lastAssignedNumber 1000

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Reading Assignment

• Quality Tips 8.2 and 8.3 page 345
• Advanced Topic 8.1 (Call by Value and Call by
  Reference) page 346
• 8.5 Preconditions and Postconditions, page 347
• Advanced Topic 8.2 (Class Invariants)
• Read self-check question 15

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Packages

• A java package is a set of related classes
  (similar to C namespace)
• Common packages

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Create your own Package

• Put a package statement on first line
• package com.horstmann.bigjava
• To avoid name clashes, common to use domain names
• Can import an entire package or a single class
• import java.util.
• import java.util.ArrayList
• Directory structure on disk will match package
  (e.g., com/horstmann/bigjava)
• Directories containing packages should be added
  to the classpath (depends on OS/IDE)
• In Eclipse, you can use the New Package icon to
  create a package, and the directory structure
  will be created for you.

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Unit Test Frameworks

• Good to create a test suite
• Goal relieve human from task of comparing
  expected and actual values
• JUnit framework available from http//junit.org
• Also included in Eclipse
• Idea design companion test class for each class
• In JUnit 3, test class must extend class TestCase
  from junit.framework
• define a method whose name starts with test,
  e.g., testSimpleCase
• If all tests pass, JUnit shows a green bar

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JUnit Framework Classes

• Test Case. Collects a set of test methods. Name
  starts with test. Has no arguments.
• Test Suite. Collection of tests run at the same
  time. May be just a single test method.
• Test Runner. Utility used to run a test suite.
  In Eclipse, shows a graphical presentation. May
  also be a command-line version.

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Setting up Eclipse projects

• JUnit tests are generally separate from
  application source code
• File gt New gt Source Folder test
• Put your classes that extend TestCase in the new
  source Folder (JUnit v3)
• Need to add junit.jar to build path

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Directory Structure
class
related test class
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Build Path
Select Project gt Properties On dialog, select
Java Build Path, Add External Jar Browse to find
junit.jar (normally under plugins) and press Add
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Run as JUnit Test
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JUnit v3 Execution
All is well!
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JUnit v4

• New Java Feature annotations
• Places a marker in code that is interpreted by
  another tool
• For JUnit, marker/annotation is _at_Test
• No need to extend TestCase

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Need different Build Path
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Slight Modification to Test class
different import
_at_Test annotation
Assert
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JUnit 4 Execution
Same as version 3
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Exercise

• Work with a partner
• Download CashRegister and the associated test
  classes from Blackboard
• Set up and run in Eclipse using both version 3
  and version 4 of JUnit
• Write a program that uses JOptionPane dialogs to
  prompt the user for a temperature and wind speed,
  displays the wind chill, then asks whether the
  user wants to do more temperatures.
• The formula for wind chill is
• 35.74 0.6215T - 35.75V (0.16)
  0.4275TV(0.16)
• In the formula, V is in the wind speed in statute
  miles per hour, and T is the temperature in
  degrees Fahrenheit.
• Write a test class to accompany your wind chill
  calculator
• NOTE See http//www.weatherimages.org/data/windch
  ill.html
• for important information