/*******************************************************************************
* Companion code for the book "Introduction to Software Design with Java",
* 2nd edition by Martin P. Robillard.
*
* Copyright (C) 2022 by Martin P. Robillard
*
* This code is licensed under a Creative Commons
* Attribution-NonCommercial-NoDerivatives 4.0 International License.
*
* See http://creativecommons.org/licenses/by-nc-nd/4.0/
*
*******************************************************************************/
package e2.chapter2;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
/**
* Represents a deck of playing cards. In this version, the cards in the
* deck are stored in a list and the list of cards in the deck can
* be obtained by client code using an immutable wrapper object.
*/
public class {
private List<Card> aCards = new ArrayList<>();
/**
* Creates a new deck of 52 cards, shuffled.
*/
public Deck() {
shuffle();
}
/**
* Reinitializes the deck with all 52 cards, and shuffles them.
*/
public void shuffle() {
aCards.clear();
for( Suit suit : Suit.values() ) {
for( Rank rank : Rank.values() ) {
aCards.add( new Card( rank, suit ));
}
}
Collections.shuffle(aCards);
}
/**
* Places pCard on top of the deck.
* @param pCard The card to place on top
* of the deck.
* @pre pCard !=null
*/
public void push() {
assert pCard != null;
aCards.add(pCard);
}
/**
* Draws a card from the deck: removes the card from the top
* of the deck and returns it.
* @return The card drawn.
* @pre !isEmpty()
*/
public Card draw() {
assert !isEmpty();
return aCards.remove(aCards.size() - 1);
}
/**
* @return True if and only if there are no cards in the deck.
*/
public boolean isEmpty() {
return aCards.isEmpty();
}
/**
* @return An unmodifiable list of all the cards in the deck.
*/
public List<Card> getCards() {
return Collections.(aCards);
}
}
A first step to have good in your software is to decide on the abstractions of your problem domain that will be represented as (Java) types.
A deck of cards is one such concept. It maps to a clear element from the problem domain, that we will want to manipulate in our program.
A first step to have good in your software is to decide on the abstractions of your problem domain that will be represented as (Java) types.
A deck of cards is one such concept. It maps to a clear element from the problem domain, that we will want to manipulate in our program.
Storing a reference to an argument to a method creates a leaking reference. Client code can still have access to the object they passed, and eventually (thus changing the internal state without using public methods).
Note that this is fine here, because Card objects are immutable, so
even if client code has access to a reference of an internal object, it
can't modify it.
Storing a reference to an argument to a method creates a leaking reference. Client code can still have access to the object they passed, and eventually (thus changing the internal state without using public methods).
Note that this is fine here, because Card objects are immutable, so
even if client code has access to a reference of an internal object, it
can't modify it.
Unless the object is immutable!
Unless the object is immutable!
For example, decks with null cards.
For example, decks with null cards.
Search for "leaking reference" on this document to see some ways.
Search for "leaking reference" on this document to see some ways.
See the Card.java example for an explanation of Javadoc's @pre
tags, which are related to Design by Contract and help avoid
defensive programming.
See the Card.java example for an explanation of Javadoc's @pre
tags, which are related to Design by Contract and help avoid
defensive programming.
The methods of this class all throw a NullPointerException
if the collections or class objects provided to them are null.
The documentation for the polymorphic algorithms contained in this class
generally includes a brief description of the implementation. Such
descriptions should be regarded as implementation notes, rather than
parts of the specification. Implementors should feel free to
substitute other algorithms, so long as the specification itself is adhered
to. (For example, the algorithm used by sort does not have to be
a mergesort, but it does have to be stable.)
The "destructive" algorithms contained in this class, that is, the
algorithms that modify the collection on which they operate, are specified
to throw UnsupportedOperationException if the collection does not
support the appropriate mutation primitive(s), such as the set
method. These algorithms may, but are not required to, throw this
exception if an invocation would have no effect on the collection. For
example, invoking the sort method on an unmodifiable list that is
already sorted may or may not throw UnsupportedOperationException.
This class is a member of the Java Collections Framework.
Good encapsulation generally requires to allow clients to modify mutable fields only through methods of the class.
In this case, it shouldn't be possible to change the content of aCards
other than by using one of Deck's methods. Otherwise, clients may do
any sort of modifications, including some that would generate
decks.
The first step towards this is to make the field private, so that it's
only visible from inside the class (i.e., not visible by client code).
However, this is not enough! References to privates fields can
leak in .
Good encapsulation generally requires to allow clients to modify mutable fields only through methods of the class.
In this case, it shouldn't be possible to change the content of aCards
other than by using one of Deck's methods. Otherwise, clients may do
any sort of modifications, including some that would generate
decks.
The first step towards this is to make the field private, so that it's
only visible from inside the class (i.e., not visible by client code).
However, this is not enough! References to privates fields can
leak in .
Unlike sets, lists typically allow duplicate elements. More formally,
lists typically allow pairs of elements e1 and e2
such that e1.equals(e2), and they typically allow multiple
null elements if they allow null elements at all. It is not inconceivable
that someone might wish to implement a list that prohibits duplicates, by
throwing runtime exceptions when the user attempts to insert them, but we
expect this usage to be rare.
The List interface places additional stipulations, beyond those
specified in the Collection interface, on the contracts of the
iterator, add, remove, equals, and
hashCode methods. Declarations for other inherited methods are
also included here for convenience.
The List interface provides four methods for positional (indexed)
access to list elements. Lists (like Java arrays) are zero based. Note
that these operations may execute in time proportional to the index value
for some implementations (the LinkedList class, for
example). Thus, iterating over the elements in a list is typically
preferable to indexing through it if the caller does not know the
implementation.
The List interface provides a special iterator, called a
ListIterator, that allows element insertion and replacement, and
bidirectional access in addition to the normal operations that the
Iterator interface provides. A method is provided to obtain a
list iterator that starts at a specified position in the list.
The List interface provides two methods to search for a specified
object. From a performance standpoint, these methods should be used with
caution. In many implementations they will perform costly linear
searches.
The List interface provides two methods to efficiently insert and
remove multiple elements at an arbitrary point in the list.
Note: While it is permissible for lists to contain themselves as elements,
extreme caution is advised: the equals and hashCode
methods are no longer well defined on such a list.
Some list implementations have restrictions on the elements that
they may contain. For example, some implementations prohibit null elements,
and some have restrictions on the types of their elements. Attempting to
add an ineligible element throws an unchecked exception, typically
NullPointerException or ClassCastException. Attempting
to query the presence of an ineligible element may throw an exception,
or it may simply return false; some implementations will exhibit the former
behavior and some will exhibit the latter. More generally, attempting an
operation on an ineligible element whose completion would not result in
the insertion of an ineligible element into the list may throw an
exception or it may succeed, at the option of the implementation.
Such exceptions are marked as "optional" in the specification for this
interface.
The List.of and
List.copyOf static factory methods
provide a convenient way to create unmodifiable lists. The List
instances created by these methods have the following characteristics:
UnsupportedOperationException to be thrown.
However, if the contained elements are themselves mutable,
this may cause the List's contents to appear to change.
null elements. Attempts to create them with
null elements result in NullPointerException.
subList views implement the
RandomAccess interface.
This interface is a member of the Java Collections Framework.
If all public methods of a class never leave an object in a corrupted state, then you have a stronger guarantee that the object won't be misused.
Similarly, complex interactions with specific constraints can be encoded in simple methods, so that client code doesn't risk missing some of these constraints.
If all public methods of a class never leave an object in a corrupted state, then you have a stronger guarantee that the object won't be misused.
Similarly, complex interactions with specific constraints can be encoded in simple methods, so that client code doesn't risk missing some of these constraints.
As long as the public members of a class stay the same, their implementation can change, for example to optimize the memory footprint of an algorithm, without affecting the rest of the code.
As long as the public members of a class stay the same, their implementation can change, for example to optimize the memory footprint of an algorithm, without affecting the rest of the code.
The idea of "encapsulation" in software design is to enclose the implementation of concepts (e.g., the workings of a playing card) inside a "capsule", or boundary (or interface), with only a minimal amount of contact points. Good encapsulation has many benefits, such as:
The idea of "encapsulation" in software design is to enclose the implementation of concepts (e.g., the workings of a playing card) inside a "capsule", or boundary (or interface), with only a minimal amount of contact points. Good encapsulation has many benefits, such as:
One can think about the encapsulation concept, with its explicit behaviors (methods), in isolation.
One can think about the encapsulation concept, with its explicit behaviors (methods), in isolation.
Chapter 2, insight #4
Ensure that the design of your classes prevents any code from modifying the data stored in an object of the class without using a method of the class. In particular, be careful to avoid leaking references to private fields of the class that refer to mutable objects
Chapter 2, insight #4
Ensure that the design of your classes prevents any code from modifying the data stored in an object of the class without using a method of the class. In particular, be careful to avoid leaking references to private fields of the class that refer to mutable objects
Chapter 2, insight #3
Hide the internal implementation of an abstraction behind an interface that tightly controls how an abstraction can be used. Declare fields of a class private, unless you have a strong reason not to. Similarly, declare any method private if it should not be explicitly part of the type's interface
Chapter 2, insight #3
Hide the internal implementation of an abstraction behind an interface that tightly controls how an abstraction can be used. Declare fields of a class private, unless you have a strong reason not to. Similarly, declare any method private if it should not be explicitly part of the type's interface
See the Card.java example for an explanation of Java's assert
statements, which are related to Design by Contract and help avoid
defensive programming.
See the Card.java example for an explanation of Java's assert
statements, which are related to Design by Contract and help avoid
defensive programming.
Returning a reference to a private field in a public method creates a leaking reference. Even if client code needs to use a public method to get the reference, it can then do any sort of modification to the field without additional calls to public methods.
To avoid leaking a reference, the method
Collections.unmodifiableList(List) wraps the argument (in this
case, the private field) in another object (the wrapper) that
doesn't allow any modifications.
This doesn't create a new copy, or make the initial field immutable, it only ensures that code that only has access to the wrapper can't modify the original list.
Note, however, that even though the reference to aCards isn't
leaked by this method, references to the Card objects stored
inside the list are leaked. For example, client code could get
the first Card from this list, and modify it directly. This isn't
a problem here, because Card is immutable.
This is the most challenging example of a leaking reference, when it involves objects within shared data structured.
Chapter 2, insight #5
To provide information about the internal data in an object without violating encapsulation, strategies include extending the interface of the class, returning copies of internal objects, or using unmodifiable views
Returning a reference to a private field in a public method creates a leaking reference. Even if client code needs to use a public method to get the reference, it can then do any sort of modification to the field without additional calls to public methods.
To avoid leaking a reference, the method
Collections.unmodifiableList(List) wraps the argument (in this
case, the private field) in another object (the wrapper) that
doesn't allow any modifications.
This doesn't create a new copy, or make the initial field immutable, it only ensures that code that only has access to the wrapper can't modify the original list.
Note, however, that even though the reference to aCards isn't
leaked by this method, references to the Card objects stored
inside the list are leaked. For example, client code could get
the first Card from this list, and modify it directly. This isn't
a problem here, because Card is immutable.
This is the most challenging example of a leaking reference, when it involves objects within shared data structured.
Chapter 2, insight #5
To provide information about the internal data in an object without violating encapsulation, strategies include extending the interface of the class, returning copies of internal objects, or using unmodifiable views
List interface. Implements
all optional list operations, and permits all elements, including
null. In addition to implementing the List interface,
this class provides methods to manipulate the size of the array that is
used internally to store the list. (This class is roughly equivalent to
Vector, except that it is unsynchronized.)
List interface. Implements
all optional list operations, and permits all elements, including
null. In addition to implementing the List interface,
this class provides methods to manipulate the size of the array that is
used internally to store the list. (This class is roughly equivalent to
Vector, except that it is unsynchronized.)
The size, isEmpty, get, set,
iterator, and listIterator operations run in constant
time. The add operation runs in amortized constant time,
that is, adding n elements requires O(n) time. All of the other operations
run in linear time (roughly speaking). The constant factor is low compared
to that for the LinkedList implementation.
Each ArrayList instance has a capacity. The capacity is
the size of the array used to store the elements in the list. It is always
at least as large as the list size. As elements are added to an ArrayList,
its capacity grows automatically. The details of the growth policy are not
specified beyond the fact that adding an element has constant amortized
time cost.
An application can increase the capacity of an ArrayList instance
before adding a large number of elements using the ensureCapacity
operation. This may reduce the amount of incremental reallocation.
Note that this implementation is not synchronized.
If multiple threads access an ArrayList instance concurrently,
and at least one of the threads modifies the list structurally, it
must be synchronized externally. (A structural modification is
any operation that adds or deletes one or more elements, or explicitly
resizes the backing array; merely setting the value of an element is not
a structural modification.) This is typically accomplished by
synchronizing on some object that naturally encapsulates the list.
If no such object exists, the list should be "wrapped" using the
Collections.synchronizedList
method. This is best done at creation time, to prevent accidental
unsynchronized access to the list:
List list = Collections.synchronizedList(new ArrayList(...));
The iterators returned by this class's iterator and
listIterator methods are fail-fast:
if the list is structurally modified at any time after the iterator is
created, in any way except through the iterator's own
remove or
add methods, the iterator will throw a
ConcurrentModificationException. Thus, in the face of
concurrent modification, the iterator fails quickly and cleanly, rather
than risking arbitrary, non-deterministic behavior at an undetermined
time in the future.
Note that the fail-fast behavior of an iterator cannot be guaranteed
as it is, generally speaking, impossible to make any hard guarantees in the
presence of unsynchronized concurrent modification. Fail-fast iterators
throw ConcurrentModificationException on a best-effort basis.
Therefore, it would be wrong to write a program that depended on this
exception for its correctness: the fail-fast behavior of iterators
should be used only to detect bugs.
This class is a member of the Java Collections Framework.