1. A Different Kind of Variable
Enumerations
A Different Kind of Variable
Enumerations appear in Java first in Java 5
They are unlike C++ because they are a class.
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2. Conventions
There are many times when you use a code that represents a value
Many times there are existing conventions for a code and its meaning
For example months
When we say that today is 3/20 you know that the 3 represents March
An enumeration allows us to name these in a way that makes the program more readable
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3. The Problem More often there are no existing conventions
Programmer makes up the convention
What kind of convention?
Choices:
Strings
Chars
Ints
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4. Example
Consider classifying students such as freshmen, sophomores, juniors, seniors, graduate, prospects, extension
We could create a code to represent these in a file that described our students
We would not want to type in the entire classification
We could misspell it and cause our extraction of juniors not to work
Takes too much space
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5. Example continued
We will usually assign a letter, unless there are too many or they start with same letter
Sophomore and senior
Then we use a number
So 1=freshman, 2=sophomore ...
The problem with that is we will see in our code statements like: s = 6
The reader then wonders what 6 represents
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6. One Solution
One approach to solve this is to use named constants: final int freshman=1,sophomore=2…;
Another solution to this problem is to use an enumeration type
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7. Enumeration Type
Enumerations are a shortened way to make a series of named constants
In this type we make up a series of names that we will call our items as a new type enum student_class {freshman, sophomore, junior, senior, grad, prospect, extension};
What we are saying is that we want a new type that has precisely 7 values
We have not yet defined a variable
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8. Code
Now we can use these in sample code enum Directions {LEFT, FORWARD, RIGHT,BACKWARD}; … Directions dir = Directions.LEFT;
Directions is now a type
It is usually placed with constants
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9. C++ and Java In C++ an enumeration is a series of named constants
In Java it is actually a special form of a class
This class does have some methods defined
These methods need: import java.lang.Enum;
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10. Enumeration Methods The method ordinal gives the ordinal position
Starts at zero
dir.ordinal() == 0 indicates that this is LEFT
The toString method gives the string representation
valueOf takes a string and gives the enumeration value
It must be an exact match otherwise IllegalArgumentException is thrown
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11. Comparisons equals gives an equality comparison
However the == is preferable
compareTo gives -1 if less, 0 if equal and +1 if greater
The other comparison operators are not allowed
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12. Representation of value
Although technically it is supposed to be hidden from us
Practically we do know it
Each value in the list is stored as an integer
First item is zero
Count up from there
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13. Booleans Booleans are a predefined enumeration with special privileges
Two values are {false, true}
The special privileges include the being result type of all comparisons
The enum reserved word is not needed
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14. Declaration note Enumerations are always declared enclosed in braces
The Java convention is that like constants they are usually all in upper case
This is just a convention
Note that bools use lower case for their constants
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15. Constants
When using an enumeration constant, it must be prefixed by the enumeration type Directions dir = Directions.LEFT;
The exception to this rule is the case constant in a switch: switch(dir){ case LEFT:
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16. Finally
Enumerations are very handy for naming constants that are related
They tend to make use of such constants more readable
They are relatively new feature
From Java 5.0
enum was a reserved word from Java 1.0 but not used
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