Java Enterprise Technology: Java language 3

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Java Enterprise TechnologyJava language 3

• CMPSC 332/609 Spring 2005
• Kurt Steinkraus

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Today

• Error handling
• Coding and naming conventions
• Responsibilities of objects

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Questions?
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Error handling

• Standard methods
• exit(-1)
• Global error flag
• Status return code
• Java (and C) add one more
• Exceptions

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Exception motivation
int readData() if(readFirstPart() -1)
return -1 if(readSecondPart() -1)
return -1 // ... return 0
if(openFile() -1) handleError() if(readData(
) -1) handleError() if(readMoreData()
-1) handleError() if(closeFile() -1)
handleError()
int readFirstPart() if(file.readBytes(...))
return -1 // ... return 0
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Exception handling

• An exception denotes an exceptional (unusual,
  error-like) situation
• Exceptions allow for centralized error handling
• Not passing/checking error code everywhere

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Exception semantics

• Try executing this code
• If an error happens, throw an exception
• If an error is thrown, catch the error and do
  something with it (possibly re-throw it)

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Exception example
void readData() throws IOException
readFirstPart() readSecondPart() // ...
try openFile() readData()
readMoreData() closeFile()
catch(IOException e) handleError()
void readFirstPart() throws IOException
if(file.readBytes(...)) throw new
IOException() // ...
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Notice throwing exceptions

• void readFirstPart() throws IOException
• if(file.readBytes(...))
• throw new IOException()
• // ...
• If an error occurs, create a new instance of an
  exception, and throw it
• You can give it some explanatory text
• throw new IOException("can't read name!")

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Catching exceptions

• When an exception is thrown, it propagates up the
  call stack until it reaches a catch block that
  handles that exception type

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Exception catching example
void readData() throws IOException
readFirstPart() readSecondPart() // ...
try openFile() readData()
readMoreData() closeFile()
catch(IOException e) handleError()
no
void readFirstPart() throws IOException
if(file.readBytes(...)) throw new
IOException() // ...
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Notice try-catch

• try
• openFile()
• readData()
• readMoreData()
• closeFile()
• catch(IOException e)
• handleError()

If an IOException isthrown anywhere in the try
block,
control flow goes tothe catch block,
and then continuesbelow
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Notice declaring exceptions

• void readData() throws IOException
• readFirstPart()
• // ...
• You must declare that a method throws an
  exception if
• It throws the exception directly
• It does not catch that exception if thrown by a
  called method (like readFirstPart)

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Two exception types

• Checked exceptions must be declared or caught
• Like IOException
• Unchecked exceptions dont have to be declared or
  caught

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Two exception types

• Checked exceptions are problems you would want to
  handle in your program
• Unchecked exceptions are problems you probably
  cant handle
• Theyre unexpected
• They denote bugs in your program
• They denote catastrophic failure

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Throwable class hierarchy
Throwable
Exception
Error
Unchecked errors
Checked errors
RuntimeException
Unchecked errors
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Kinds of Throwable

• Error subclasses unchecked
• Hardware or java executable problems
• Exception subclasses checked
• Normal exceptional conditions, e.g., file not
  found, disk full
• RuntimeException subclasses unchecked
• Bugs in your program, e.g., divide by zero

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Finding an exception to throw

• There are a lot of existing exceptions in the
  java standard library
• Standard ones
• NullPointerException
• IllegalArgumentException
• IOException
• You can create your own exceptions too

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Creating custom exceptions

• public class MyException
• extends RuntimeException
• public MyException() super()
• public MyException(String str)
• super(str)
• // any extra fields or methods
• // used to describe what went wrong

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finally block

• Sometimes you want some code to execute no matter
  what
• Normal execution
• Caught exception
• Uncaught exception
• Use a finally block

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finally block example

• openFile()
• try
• // do things to the opened file
• // ...
• finally
• closeFile()

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Exceptions pros and cons

• Pro
• Centralized error handling
• Cleaner code

• Con
• Cant succinctly handle lots of checked
  exceptions
• Cant succinctly ignore exceptions
• Messier code

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Questions on error handling?
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Java coding conventions

• Conventions used in standard library (and you can
  look at the source!)
• http//java.sun.com/docs/codeconv/
• Try to use Suns coding conventions
• Come up with variations if you like, but have
  good reasons and be consistent

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Naming conventions

• Packages
• If for wide distribution, use a unique prefix,
  preferably inverted Internet domain name
• e.g., us.steinkraus.examples

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Naming conventions

• Classes
• Mixed case, each word starts with capital
• e.g., String, PoliteSpeaker
• Interfaces
• Samewe want to know that were talking about an
  object with a particular type, but it shouldnt
  matter if it is a class or interface

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Naming conventions

• Methods
• Mixed case, lowercase first letter, second and
  following words start with capital
• Active verb followed by noun
• e.g., getValue(), processData()
• Solitary noun means tiny accessor method
• e.g., key(), intValue()

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Naming conventions

• Variables
• Mixed case, lowercase first letter, second and
  following words start with capital
• Noun
• e.g., count, windowWidth
• Single-letter names are OK for small-scope
  temporary variables

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Naming conventions

• Constants
• All capitals, words separated by underscores
• e.g., MAX_WIDTH, PI

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My personal conventions

• Hungarian notation (type prefixes)
• m_ indicates member variables
• s_ indicates static variables
• I cant follow conventions in examples
• You dont have to do what I do
• Most important thing is consistency

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Questions about naming conventions?
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java.lang.Object

• Every class implicitly extends Object
• This has benefits
• Collections and other code can operate on any
  class if they operate on Objects
• Every class gets Object interface
• But what is this Object interface?

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Object methods

• protected Object clone()
• public boolean equals(Object o)
• protected void finalize()
• public Class getClass()
• public int hashCode()
• public String toString()
• // synchronization methods for a future class

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clone()

• protected method
• Meant to provide a way of creating a (deep) copy
  of a class without knowing its type
• Associated Cloneable interface
• Semantics if an Object implements Cloneable,
  then it can be cloned

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Using clone?

• List l someList()
• List l2 new LinkedList()
• for(Iterator it l.iterator()
• it.hasNext() )
• Object obj it.next()
• Object obj2 ((Cloneable)obj).clone()
• l2.add(obj2)

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Cant use clone that way

• Cloneable is just a tagging interface
• It doesnt define any methods
• clone() is in Object
• Cloneable just says clone() works
• We shouldnt coerce to Cloneable

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Using clone, take two?

• List l someList()
• List l2 new LinkedList()
• for(Iterator it lt.iterator()
• it.hasNext() )
• Object obj it.next()
• // we could check obj instanceof Cloneable
• // but it would fail the same either way
• Object obj2 obj.clone()
• l2.add(obj2)

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Cant use clone that way

• clone() is protected and can only be accessed by
• Methods in same package
• Methods in subclass

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Using clone, or not

• clone() was probably originally introduced
  because someone had a problem and didnt think
  enough
• Now it has been discovered to have rather broken
  semantics
• Dont use it

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finalize()

• Called when the object is about to be
  garbage-collected
• Used to do last-minute cleanup
• Not guaranteed to be called
• There are better ways to clean up
• Ive never used it

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getClass()

• Used to get a Class object
• Used with reflection API
• Reflection looking back at oneself
• Figuring out what a class looks like at runtime
  rather than compile time
• Sometimes useful for debugging

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toString()

• Default implementation returns a String giving
  information about class and memory address of an
  object
• default toString is unique for each object
• java.lang.Object_at_b6ece5
• You should override toString for any class meant
  to be instantiated
• Very useful for debugging

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equals and hashCode

• Two of the most important Object methods
• Two of the trickiest to override correctly
• And not just because of typos
• equals(MyClass c) vs. equals(Object o)
• hashcode() vs. hashCode()

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equals(Object o)

• always means is the same object, i.e. the
  same object in memory
• .equals() means is an equal object, whatever
  equal means
• default implementation is
• e.g., comparing Strings, use .equals()

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equals(Object o) contract

• equals is an equivalence relation
• equals is consistent across calls
• x.equals(null) returns false

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Equivalence relation

• Reflexive
• x.equals(x) true
• Symmetric
• x.equals(y) y.equals(x)
• Transitive
• x.equals(y) true, y.equals(z) true ?
  x.equals(z) true

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Example Point class

• public class Point
• protected int x, y
• // constructor, access methods, ...
• public boolean equals(Object o)
• if(!(o instanceof Point))
• return false
• Point p (Point)o
• return ((x p.x) (y p.y))

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Equivalence and inheritance

• It is relatively easy to satisfy the equivalence
  relation criteria for a single class
• It is non-trivial to satisfy the equivalence
  relation for inheritance hierarchies

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Example ColorPoint class

• public class ColorPoint extends Point
• private Color color
• // constructor, access methods, ...
• public boolean equals(Object o)
• // ???
• How do we override equals?

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First attempt at equals

• public boolean equals(Object o)
• if(!(o instanceof ColorPoint))
• return false
• ColorPoint cp (ColorPoint)o
• return (super.equals(cp)
• color.equals(cp.color))

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Symmetry violated

• Point p new Point(3, 4)
• ColorPoint cp
• new ColorPoint(3, 4, Color.RED)
• boolean fTrue p.equals(cp)
• boolean fFalse cp.equals(p)

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Second attempt at equals

• public boolean equals(Object o)
• if(o instanceof ColorPoint)
• ColorPoint cp (ColorPoint)o
• return (super.equals(o)
• color.equals(cp.color))
• else if(o instanceof Point)
• return super.equals(o)
• else
• return false

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Transitivity violated

• ColorPoint cp1
• new ColorPoint(3, 4, Color.BLUE)
• Point p new Point(3, 4)
• ColorPoint cp2
• new ColorPoint(3, 4, Color.RED)
• boolean fTrue1 cp1.equals(p)
• boolean fTrue2 p.equals(cp2)
• boolean fFalse cp1.equals(cp2)

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Solutions

• Make ColorPoints equals not compare colors
• But this negates the whole point of ColorPoint
• Make ColorPoint not inherit from Point
• It doesnt really satisfy the is-a relation

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Questions about equality?
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hashCode()

• Provides a good hash on the object
• Used by HashSet, HashMap, etc.
• Default implementation returns the objects
  location in memory

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equals and hashCode??Why are we talking about
these together?
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Aside hashtable innards
hashtable
(0,1) hash 0
?
(6,9) hash 3
(6,9) hash 3
(7,6) hash 5
(1,-2) hash 5
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hashCode() contract

• hashCode is consistent across calls
• hashCode agrees with equals
• If two objects are equal, then they have the same
  hash
• As much as possible, unequal objects have unequal
  hashes
• Advisable but not requiredcould give everything
  a hash of 0

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Review of Object methods

• protected Object clone()
• public boolean equals(Object o)
• protected void finalize()
• public Class getClass()
• public int hashCode()
• public String toString()
• // synchronization methods for a future class

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Questions?
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