Enums and for-each Loops

Java 5.0 introduces a new “enhanced” form of the for loop that is designed to be
convenient for processing data structures. A data structure is a collection of data
items, considered as a unit. For example, a list is a data structure that consists
simply of a sequence of items. The enhanced for loop makes it easy to apply the
same processing to every element of a list or other data structure. However, one of
the applications of the enhanced for loop is to enum types, and so we consider it
briefly here.
The enhanced for loop can be used to perform the same processing on each of the
enum constants that are the possible values of an enumerated type. The syntax for
doing this is:
for ( enum−type−name variable−name : enum−type−name.values() )
statement
or
for ( enum−type−name variable−name : enum−type−name.values() ) {
statements
}
If MyEnum is the name of any enumerated type, then MyEnum.values() is a method
call that returns a list containing all of the values of the enum. (values() is a static
member method in MyEnum and of any other enum.) For this enumerated type, the
for loop would have the form:
for ( MyEnum variable−name : MyEnum.values() )
statement
The intent of this is to execute the statement once for each of the possible values of
the MyEnum type. The variable-name is the loop control variable. In the statement,
it represents the enumerated type value that is currently being processed. This variable
should not be declared before the for loop; it is essentially being declared in the
loop itself.
To give a concrete example, suppose that the following enumerated type has been
defined to represent the days of the week:
enum Day { MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY }
Then we could write:
for ( Day d : Day.values() ) {
System.out.print( d );
System.out.print(" i s day number ");
System.out.println( d.ordinal() );
}
Day.values() represents the list containing the seven constants that make up the
enumerated type. The first time through this loop, the value of d would be the first
enumerated type value Day.MONDAY, which has ordinal number 0, so the output
would be “MONDAY is day number0”. The second time through the loop, the value
of d would be Day.TUESDAY, and so on through Day.SUNDAY. The body of the loop
is executed once for each item in the list Day.values(), with d taking on each of those
values in turn. The full output from this loop would be:
MONDAY is day number 0
TUESDAY is day number 1
WEDNESDAY is day number 2
THURSDAY is day number 3
FRIDAY is day number 4
SATURDAY is day number 5
SUNDAY is day number 6
Since the intent of the enhanced for loop is to do something “for each” item in a
data structure, it is often called a for-each loop. The syntax for this type of loop is
unfortunate. It would be better if it were written something like “foreach Day d in
Day.values()”, which conveys the meaning much better and is similar to the syntax
used in other programming languages for similar types of loops. It’s helpful to think
of the colon (:) in the loop as meaning “in.”