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<h1>JMRI: Using Enums</h1>
<h2>The Minimal Enum</h2>
<p>The minimal enum is really small. Here's an example of embedding an enum in a class to
represent seven different constants:</p>
<pre style="font-family: monospace;">
class MyClass {
public enum Day {
SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
THURSDAY, FRIDAY, SATURDAY
}
void checkParty(Day day) {
if (day == Day.FRIDAY) {
doParty();
}
}
}
</pre>
<ul>
<li>This shows a lightweight enum embedded in one class, but you can also define them
independently.</li>
<li>Another class can refer to this as <code>MyClass.Day</code> and
<code>MyClass.Day.FRIDAY</code></li>
</ul>
<p>Examples of use:</p>
<pre style="font-family: monospace;">
for (Day day : Day.values()) {
// do what you want
}
</pre>
<pre style="font-family: monospace;">
String whichDay = "THURSDAY";
Day day = Day.valueOf(whichDay);
checkParty(day); // disappointment!
checkParty(Day.TUESDAY); // disappointment!
</pre>
<p>Note there are no meaningful integer values for this.</p>
<h2>Migrating Integer Constants to Enum</h2>
<p>If you have a set of constants like</p>
<pre style="font-family: monospace;">
static final int DOG = 1;
static final int CAT = 2;
static final int FISH = 3;
static final int BIRD = 12;
</pre>
<p>converting those to an enum can make them more type-safe.</p>
<pre style="font-family: monospace;">
public enum Pet {
DOG(1),
CAT(2),
FISH(3),
BIRD(12);
private final int value;
Pet(int v) { value = v; }
int asInt() { return value; }
}
</pre>
<p>To use this, just replace "int" with "Pet" where-ever one is defined or passed as a
parameter. Then compile and see what else needs to be changed; if you're not doing arithmetic
with the constants (in which case maybe they're not enums), there shouldn't be much else that
needs to be changed.</p>
<pre style="font-family: monospace;">
void check(int val) {
switch (val) {
case DOG:
// do stuff and return
case CAT:
// do stuff and return
default:
// do stuff and return
}
}
</pre>
<p>becomes</p>
<pre style="font-family: monospace;">
void check(Pet val) {
switch (val) {
case DOG:
// do stuff and return
case CAT:
// do stuff and return
default:
// do stuff and return
}
}
</pre>
<p>(The only change was in the 1st line)</p>
<h2>Adding String Values</h2>
<p>Sometimes you want the elements to also have user-readable names separate from the names
of the individual items. For example, you might want to call it TUESDAY in the code, but have
it provide "Tuesday" for pretty printing.</p>
<pre style="font-family: monospace;">
public enum NamedPet {
DOG(1, "Dog"),
CAT(2, "Cat"),
FISH(3, "Fish"),
BIRD(12, "Bird");
private final int value;
private final String name;
Pet(int v, string s) {
value = v;
name = s;
}
int asInt() { return value; }
String toString() { return name; }
}
</pre>
<p>Note that you might want to internationalize these in the constructor. See <a href=
"https://github.com/JMRI/JMRI/blob/master/java/src/jmri/LocoAddress.java">java/src/jmri/LocoAddress.java</a>
for an example of this.</p>
<p>In general, you should <u>not</u> build a lot of code to convert between ints and Enums or
between Strings and enums. That's a code smell that indicates something wrong with how you're
using the enums. If users have to select a specific one, for example, provide a combobox of
values, don't have them type a String that you then have to error-check and convert.</p>
<p>The one exception to this is the ConvertXML persistance system, which wants to convert
your enum values to and from String values to store and load in XML files, and may also need
to convert from the numeric values for historical reasons. See below.</p>
<h2>ConvertXML</h2>
<p>Enums can provide interfaces. We use the <code>StringConvertibleEnum</code> and
<code>IntConvertibleEnum</code> interfaces as flags.</p>
<h4>Migration</h4>
Start writing a new schema that only allows the specific elements, but make sure your code
can take the old numeric values. Since the file contains the schema it obeys, this is OK. But
it might require a lot of migrations if you do this between test releases....
<h4>Schema limiting to a set of specific enum values</h4>
<pre style="font-family: monospace;">
&lt;xs:attribute name="connection" default="unspecified" &gt;
&lt;xs:simpleType&gt;
&lt;xs:restriction base="xs:token" &gt;
&lt;xs:enumeration value="unspecified"/&gt;
&lt;xs:enumeration value="plug"/&gt;
&lt;xs:enumeration value="wire"/&gt;
&lt;xs:enumeration value="solder"/&gt;
&lt;xs:enumeration value="LED"/&gt;
&lt;xs:enumeration value="bulb"/&gt;
&lt;xs:enumeration value="other"/&gt;
&lt;/xs:restriction&gt;
&lt;/xs:simpleType&gt;
&lt;/xs:attribute&gt;
</pre>
<h4>Schema also permitting older numeric values</h4>
<pre style="font-family: monospace;">
&lt;xs:attribute name="size"&gt;
&lt;xs:simpleType&gt;
&lt;xs:union&gt;
&lt;xs:simpleType&gt;
&lt;xs:restriction base="xs:positive-integer"&gt;
&lt;xs:maxInclusive="10"/&gt;
&lt;/xs:restriction&gt;
&lt;/xs:simpleType&gt;
&lt;xs:simpleType&gt;
&lt;xs:restriction base="xs:NMTOKEN"&gt;
&lt;xs:enumeration value="small"/&gt;
&lt;xs:enumeration value="medium"/&gt;
&lt;xs:enumeration value="large"/&gt;
&lt;/xs:restriction&gt;
&lt;/xs:simpleType&gt;
&lt;/xs:union&gt;
&lt;/xs:simpleType&gt;
&lt;/xs:attribute&gt;
</pre>This can also be used for elements (which is the generally prefered approach)
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