CSci 581: Obj.-Oriented Design & Programming
Fall Semester 1997
Lecture Notes

Similarly, the variables (data fields) declared in a class may be either instance variables or class variables. As in the case of methods, an instance variable is associated with an instance of the class; each instance has its own copy of the variable. A class variable is associated with the class as a whole; there is only one copy of the variable for the entire class. As with methods, the keyword static denotes class variables.

An instance method has 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. 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.

The components of a class can be designated as public or private. The public components of the class are accessible from anywhere in the program. The 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's methods. This hides, or encapsulates, the data fields within class instance.

In addition to the primitive types, we can declare a Java variable to have a class as its type. These variables are actually containers for references to (i.e., addresses of) the instances of the class. 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. In general, avoid public variables.

Implementing ADTs as Java Classes

To implement an ADT in Java, we can do the following:

1. Use the Java class construct to represent the entire ADT. If we want to allow access to the Stack class from anywhere in the program, we will make the class public.

   For the Stack ADT, we can use the following structure for the class:

   public class Stack
   {   // implementation of instance methods and data here
   }
   

2. Use an instance of the Java class to represent an instance of the ADT and, hence, variables of the class type to hold references to instances.

   For example, to declare a variable that can hold a reference to a Stack instance, we can use the following declaration:

   Stack stk;
   

3. Represent the ADT's constructor by Java constructor methods.

   A Java constructor is a method with the same name as the class. Upon creation of an instance of the class, the constructor initializes the instance's state. 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 it, and then returns a reference to the new instance.

   For example, we can represent the ADT operation create by the constructor method Stack.

   public class Stack
   {   public Stack(int size)
       {   // initialization code
       }
   
       // rest of Stack methods and data ...
   }
   

   A user of the Stack can then declare a variable and initialize it to hold a reference to a new stack with a capacity of 100 items as follows:

   Stack stk = new Stack(100);
   

   The expression new Stack(100) allocates a Stack instance in the heap storage and calls the constructor above to initialize the data fields encapsulated within the instance.

4. Represent the ADT operations by instance methods of the class. Thus the state of the ADT instance, which is given explicitly in the ADT signatures, becomes an implicit argument of all method calls. Mutators also have the state as an implicit return.

   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 Stack ADT we can represent the operations as instance methods of class Stack. The explicit Stack 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(); 
   }
   

5. Make the constructors, mutators, accessors, and destructors public methods of the class. That is, precede the method's definition by the keyword public.

6. Represent the ADT mutator operations by Java procedure (i.e., void) methods. These modify the encapsulated state of the class instance (which is the implicit argument of the methods).

   public void pop()
   {   //  code to implement operation
   }
   

7. Represent the ADT accessor operations by Java function methods of the proper return type. These functions can access the encapsulated state of the class instance (which is the implicit argument of these methods) and compute a value to be returned.

   public boolean empty()
   {   //  code to implement operation
   }
   

8. If necessary for deallocation of internal resources, represent the ADT destructor methods by explicit Java procedures; otherwise, just allow the automatic garbage collection to reclaim instances that are no longer being used.

   public void destroy()
   {   //  code to free resources
   }
   

9. Use 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.

   public class Stack
   {   //  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
   }
   

10. Do not use public data fields in the class. They violate the principle of information hiding. Instead introduce appropriate accessor and mutator methods to allow manipulation of the hidden state.

11. Include, as appropriate, private methods to aid in implementation. Functionality common to several methods can be placed in separate functions and procedures as appropriate. However, since these are private, they can only be accessed from within the class and thus can be changed without effecting the public interface of the class.

Java does not currently have a parameterized class facility (like the C++ template or Ada generic mechanisms). We must handle the type parameters of the ADT in other ways.

For example, in the next section we represent the set Item of the Stack 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 Stack. With this definition, any data of a reference type can appear in the stack, but values of the primitive types cannot.

Java Implementation of the Stack

In this section, we give an implementation of the Stack that uses an array of objects and an integer "pointer" to represent the stack. 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 Stack ADT
public class Stack {
    // Interface Invariant:  Once created and until destroyed, this
    //     instance has a valid and consistent internal state

    public Stack(int size)
    // Pre:   size >= 0
    // Post:  initialized empty Stack instance with capacity size
    {   stk = new Object[size];
        capacity = size;
	topItem = 0;
    }

    public void push(Object I)
    // Pre:   this instance is not full
    // Post:  I added as the new top of this instance's stack
    {   stk[topItem] = I;
        topItem++;
    }

    public void pop()
    // Pre:   this instance is not empty
    // Post:  item at top of stack removed from this instance
    {   topItem--;
        stk[topItem] = null;
    }

    public Object top()
    // Pre:   this instance is not empty
    // Post:  item at top of this instance's stack returned
    {   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 Invariants:  0 <= topItem <= capacity
    //     stack is in array section stk[0..topItem-1]
    //         with the top at stk[topItem-1], etc.

    private int topItem;   // Pointer to next index for insertion
    private int capacity;  // Maximum number of items in stack
    private Object[] stk;  // the stack
}

A Date (Day) ADT

Consider an ADT for storing and manipulating calendar dates. We will call the ADT Day to avoid confusion with the Date class in the Java API. This ADT is based on the Day class defined in Chapter 4 of the first edition of the book Core Java.

Logically, a calendar date consists of three pieces of information: a year designator, a month designator, and a day of the month designator. A secondary piece of information is the day of the week. In this ADT interface definition, we use integers (i.e., Java int) to designate these pieces of information.

Constructor

create(int y, int m, int d) -> Day D'

Precondition:

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.

Postcondition:

D' is a valid new instance of Day with year y, month m, and day d.

Mutators

setDay(Day D, int y, int m, int d) -> Day D'

Precondition:

D is a valid instance of Day.
y != 0 && 1 <= m <= 12 && 1 <= d <= #days in month.
(y,m,d) does not fall in the gap formed by the change to the modern (Gregorian) calendar.

Postcondition:

D' is a valid instance of Day.
D'= D except with year y, month m, and day d.

Question: Should we include setDay, setMonth, and setYear operations? What problems might arise?

advance(Day D, int n) -> Day D'

Precondition:

D is a valid instance of Day.

Postcondition:

D' is a valid instance of Day.
D' = D with the date moved n days later. (Negative n moves to an earlier date.)

Accessors

getDay(Day D) -> int d

Precondition:

D is a valid instance of Day.

Postcondition:

d is day of the month from D, where 1 <= d <= #days in month getMonth(D).
(D is unchanged.)

getMonth(Day D) -> int m

Precondition:

D is a valid instance of Day.

Postcondition:

m is the month from D, where 1 <= m <= 12.
(D is unchanged.)

getYear(Day D) -> int y

Precondition:

D is a valid instance of Day.

Postcondition:

y is the year from D, where y != 0.
(D is unchanged.)

getWeekday(Day D) -> int wd

Precondition:

D is a valid instance of Day.

Postcondition:

wd is the day of the week upon which D falls: 0 = Sunday, 1 = Monday, ..., 6 = Saturday.
(D is unchanged.)

equals(Day D, Day D1) -> boolean eq

Precondition:

D and D' are valid instances of Day.

Postcondition:

eq is true if and only if D and D' denote the same calendar date.
(D and D' are unchanged.)

daysBetween(Day D, Day D1) -> int d

Precondition:

D and D' are valid instances of Day.

Postcondition:

d is the number of calendar days from D1 to D, i.e., equals(D,advance(D1,d)) would be true.
(D is unchanged.)

toString(Day D) -> String s

Precondition:

D is a valid instance of Day.

Postcondition:

s is the date D expressed in the format "Day[getYear(D),getMonth(D),getDay(D)]".
(D is unchanged.)

Destructor

destroy(Day D) ->

Precondition:

D is a valid instance of Day.

Postcondition:

D no longer exists.

Java Class Implementation for Day

// Parts of this come unmodified from _Core_Java_ (First Edition).
// Not all the commenting has been made consistent.

import java.util.*;

public class Day 
{
    // Interface Invariant:  Once created and until destroyed, this
    //    instance contains a valid date.  getYear() != 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.  

  // 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)
    {   java.util.Date today = new java.util.Date();
        year  = today.getYear() + 1900;
        month = today.getMonth() + 1;
        day   = today.getDay();
    }
 
    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 year, month
    //             and day 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 0 <= getWeekday() <= 6;
    //            0 == Sunday, 1 == Monday, ..., 6 == Saturday
    {   //  calculate day of week
        return (toJulian() + 1)% 7;
    }

    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]"
    {   //  implementation code
        return "Day[" + year + "," + month + "," + day + "]";
    }

  //  Destructors -- None needed

  //  Private Methods -- Mostly borrowed from _Core_Java_

 
   /**
    * Computes the number of days between two dates
    * @return true iff this is a valid date
    */
   private boolean isValid()
   {  Day t = new Day();
      t.fromJulian(this.toJulian());
      return t.day == day && t.month == month 
         && t.year == year;
   }

   private int toJulian()
   /**
    * @return The Julian day number that begins at noon of 
    * this day
    * Positive year signifies A.D., negative year B.C. 
    * Remember that the year after 1 B.C. was 1 A.D.
    *
    * A convenient reference point is that May 23, 1968 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)
    /**
     * Converts a Julian day to a calendar date
     * @param j  the Julian date
     * 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)
        /* cross-over to Gregorian Calendar produces this
           correction
         */ 
        {   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; 
}

Client-Supplier Relationship

The design and implementation of ADTs (i.e., classes) must be approached from two points of view simultaneously:

supplier

the developers of the ADT -- the providers of the services.

client

the users of the ADT -- the users of the services (e.g., the designers of other ADTs).

Client-supplier relationship:

   ________________             ________________ 
  |                |           |                |
  |     Client     |===USES===>|    Supplier    |
  |________________|           |________________|

     (ADT user)                    (ADT)

The supplier's concerns include:

• efficient and reliable algorithms and data structures,
• convenient implementation,
• easy maintenance.

The clients' concerns include:

• accomplishing their own tasks,
• using the supplier ADT without effort to understand its internal details,
• having a sufficient, but not overwhelming, set of operations.

As we have noted previously, the interface of an ADT is the set of features (i.e., public operations) provided by a supplier to clients.

A precise description of a supplier's interface forms a contract between clients and supplier.

The client-supplier contract:

1. gives the responsibilities of the client. These are the conditions under which the supplier must deliver results -- when the preconditions of the operations are satisfied (i.e., the operations are called correctly).
2. gives the responsibilities of the supplier. These are the benefits the supplier must deliver -- make the postconditions hold at the end of the operation (i.e., the operations deliver the correct results).

The contract

• protects the client by specifying how much must be done by the supplier.
• protects the supplier by specifying how little is acceptable to the client.

  If we are both the clients and suppliers in a design situation, we should consciously attempt to separate the two different areas of concern, switching back and forth between our supplier and client "hats".

  Design Criteria for ADT (Class) Interfaces

  We can use the following design criteria for evaluating ADT (class) interfaces. Of course, some of these criteria conflict with one another; a designer must carefully balance the criteria to achieve a good interface design.

  Cohesion:

  All operations must logically fit together to support a single, coherent purpose. The ADT (class) should describe a single abstraction.

  Simplicity:

  Avoid needless features. The smaller the interface the easier it is to use the ADT (class).

  No redundancy:

  Avoid offering the same service in more than one way; eliminate redundant features.

  Atomicity:

  Do not combine several operations if they are needed individually. Keep independent features separate. All operations should be primitive, that is, not be decomposable into other operations also in the public interface.

  Completeness:

  All primitive operations that make sense for the abstraction should be supported by the ADT (class).

  Consistency:

  Provide a set of operations that are internally consistent in

  • naming convention (e.g., in use of prefixes like "set" or "get", in capitalization, in use of verbs/nouns/adjectives),
  • use of arguments and return values (e.g., order and type of arguments),
  • behavior (i.e., make operations work similarly).

  Avoid surprises and misunderstandings. Consistent interfaces make it easier to understand the rest of a system if part of it is already known.

  Reusability:

  Do not customize ADTs (classes) to specific clients, but make them general enough to be reusable in other contexts.

  Robustness with respect to modifications:

  Design the interface of an ADT (class) so that it remains stable even if the implementation of the ADT changes.

  Convenience:

  Where appropriate, provide additional operations (e.g., beyond the complete primitive set) for the convenience of users of the ADT (class). Add convenience operations only for frequently used combinations after careful study.

  Acknowledgments

  Some of the material here is based on the presentation in the following books:

  • Gary Cornell and Cay S. Horstmann. Core Java (First Edition), Sun Microsystems Press (Prentice-Hall), 1996.
  • Cay S. Horstmann. Mastering Object-Oriented Design in C++, Wiley, New York, 1995.
  • Hanspeter Mossenbock. Object-Oriented Programming in Oberon-2, Springer-Verlag, Berlin, 1995.
  • Nell Dale and Henry M. Walker. Abstract Data Types: Specifications, Implementations, and Applications, D. C. Heath, 1996.

  The first version of these lecture notes were written for use in the first Java-based version of CSCI 211 (File Systems) during the fall semester of 1996.

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  Copyright © 1997, H. Conrad Cunningham
  Last modified: 17 October 1998.