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Lecture 9 - CS 159 enumerated types

Prof. Alvin Chao

Due this week:

Thu 11pm Quiz Loops
Fri Exam 1(in class)
Next Week�Wed Lab 4B�Fri Lab 5�Fri Reading quiz Testing

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Enumerated Types

Definition:

A definition of a set that is written by listing the elements (i.e., an extensive definition of a set)
The set of elements so defined

Examples:

Months of the Year (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec)
Marital Statuses (Never Been Married, Engaged, Married, Divorced, Widowed)
Suits in a Deck of Cards (Clubs Diamonds, Hearts, Spades)
Cards in a Deck of Cards (Ace, 2, ..., 10, Jack, Queen, King)

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Enumerated Types in Java

Value Type or Reference Type?

Enumerated types are reference types�

Ordered or Unordered?

Enumerated types are ordered (though the order may not be meaningful/useful)

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Design + Implementation

Identify the Elements:

List all of the elements in the set

Identify the Properties of the Elements:

Declare the names and types of attributes

Identify the Behaviors of the Elements:

Declare the signatures of methods

Refine the Design:

Think about ways the design can be improved before your start writing code

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Properties of enum vs. instance

Properties of the Instances:

Attributes and behaviors of individual instances (in Java, the non-static variables and methods)

Properties of the enum:

Attributes and behaviors of the set of instances (in Java, the static variables and methods)

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Methods available to enum in Java

Though it is not immediately obvious from the documentation for the Java API, all enumerated types have the following methods "implemented for them".�Static Methods:� - values() returns an array containing all of the objects in the set� - valueOf(String) returns the instance with the specified identifier (or throws IllegalArgumentException)�Non-Static Methods:� - int compareTo(E other) which uses the order of the elements� - boolean equals(E other)� - ordinal() returns the ordinal value of the instance (0-based)� - toString() returns a String representation of the identifier

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Example

Suppose:

We are designing and implementing a product that needs a calendar

An Obvious enum:

The months of the year

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Example - Initial Encapsulation

public enum Month
{
JANUARY, FEBRUARY, MARCH, � APRIL, MAY, JUNE, JULY, � AUGUST, SEPTEMBER, OCTOBER, � NOVEMBER, DECEMBER;
}

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Using Initial Encapsulation

public class MonthDriver
{
public static void main(String[] Args)
{
Month[] all, fun;
fun = new Month[3];
fun[0] = Month.JUNE;
fun[1] = Month.JULY;
fun[2] = Month.AUGUST;
System.out.println("\nThe fun months:");
for (int i=0; i<fun.length; i++)
{
System.out.println(fun[i].toString());
}
System.out.println("\nAll months after May:");
all = Month.values();
for (int i=0; i<all.length; i++)
{
if (all[i].compareTo(Month.MAY) > 0)
{
System.out.println(all[i].toString());
}
}
}
}

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Adding Attributes

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Adding Methods

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Final Design

public enum Month
{
JANUARY ("January", 31),
FEBRUARY ("February", 28),
MARCH ("March", 31),
APRIL ("April", 30),
MAY ("May", 31),
JUNE ("June", 30),
JULY ("July", 31),
AUGUST ("August", 31),
SEPTEMBER ("September",30),
OCTOBER ("October", 31),
NOVEMBER ("November", 30),
DECEMBER ("December", 31);
private final int days;
private final String name;
private Month(String name, int days)
{
this.name = name;
this.days = days;
}

public String getAbbreviation()
{
String result;
if (name.length() > 3) result = name.substring(0,3)+".";
else result = name;
return result;
}
public int getNumber()
{
return ordinal()+1; // ordinal() returns the position in the declaration
}
public int getLength()
{
return days;
}
public static Month parseMonth(String s)
{
Month[] all;
all = Month.values();
for (int i=0; i<all.length; i++)
{
if (s.equalsIgnoreCase(all[i].name)
|| s.equalsIgnoreCase(all[i].getAbbreviation()))
{
return all[i];
}
}
return null;
}
public String toString()
{
return name;
}
}

public enum Month
{
JANUARY ("January", 31),
FEBRUARY ("February", 28),
MARCH ("March", 31),
APRIL ("April", 30),
MAY ("May", 31),
JUNE ("June", 30),
JULY ("July", 31),
AUGUST ("August", 31),
SEPTEMBER ("September",30),
OCTOBER ("October", 31),
NOVEMBER ("November", 30),
DECEMBER ("December", 31);
private final int days;
private final String name;
private Month(String name, int days)
{
this.name = name;
this.days = days;
}
public String getAbbreviation()
{
String result;
if (name.length() > 3) result = name.substring(0,3)+".";
else result = name;
return result;
}
public int getNumber()
{
return ordinal()+1; // ordinal() returns the position in the declaration
}
public int getLength()
{
return days;
}
public static Month parseMonth(String s)
{
Month[] all;
all = Month.values();
for (int i=0; i<all.length; i++)
{
if (s.equalsIgnoreCase(all[i].name)
|| s.equalsIgnoreCase(all[i].getAbbreviation()))
{
return all[i];
}
}
return null;
}
public String toString()
{
return name;
}
}

CommentsNumbers

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Using the Month enum

public class MonthDriver
{
public static void main(String[] Args)
{
int total;
Month[] fun;
fun = new Month[3];
fun[0] = Month.JUNE;
fun[1] = Month.JULY;
fun[2] = Month.AUGUST;
total = 0;
System.out.println("\nThe fun months:");
for (int i=0; i<fun.length; i++)
{
System.out.printf("%5s %3d\n",
fun[i].getAbbreviation(), fun[i].getLength());
total += fun[i].getLength();
}
System.out.printf("\n%5s %3d", " ", total);
}
}

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Alternatives to enums

A "Set" of public static final String Objects:

They can only be validated at run-time
It is impossible to iterate over the group

An Array of public static final String Objects:

They can only be validated at run-time
They need to be referred to by index number

A "Set" of public static final int Values:

They can only be validated at run-time
They aren't useful for user interaction (e.g., they are meaningless when printed)

A Mixed Approach:

They can only be validated at run-time
Inconsistencies can arise