Transcript Lecture slides

CSC 243 – Java Programming,
Spring 2014
March, 2014
Week 8ish, java.lang.Clonable &
java.io.Serializable Interfaces
java.lang.Cloneable
• Cloning is akin to a copy constructor in C++.
• java.lang.Object defines this method
• protected Object clone() throws
CloneNotSupportedException
• java.lang.Cloneable is a marker interface
• It does not define any method.
• A class that inherits Cloneable guarantees that it
provides a public clone() method that returns a copy of
the object on which clone() is invoked.
Shallow versus deep copy
• Shallow copy clones the elements of a container by
copying references to contained objects. It does not
clone contained objects.
• LinkedList<String> dictionary = LIST WITH ELEMENTS
• LinkedList<String> listcopy = dictionary.clone();
• Deep copy clones the elements, recursively.
• LinkedList<IFillWord> original = SOME ELEMENTS
• LinkedList<IFillWord> duplicate = new LinkedList<IFillWord> ();
• for (IFillWord game : original) {
» duplicate.add(game.clone());
• }
Covariance of subclass & return
type
• Interfaces and classes that subclass Cloneable can refine its
return type to a subclass of the base class clone() method,
and loosen protection to public.
• Object has protected Object clone()
• IFillWord has public IFillWord clone()
• FillWordBasic has public FillWordBasic clone()
• FillWordGrows has public FillWordGrows clone()
• Internally clone() constructs a new object and copies
fields from the original object using shallow or deep
copy. Our classes use deep copy on the board and
random number generator.
java.io.Serializable
• Serializable is also a marker interface.
• It guarantees that its object fields consist of primitive
types, Serializable objects, and arrays of these two
types.
• Serialization ignores fields marked as transient.
• A Serializable object easily converts to and from a byte stream
(a byte []) for transmission over a network or storage in a file.
• There are special methods you can define for
Serializable objects with some non-Serializable fields.
We will ignore these for now.
Serializable Object graphs
• When multiple objects contain fields that point to
each other, that is an object graph.
• Java input-output classes have automatic means for
serializing an entire object graph, as long as it
consists strictly of Serializable objects, primitive
types, and arrays of these.
• Transmitting or storing object graphs is almost easy.
• Field private static final long serialVersionUID =
A_LONG_VALUE ; is used to tag the version number of a
Serialized object, in case a later version of its class
changes field types.
FillWordHelper fields in
Assignment 3
protected transient List<String> dictionary
protected char [][] board
protected int wordSizeLimit
protected RandomCloneable randomNumGenerator
protected boolean isacross
protected String wordToPlay
protected int score
protected int swaps
We will clone() using deep copy on randomNumGenerator &
board. Random is Serializable. I have subclassed it in
RandomCloneable to make randomNumGenerator Cloneable.
We will serialize every field except dictionary. We do not want to
store or transmit a copy of the dictionary on every step.
Applications for last year
• We usedserialization & deserialization to save
& restore games in files in Assignment 3.
• We could use clone() to maintain an undo –
redo stack of copies of games.
• We have used clone() in a past grad O-O
course to allow artificial software players (AIs)
to play thousands of games in searching for
best moves by cloning ScrabbleGame state.