These notes form a Java-specific supplement to the Lecture Notes on Data Abstraction. This discussion here is meant to be read after completion of the Data Abstraction notes.
As a language construct, a Java class is similar to a
user-defined struct type in C or user-defined record
type in Pascal. A class is a template for
constructing data items that have the same structure but differing
values (states). We say that an item constructed by a class is a
class instance (or, as we see later, an object).
Like the C structure type or Pascal record type, a Java class can consist of several components. In Pascal, all the components are data fields. However, in Java, functions and procedures may be included as components of a class. These procedures and functions are called methods.
Caveat: The discussion of Java in these notes does not use generic type parameters.
A method declared in a class may be either a class method or instance method.
We declare a method as a class method by giving the keyword
static in the header of its definition.
For example, a main method of a program is a class method
of the class in which it is defined.
public static void main(String[] args)
{ // beginning code for the program
}
If we do not include the keyword static in the
header of a method definition, the method is an instance
method. For example, consider methods to implement the
push and top operations of a class that
implements a stackb.
public void pop()
{ // code for pop operation
}
public Object top()
{ // code for top operation
}
Note that pop() is a procedure (i.e., it has
return type void) method and top is a
function method.
Scala note: The Scala language does not have static members of
classes. However, the methods of a Scala singleton
object have basically the same characteristics described
above for "class methods". Often the "class methods" appear in the
companion object for a class. i.e., the
object with the same name as the class.
In a similar fashion, the variables (data fields) declared in a class may be either class variables or instance variables.
static is used to declare a
class variable.
static denotes an
instance variable.
An instance method has direct access to the instance variables of the
class instance (object) to which it is applied. The instance's
variables are implicit arguments of the method calls. (If needed to
distinguish among names, the builtin variable this can be
used to refer to the instance to which the method is applied.) The
instance methods also have access to the class variables (if any).
Class methods only have access to the class variables. The methods do not have any implicit arguments. In fact, class methods can be called without any instances of the class being in existence.
Scala note: The Scala language does not have static members of
classes. However, the variables of a Scala singleton
object have basically the same characteristics described
above for "class variables". Often the "class variables" appear in the
companion object for a class. i.e., the
object with the same name as the class.
The components of a class can be designated as public or
private.
public components of the class are accessible
from anywhere in the program (i.e., from any package).
private components are only accessible from
inside the class.
As a general rule, the data fields of a class should be private instance variables, meaning that they are associated with a specific instance and are only accessible by the instance methods. This hides, or encapsulates, the data fields within the class instance.
Note: Actually, the instance methods of a Java class can access the instance variables of any instance of that class, not just the current instance.
In general, avoid public instance variables. They break the principle of information hiding, leading to potential entanglements among modules.
A public method of a class is a service provided by that instance to other parts of a program. The private methods of a class can be used in implementing the public methods.
Class methods and variables should be used sparingly. These are more or less the types of subprograms and global variables found in languages like Pascal and C. Their excessive use can greatly reduce the potential benefits that can be realized from object-oriented techniques.
Java note: There are two other types of accessibility, "friendly" and
protected, but public and
private are sufficient for our purposes in discussion of
ADT implementations.
Scala note: Although similar in concept to that of Java, the
accessibility features of Scala differ somewhat. By default, all
features are public in Scala, but accessibility can restricted in a
more fine-grained manner than in Java. The unmodified keyword
private has the same meaning as in Java.
A Java variable is a strongly typed "container" in memory that is declared to hold either:
int), floating point numbers
(double), booleans (boolean), and single
characters (char).
Java note: Although arrays are not class instances, array variables hold a reference to an instance of the array.
The class instances themselves are stored in the dynamically managed heap memory area. Java allocates memory from the heap to hold newly constructed instances of a class. Java's garbage collector reclaims the memory for instances that are no longer needed by the program.
Note: Recent versions of Java can sometimes hide the differences
between primitive values and references by automatically "boxing"
primitive values as instances of the corresponding wrapper classes
(e.g., int values as Integer
instances). Scala goes further in that primitives and references are
in the same type hierarchy. However, both languages run on the Java
Virtual Machine, which makes a distinction between primitive values
and references (i.e., pointers), so it is not possible to avoid the
distinction entirely.
If only one implementation of an ADT is needed, the following techniques can be used to implement an ADT using Java.
The implementation techniques discussed in this section implement the ADT in an imperative way. That is, instead of returning a new instance of the ADT with a modified state, a mutator operation usually modifies the state of the existing instance.
Caveat: The discussion of Java in these notes does not use generic
type parameters. For the StackB ADT, the type of the
items stored in the stack can be a parameter of the
StackB class.
class construct to represent
the entire ADT. If we want to allow access to the class from
anywhere in the program, we will make the class public.
For the StackB ADT, we can use the following structure for
the class:
public class StackB
{ // implementation of instance methods and data here
}
For example, to declare a variable that can hold a reference to a
StackB instance, we can use the following declaration:
StackB stk;
The class and its methods should be documented with the invariants, preconditions, and postconditions.
A Java constructor is a method with the same name as the class. It does not have a return type specified. Upon creation of an instance of the class, the constructor initializes the instance's state so that the class invariants are established.
A constructor is normally invoked by the Java operator
new. The operator new allocates memory on
the heap for the instance, calls the constructor to initialize the new
instance, and then returns a reference to the new instance.
For example, we can represent the ADT operation
create by the constructor method StackB.
public class StackB
{ public StackB(int size)
{ // initialization code
}
// rest of StackB methods and data ...
}
A user of the StackB class can then declare a variable
and initialize it to hold a reference to a new stack with a capacity
of 100 items as follows:
StackB stk = new StackB(100);
The expression new StackB(100) allocates a
StackB instance in the heap storage and calls the
constructor above to initialize the data fields encapsulated within
the instance.
We can apply a method to a class instance by using the selector (i.e.,
"dot") notation. This notation is similar to the notation for
accessing record components in Pascal.
For example, in the case of the StackB ADT we can
represent the operations as instance methods of class
StackB. The explicit StackB parameters and
return values of the operations thus become implicit.
Suppose we want to push an item x onto the stk
created above. We can do that with the following code:
if (!stk.full())
stk.push(x);
We can then examine the top item and remove it:
if (!stk.empty())
{ it = stk.top();
stk.pop();
}
public methods of the class. That
is, precede the method's definition by the keyword
public.
void) methods, except those mutator operations that
explicitly require new instances to be generated (e.g., a copy or
clone operation).
For example, the pop method of StackB would
have the following structure:
public void pop()
{ // code to implement operation
}
A mutator method modifies the encapsulated state of the class instance (which is the implicit argument of the method). In any circumstance in which its precondition and the class invariants hold on entry, the method must establish its postcondition and reestablish the invariants upon exit. (The invariant might not hold in the middle of the method's execution.)
Comment: Implementing mutator operations as procedure calls that modify the stored state is really an optimization. All mutators can be implemented in the applicative style, returning a modified copy of the instance. This implementation might, however, be quite inefficient in use of processor time and memory.
clone), implement the corresponding Java methods to
return new instances of the class rather than to modify the current
instance (i.e., their implicit arguments).
Any mutator method must, of course, establish its postcondition and reestablish the invariants for the current instance. In addition, these applicative mutators must also establish the invariants for the new instance returned.
For example, the empty method of StackB would
have the following structure:
public boolean empty()
{ // code to implement operation
}
An accessor method accesses the encapsulated state of the class instance (which is the implicit argument) and computes a value to be returned. In any circumstance in which its precondition and the class invariants hold on entry, the method must establish its postcondition and reestablish the invariants upon exit. (The invariant might not hold in the middle of the method's execution.)
For example, in the StackB class, we might include an
explicit destroy operation that releases the storage resources and
disables further use of the instance.
public void destroy()
{ // code to free resources
}
Note: The Java framework allows a finalize() method to be
included in each class. This method is called implicitly whenever the
garbage collector detects that the instance is no longer in use.
However, since it is difficult to predict when (if ever) this method
will be executed, it is safer to include explicit destructors when
resources are in short supply and must be explicitly managed.
private data fields of the Java class to
represent the encapsulated state of the instance needed for a
particular implementation. By making the data fields
private they are still available to the instance's
methods, but are not visible outside the class.
For example, the StackB class might have the following
data fields:
public class StackB
{ // public operations of class instance
// encapsulated data fields of class instance
private int topItem; // Pointer to next index for insertion
private int capacity; // Maximum number of items in stack
private Object[] stk; // the stack
}
public data fields in the class.
These violate the principle of information hiding. Instead
introduce appropriate accessor and mutator methods to allow
manipulation of the hidden state.
private methods to aid in
implementation.
Functionality common to several methods can be placed in separate
functions and procedures as needed. However, since these are
private, they can only be accessed from within the class
and thus can be changed without affecting the public interface of the
class.
For example, it is frequently useful to add public
toString and clone methods. The
toString method returns a Java String
reflecting the "value" of the instance in a format suitable for
printing. The clone method creates a new instance that
has the same value as the current instance.
static) variables. Since a class variable is
shared among all instances of the class, it may be difficult to
preserve the invariants for individual instances as the value of the
class variable changes.
However, it is a good programming practice to use class
constants where appropriate. These are data fields
declared with both the static and final
modifiers. Their values may be initialized but cannot be changed
thereafter.
These constants may be declared private if usage is to be
restricted to the class or public if the users of the
class also need access.
By convention, the names of constants are normally written with all
uppercase letters. For example, the following defines a symbolic name
for the integer code used for Sunday as a day of the week in the
Day class defined later.
public static final int SUNDAY = 1;
Caveat: When this set of notes was originally written, Java did not
yet have generics. So the examples below handle the type parameters of
the ADT in other ways. A Java generic provides a class facility that
can be parameterized with types (like the C++ template or
Ada generic mechanisms).
For example, in the implementation below we represent the set
Item of the StackB ADT by the class
Object. As we will see when we discuss inheritance, the
Object type will allow us to store an instance of any
class on the StackB. With this definition, any data of a
reference type can appear in the stack, but values of the primitive
types cannot. A better implementation would have Item as
type parameter of the class.
The next section gives a Java implementation of the
StackB ADT. A similar constructive definition and two
implementations of a Queue
ADT are available in a separate document.
In this section, we give an implementation of the StackB
ADT that uses an array of objects and an integer "pointer" to
represent the stack. (This implementation does not use Java generic
classes.)
This implementation is not robust; each operation assumes that its precondition holds. A more robust implementation might check whether the precondition holds and throw an exception if it does not.
Remember that the invariants are implicitly pre- and postconditions of all mutator and accessor methods, postconditions of the constructor, and preconditions of the destructor.
// A Bounded Stack ADT
public class StackB
{ // Interface Invariant: Once created and until destroyed, this
// stack instance has a valid and consistent internal state
public StackB(int size)
// Pre: size >= 0
// Post: initialized new instance with capacity size && empty()
{ stk = new Object[size];
capacity = size;
topItem = 0;
}
public void push(Object item)
// Pre: not full()
// Post: item added as the new top of this instance's stack
{ stk[topItem] = item;
topItem++;
}
public void pop()
// Pre: not empty()
// Post: item at top of stack removed from this instance
{ topItem--;
stk[topItem] = null;
}
public Object top()
// Pre: not empty()
// Post: return item at top of this instance's stack
{ return stk[topItem-1];
}
public boolean empty()
// Pre: true
// Post: return true iff this instance's stack has no elements
{ return (topItem <= 0);
}
public boolean full()
// Pre: true
// Post: return true iff this instance's stack is at full capacity
{ return (topItem >= capacity);
}
public void destroy()
// Pre: true
// Post: internal resources released; stack effectively deleted
{ stk = null;
capacity = 0;
topItem = 0;
}
// Implementation Invariant for informal model:
// 0 <= topItem <= capacity &&
// stack is in array section stk[0..topItem-1]
// with the top at stk[topItem-1], etc.
// Implementation Invariant for more formal model representing stack
// as tuple (integer max, sequence stkseq)
// m == capacity && 0 <= topItem <= capacity &&
// stackInArray(stk,topItem,stkseq)
// where stackInArray(arr,t,ss) = if t == 0 then ss == []
// else arr[t-1] == head(ss)
// && stackInArray(arr,t-1,tail(ss))
private int topItem; // Pointer to next index for insertion
private int capacity; // Maximum number of items in stack
private Object[] stk; // the stack
}
If several different implementations of an ADT are needed, then the Java specification of an ADT's interface should be separated from the class implementation. The interface specification can be reused among several classes and various implementations of the interface can be used interchangeably.
This can be done as follows:
interface that specifies the
type signatures for the ADT's mutator and accessor (and, if needed,
destructor) operations. These method signatures should have
the same characteristics as described above in the discussion of
class-based specification.
interface by
the interface invariants, preconditions, and postconditions that must
be supported by any implementation of the ADT. There are no
implementation invariants for an interface, but individual classes
that implement the interface will have them.
For example, a bounded stack interface might be specified
as follows:
public interface StackADT
{ // Interface Invariant: Once created and until destroyed, this
// stack instance has a valid and consistent internal state
public void push(Object item);
// Pre: not full()
// Post: item added as the new top of this instance's stack
...
public Object top();
// Pre: not empty()
// Post: return item at top of this instance's stack
...
}
interface.
For example, an array-based StackADT could be implemented
similarly to the StackB definition given in the previous
section.
public class StackInArray implements StackADT
{ // Interface Invariant: Once created and until destroyed, this
// stack instance has a valid and consistent internal state
public StackInArray(int size)
// Pre: size >= 0
// Post: initialized new instance with capacity size && empty()
{ stk = new Object[size];
capacity = size;
topItem = 0;
}
public void push(Object item)
// Pre: not full()
// Post: item added as the new top of this instance's stack
{ stk[topItem] = item;
topItem++;
}
...
public Object top()
// Pre: not empty()
// Post: return item at top of this instance's stack
{ return stk[topItem-1];
}
...
// Implementation Invariant for informal model:
// 0 <= topItem <= capacity &&
// stack is in array section stk[0..topItem-1]
// with the top at stk[topItem-1], etc.
// Implementation Invariant for more formal model representing stack
// as tuple (integer max, sequence stkseq)
// m == capacity && 0 <= topItem <= capacity &&
// stackInArray(stk,topItem,stkseq)
// where stackInArray(arr,t,ss) =
// if t == 0 then ss == []
// else arr[t-1] == head(ss)
// && stackInArray(arr,t-1,tail(ss))
private int topItem; // Pointer to next index for insertion
private int capacity; // Maximum number of items in stack
private Object[] stk; // the stack
}
interface
type to hold instances of any concrete class that implements the
interface. Any of the operations defined in the
interface can be applied to the instance to which this
variable refers.
For example, a variable of type StackADT can hold
instances of any concrete class that implements the interface
StackADT.
StackADT theStack = new StackInArray(100);
theStack.push("Hello World");
For an ADT specification and implementations that follow this approach, see the description of the Ranked Sequence ADT case study given in a separate document. In addition to Java interfaces, the Ranked Sequence case study uses other Java features such as exceptions, enumerations, packages, and Javadoc annotations.
The following implementation of the Day ADT is adapted
from the like-named class in Chapter 4 of the book Core Java
1.2: Volume I - Fundamentals (Fourth Edition) by Cay
S. Horstmann and Gary Cornell (Sun Microsystems Press/Prentice Hall,
1999).
This implementation represents the calendar as three integers. It converts the dates to and from Julian dates to do some of the operations.
// This class implementation is adapted from the Day class in
// Horstmann and Cornell, Core Java 1.2: Volume I - Fundamentals
// (Fourth Edition), Prentice Hall, 1999.
import java.util.*;
import java.io.*;
public class Day
{
// Interface Invariant: Once created and until destroyed, this
// instance contains a valid date. gateau() != 0 &&
// 1 <= getMonth() <= 12 && 1 <= getDay() <= #days in getMonth().
// Also calendar date getMonth()/getDay()/getYear() does not
// fall in the gap formed by the change to the modern
// (Gregorian) calendar.
// Constants for days of the week
public static final int SUNDAY = 1;
public static final int MONDAY = 2;
public static final int TUESDAY = 3;
public static final int WEDNESDAY = 4;
public static final int THURSDAY = 5;
public static final int FRIDAY = 6;
public static final int SATURDAY = 7;
// Constructors
public Day()
// Pre: true
// Post: the new instance's day, month, and year set to today's
// date (i.e., the date of creation of the instance)
//
// Implementation uses GregorianCalendar class from the Java API
// to get today's date.
//
{ GregorianCalendar todaysDate = new GregorianCalendar();
year = todaysDate.get(Calendar.YEAR);
month = todaysDate.get(Calendar.MONTH) + 1;
day = todaysDate.get(Calendar.DAY_OF_MONTH);
}
public Day(int y, int m, int d)
throws IllegalArgumentException
// Pre: y != 0 && 1 <= m <= 12 && 1 <= d <= #days in month m
// (y,m,d) does not fall in the gap formed by the
// change to the modern (Gregorian) calendar.
// Post: the new instance's day, month, and year set to y, m,
// and d, respectively
// Exception: IllegalArgumentException if y m d not a valid date
{ year = y;
month = m;
day = d;
if (!isValid())
throw new IllegalArgumentException();
}
// Mutators
public void setDay(int y, int m, int d)
throws IllegalArgumentException
// Pre: y != 0 && 1 <= m <= 12 && 1 <= d <= #days in month m
// (y,m,d) does not fall in the gap formed by the
// change to the modern (Gregorian) calendar.
// Post: this instance's day, month, and year set to y, m,
// and d, respectively
// Exception: IllegalArgumentException if y m d not a valid date
{ year = y;
month = m;
day = d;
if (!isValid())
throw new IllegalArgumentException();
}
public void advance(int n)
// Pre: true
// Post: this instance's date moved n days later. (Negative n
// moves to an earlier date.)
{ fromJulian(toJulian() + n);
}
// Accessors
public int getDay()
// Pre: true
// Post: returns the day from this instance, where
// 1 <= getDay() <= #days in this instance's month
{ return day;
}
public int getMonth()
// Pre: true
// Post: returns the month from this instance's date, where
// 1 <= getMonth() <= 12
{ return month;
}
public int getYear()
// Pre: true
// Post: returns the year from this instance's date, where
// getYear() != 0
{ return year;
}
public int getWeekday()
// Pre: true
// Post: returns the day of the week upon which this instance
// falls, where 1 <= getWeekday() <= 7;
// 1 == Sunday, 2 == Monday, ..., 7 == Saturday
{ // calculate day of week
return (toJulian() + 1) % 7 + 1;
}
public boolean equals(Day dd)
// Pre: dd is a valid instance of Day
// Post: returns true if and only if this instance and instance
// dd denote the same calendar date
{ return (year == dd.getYear() && month == dd.getMonth()
&& day == dd.getDay());
}
public int daysBetween(Day dd)
// Pre: dd is a valid instance of Day
// Post: returns the number of calendar days from the dd
// instance's date to this instance's date, where
// equals(dd.advance(n)) would hold
{ // implementation code
return toJulian() - dd.toJulian();
}
public String toString()
// Pre: true
// Post: returns this instance's date expressed in the format
// "Day[year,month,day]"
{
return "Day[" + year + "," + month + "," + day + "]";
}
// Destructors -- None needed
// Private Methods
private boolean isValid()
// Pre: true
// Post: returns true iff this is a valid date
{ Day t = new Day();
t.fromJulian(this.toJulian());
return t.day == day && t.month == month
&& t.year == year;
}
private int toJulian()
// Pre: true
// Post: returns Julian day number that begins at noon of this day
//
// A positive year signifies A.D., negative year B.C.
// Remember that the year after 1 B.C. was 1 A.D. (i.e., no year 0).
//
// A convenient reference point is that May 23, 1968, at noon
// is Julian day 2440000.
//
// Julian day 0 is a Monday.
//
// This algorithm is from Press et al., Numerical Recipes
// in C, 2nd ed., Cambridge University Press 1992.
//
{ int jy = year;
if (year < 0)
jy++;
int jm = month;
if (month > 2)
jm++;
else
{ jy--;
jm += 13;
}
int jul = (int) (java.lang.Math.floor(365.25 * jy)
+ java.lang.Math.floor(30.6001*jm) + day + 1720995.0);
int IGREG = 15 + 31*(10+12*1582);
// Gregorian Calendar adopted Oct. 15, 1582
if (day + 31 * (month + 12 * year) >= IGREG)
// change over to Gregorian calendar
{ int ja = (int)(0.01 * jy);
jul += 2 - ja + (int)(0.25 * ja);
}
return jul;
}
private void fromJulian(int j)
// Pre: true
// Post: this calendar Day is set to Julian date j
//
// This algorithm is from Press et al., Numerical Recipes
// in C, 2nd ed., Cambridge University Press 1992
//
{ int ja = j;
int JGREG = 2299161;
/* the Julian date of the adoption of the Gregorian
calendar
*/
if (j >= JGREG)
/* correct for crossover to Gregorian Calendar */
{ int jalpha = (int)(((float)(j - 1867216) - 0.25)
/ 36524.25);
ja += 1 + jalpha - (int)(0.25 * jalpha);
}
int jb = ja + 1524;
int jc = (int)(6680.0 + ((float)(jb-2439870) - 122.1)
/365.25);
int jd = (int)(365 * jc + (0.25 * jc));
int je = (int)((jb - jd)/30.6001);
day = jb - jd - (int)(30.6001 * je);
month = je - 1;
if (month > 12)
month -= 12;
year = jc - 4715;
if (month > 2)
--year;
if (year <= 0)
--year;
}
// Implementation Invariants:
// year != 0 && 1 <= month <= 12 && 1 <= day <= #days in month
// (year,month,day) not in gap formed by the change to the
// modern (Gregorian) calendar
private int year;
private int month;
private int day;
}
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