CS242 Advanced Programming Concepts in Java

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CS242Advanced Programming Concepts in Java

• 11/01/07
• I/O and Cloning

Prof. Searleman jets_at_clarkson.edu
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Outline

• Java I/O
• Binary I/O StreamReader, StreamWriter
• Text streams unicode
• Cloning
• Reading
• - Java Technical Tips, Cloning objects
• (http//java.sun.com/developer/JDCTechTips/2001/tt
  0306.html)
• - Effective Java Item 10
• - OODP, p. 112-113 Chap. 7, pp. 280 - 285
• HW5 posted, team (of 2), due 11/13/07
• Review session for Exam1 tonight, 700 pm, ITL

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Binary Data

• Use DataOutputStream to create a binary file
  then use DataInputStream to read in data from
  that binary file
• Use Object Input/Output streams for
  serialization
• Use Zip or GZip Input/Output streams for file
  compression

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• // Example create a gzipped output file
• DataOutputStream outFile
• new DataOutputStream(
• new BufferedOutputStream(
• new GZIPOutputStream(
• new FileOutputStream(mydata.gz)))
• int c
• while((c in.read()) ! -1)
• outFile.write(c)
• outFile.close()
• // Exercise read in and print the data in this
  file

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Text Streams

• consider the text 1342
• Unicode 00 31 00 33 00 34 00 32
• most environments use their own char encoding
  e.g. windows 31 33 34 32
• Java provides conversion filters from Unicode to
  the char encoding used by the local OS

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Input/Output StreamReader revisited

• InputStreamReader converts an input stream
  containing bytes in a particular char encoding
  into a reader that emits Unicode chars
• OutputStreamReader is similar

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• e.g. read keystrokes from console (recall that
  System.in is an InputStream), and convert to
  Unicode
• A..Z \u0041 .. \u005a
• a..z \u0061 .. \u007a
• 0..9 \u0030 .. \u0039
• InputStreamReader isr
• new InputStreamReader(System.in)
• int x isr.read()
• user types 4 what is x (in decimal)?

52 (hex 34)
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Other encodings
new InputStreamReader( new FileInputStream(myd
ata, 8859_5)
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Cloning making safe copies

• Item 10 Override clone judiciously
• Cloneable interface
• clone() method
• shallow vs. deep cloning
• in addition to the Effective Java text, also
• read OODP, p. 112-113 Chap. 7, pp. 280 - 285

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• // consider the following class
• public class Swimmer
• private String name
• private float swimTime
• private Date eventDate
• public Swimmer()
• public String getName() return name
• public float getSwimTime() return swimTime
  
• public Date getEventDate() return eventDate
  

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• // application programmer writes
• Swimmer cu1 new Swimmer()
• Date d cu1.getEventDate()
• d.setYear(1982) // problem!! Date is mutable
• Its not just that setYear() is deprecated (we
  can adjust for that by using calendar class) the
  more serious problem is that we have broken
  encapsulation
• This is true even if the fields are made final

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Swimmer
String
Kristen
name
swimTime
26.31
eventDate
Date
year
2007
1982
etc.
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Aliasing problem

• both d and cu1.eventDate refer to the same exact
  object
• so, if you change one, you change the other
• solution accessor should return a clone
  (copy) of the event date
• is this a problem for the name field?
• clone() makes a copy of an object the question
  is what kind of copy?

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Object cosmic superclass

• public final Class getClass()
• public int hashCode()
• public boolean equals(Object obj)
• public String toString()
• protected Object clone() throws
• CloneNotSupportedException

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protected Object clone() throws
CloneNotSupportedException

• Creates and returns a copy of this object. The
  precise meaning of "copy" may depend on the class
  of the object. The general intent is that, for
  any object x
• x.clone() ! x
• will be true, and that the expression
• x.clone().getClass() x.getClass()
• will be true, but these are not absolute
  requirements. While it is typically the case
  that
• x.clone().equals(x)
• will be true, this is not an absolute
  requirement.

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Object.clone()

• makes a field-by-field, bit-by-bit copy of a
  cloneable object
• throws CloneNotSupportedException for any class
  which doesnt implement cloneable
• java.lang.Object declares its clone() protected
  and native gt can call Object.clone() the
  native method that duplicates the object only
  from within the object being cloned

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Cloneable interface

• the Cloneable interface is a marker interface
  it has no methods!
• the purpose of a marker interface is allow you to
  use instanceof in a type inquiry
• if (x instanceof Cloneable)
• repeat clone() is not in the Cloneable interface
  (unlike Comparable) the clone() method is in
  class Object

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Cloning dice

• Die d1 new Die()
• Die d2 (Die) d1.clone() // ERROR
• need to do the following
• implement Cloneable interface
• override Object.clone() method

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• public class Die
• implements Cloneable, Comparable
• public Object clone()
• try
• return super.clone()
• catch (CloneNotSupportedException e)
• throw new InternalError(e.toString())
• // etc.

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Cloning swimmer

• Its not enough to use the Object cloning method,
  since it only makes a shallow copy

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• public class Swimmer
• implements Cloneable, Comparable
• public Object clone()
• try
• return super.clone()
• catch (CloneNotSupportedException e)
• throw new InternalError(e.toString())
• // etc.

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Cloning swimmers

• Swimmer s1 new Swimmer()
• Swimmer s2 (Swimmer) s1.clone()
• Whats the problem?
• Have used the bitwise, shallow copy. So the
  references still point to the same exact objects.
• Not a problem for immutable objects, but is
  problematical for mutable objects

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Cloning swimmer

• call Object.clone() to make a bitwise copy of the
  swimmer
• cast the result to Swimmer so that the eventDate
  instance variable can be cloned
• clone the eventDate
• return the result

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• public class Swimmer
• implements Cloneable, Comparable
• public Object clone()
• try
• Swimmer s (Swimmer) super.clone()
• s.eventDate (Date) eventDate.clone()
• return s
• catch (CloneNotSupportedException e)
• throw new InternalError(e.toString())

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It is up to the class designer to determine
whether

• the default clone() is good enough
• Shallow cloning
• the default clone() can be patched up by calling
  clone on each non-primitive, mutable instance
  variable
• Deep cloning
• give up dont use clone

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Case 1 shallow cloning

• Default clone is good enough
• All instance variables are either primitive
  values or immutable objects, and every superclass
  up to Object is well-behaved
• implement Cloneable interface
• in the class, define a method
• public Object clone() that calls
• super.clone() and returns that copy

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• public class X
• implements Cloneable
• public Object clone()
• try
• return super.clone() // shallow clone
• catch (CloneNotSupportedException e)
• throw new InternalError(e.toString())
  

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Case 2 deep cloning

• The default clone can be patched up
• implement Cloneable interface
• in your class, define a method
• public Object clone()
• that calls super.clone() to make a shallow copy,
  plus makes deep copies of fields that refer to
  mutable objects by calling clone() individually
  on those fields

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• public class X implements Cloneable
• private int a // primitive
• private String b // reference to immutable
  object
• private Y c // where class Y has mutator
  methods
• private Z d // where class Z has mutator
  methods
• public Object clone() // deep clone
• try
• X other (X) super.clone() // fields a
  b OK
• other.c (Y) c.clone() // fix c by making
  a copy
• other.d (Z) d.clone() // fix d by making
  a copy
• return other // return the deep clone
• catch (CloneNotSupportedException e)
• throw new InternalError(e.toString())

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Case 2 deep cloning requirements

• the example depends on classes Y and Z having
  the ability to clone
• cannot fix any field which is final

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Case 3 dont use cloning

• even if all fields are primitive and/or
  immutable objects, there may still be problems
• e.g. timestamps, unique serial ,
• Consider providing a copy constructor or a static
  factory method
• (cf. Effective Java Programming Language Guide,
  pp. 45-52)

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• public class Swimmer implements Cloneable
• private String name
• private float swimTime
• private Date eventDate
• public Swimmer()
• public String getName() return name
• public float getSwimTime() return swimTime
  
• public Object clone() / case 2 deep clone
  /
• public Date getEventDate()
• return (Date) eventDate.clone()
• // note Date is cloneable

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What about arrays?

• public class CloneDemo6
• public static void main(String args)
• int vec1 new int1, 2, 3
• int vec2 (int)vec1.clone()
• System.out.println(vec20 " "
• vec21 " " vec22)

Note all arrays are considered to implement the
cloneable interface
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Cloning an array of objects

• public class DiceGame
• private Die dice new Die5
• for (int i 0 i lt dice.length i)
• dice0 new Die(i6)
• Need a deep copy to clone DiceGame

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To clone an array of Objects

• clone the array, then
• clone each object in the array
• Similar for cloning Collections

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• public class A implements Cloneable
• private Map map
• public A()
• map new HashMap()
• map.put("key1", "value1")
• map.put("key2", "value2")
• public Object clone()
• try
• A other (A)super.clone()
• other.map (HashMap)map.clone()
• return other
• catch (CloneNotSupportedException e)
• throw new InternalError(e.toString())
  

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• public class CloneDemo5
• public static void main(String args)
• A obj1 new A()
• A obj2 (A)obj1.clone()
• obj1.map.remove("key1")
• System.out.println(obj2.map.get("key1"))
  

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• public class CloneDemo8
• private int a
• private int b
• private final long c
• public CloneDemo8(int a, int b)
• this.a a
• this.b b
• this.c System.currentTimeMillis()
• // how to clone this so that the copy has
• // the correct timestamp? cant

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• // instead of cloning, provide a copy
• // constructor
• public CloneDemo8(CloneDemo8 obj)
• this.a obj.a
• this.b obj.b
• this.c System.currentTimeMillis()

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Things to remember

• cloning is a way to create and initialize an
  object, but its fundamentally different from
  calling a constructor
• MUST call super.clone (not a constructor)
• final fields are problematical
• must consider whether shallow cloning is
  sufficient, or a deep clone is required
• arrays collections of objects require deep
  cloning
• consider providing a copy constructor