Effective Java

// Implementing a fromString method on an enum type
private static final Map stringToEnum =
Stream.of(values()).collect(
toMap(Object::toString, e -> e));
// Returns Operation for string, if any
public static Optional fromString(String symbol) {
return Optional.ofNullable(stringToEnum.get(symbol));
}
Note that the 
Operation
constants are put into the 
stringToEnum
map from a
static field initialization that runs after the enum constants have been created. The
previous code uses a stream (Chapter 7) over the array returned by the 
values()
method; prior to Java 8, we would have created an empty hash map and iterated
over the values array inserting the string-to-enum mappings into the map, and you
can still do it that way if you prefer. But note that attempting to have each constant
put itself into a map from its own constructor does 
not
work. It would cause a
compilation error, which is good thing because if it were legal, it would cause a
NullPointerException
at runtime. Enum constructors aren’t permitted to access
the enum’s static fields, with the exception of constant variables (Item 34). This
restriction is necessary because static fields have not yet been initialized when
enum constructors run. A special case of this restriction is that enum constants
cannot access one another from their constructors.
Also note that the 
fromString
method returns an 
Optional
. This
allows the method to indicate that the string that was passed in does not represent
a valid operation, and it forces the client to confront this possibility (Item 55). 
A disadvantage of constant-specific method implementations is that they
make it harder to share code among enum constants. For example, consider an
enum representing the days of the week in a payroll package. This enum has a
method that calculates a worker’s pay for that day given the worker’s base salary
(per hour) and the number of minutes worked on that day. On the five weekdays,
any time worked in excess of a normal shift generates overtime pay; on the two
weekend days, all work generates overtime pay. With a 
switch
statement, it’s easy

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ITEM 34: USE ENUMS INSTEAD OF INT CONSTANTS
165
to do this calculation by applying multiple case labels to each of two code frag-
ments: