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MQTT, SECSI doc updates
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| <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> | ||
| <html lang="en"> | ||
| <head> | ||
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| <title> | ||
| JMRI: Using Enums | ||
| </title> | ||
| <meta name="author" content="YOUR NAME HERE"> | ||
| <meta name="keywords" content="SOME KEYWORDS"> | ||
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| <!--#include virtual="/Style.shtml" --> | ||
| </head> | ||
| <!--#include virtual="/Header.shtml" --> | ||
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| <body> | ||
| <div id="mBody"> | ||
| <!--#include virtual="Sidebar.shtml" --> <!-- select the local or global Sidebar file --> | ||
| <div id="mainContent"> | ||
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| <h1>JMRI: Using Enums</h1> | ||
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| <h2>The Minimal Enum</h2> | ||
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| <p>The minimal enum is really small. Here's | ||
| an example of embedding an enum in a class to | ||
| represent seven different constants:</p> | ||
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| <pre style="font-family: monospace;"> | ||
| class MyClass { | ||
| public enum Day { | ||
| SUNDAY, MONDAY, TUESDAY, WEDNESDAY, | ||
| THURSDAY, FRIDAY, SATURDAY | ||
| } | ||
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| void checkParty(Day day) { | ||
| if (day == Day.FRIDAY) { | ||
| doParty(); | ||
| } | ||
| } | ||
| } | ||
| </pre> | ||
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| <ul> | ||
| <li>This shows a lightweight enum embedded in one class, | ||
| but you can also define them independently. | ||
| <li>Another class can refer to this as | ||
| <code>MyClass.Day</code> and <code>MyClass.Day.FRIDAY</code> | ||
| </ul> | ||
| <p> | ||
| Examples of use:</p> | ||
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| <pre style="font-family: monospace;"> | ||
| for (Day day : Day.values()) { | ||
| // do what you want | ||
| } | ||
| </pre> | ||
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| <pre style="font-family: monospace;"> | ||
| String whichDay = "THURSDAY"; | ||
| Day day = Day.valueOf(whichDay); | ||
| checkParty(day); // disappointment! | ||
| checkParty(Day.TUESDAY); // disappointment! | ||
| </pre> | ||
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| <p> | ||
| Note there are no meaningful integer values for this.</p> | ||
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| <h2>Migrating Integer Constants to Enum</h2> | ||
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| <p>If you have a set of constants like</p> | ||
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| <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> | ||
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| <p>converting those to an enum can make them more type-safe.</p> | ||
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| <pre style="font-family: monospace;"> | ||
| public enum Pet { | ||
| DOG(1), | ||
| CAT(2), | ||
| FISH(3), | ||
| BIRD(12); | ||
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| private final int value; | ||
| Pet(int v) { value = v; } | ||
| int asInt() { return value; } | ||
| } | ||
| </pre> | ||
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| <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> | ||
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| <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> | ||
|
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| <p>becomes</p> | ||
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| <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> | ||
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| <p>(The only change was in the 1st line)</p> | ||
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| <h2>Adding String Values</h2> | ||
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| <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> | ||
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| <pre style="font-family: monospace;"> | ||
| public enum NamedPet { | ||
| DOG(1, "Dog"), | ||
| CAT(2, "Cat"), | ||
| FISH(3, "Fish"), | ||
| BIRD(12, "Bird"); | ||
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| 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> | ||
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| <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> | ||
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| <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> | ||
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| <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> | ||
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| <h2>ConvertXML</h2> | ||
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| <p> | ||
| Enums can provide interfaces. We use the | ||
| <code>StringConvertibleEnum</code> and | ||
| <code>IntConvertibleEnum</code> | ||
| interfaces as flags. | ||
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| <h4>Migration</h4> | ||
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| 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. | ||
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| But it might require a lot of migrations if you do this | ||
| between test releases.... | ||
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| <h4>Schema limiting to a set of specific enum values</h4> | ||
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| <pre style="font-family: monospace;"> | ||
| <xs:attribute name="connection" default="unspecified" > | ||
| <xs:simpleType> | ||
| <xs:restriction base="xs:token" > | ||
| <xs:enumeration value="unspecified"/> | ||
| <xs:enumeration value="plug"/> | ||
| <xs:enumeration value="wire"/> | ||
| <xs:enumeration value="solder"/> | ||
| <xs:enumeration value="LED"/> | ||
| <xs:enumeration value="bulb"/> | ||
| <xs:enumeration value="other"/> | ||
| </xs:restriction> | ||
| </xs:simpleType> | ||
| </xs:attribute> | ||
| </pre> | ||
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| <h4>Schema also permitting older numeric values</h4> | ||
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| <pre style="font-family: monospace;"> | ||
| <xs:attribute name="size"> | ||
| <xs:simpleType> | ||
| <xs:union> | ||
| <xs:simpleType> | ||
| <xs:restriction base="xs:positive-integer"> | ||
| <xs:maxInclusive="10"/> | ||
| </xs:restriction> | ||
| </xs:simpleType> | ||
| <xs:simpleType> | ||
| <xs:restriction base="xs:NMTOKEN"> | ||
| <xs:enumeration value="small"/> | ||
| <xs:enumeration value="medium"/> | ||
| <xs:enumeration value="large"/> | ||
| </xs:restriction> | ||
| </xs:simpleType> | ||
| </xs:union> | ||
| </xs:simpleType> | ||
| </xs:attribute> | ||
| </pre> | ||
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| This can also be used for elements (which is the generally prefered approach) | ||
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| <!--#include virtual="/Footer.shtml" --> | ||
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| </div><!-- closes #mainContent--> | ||
| </div> <!-- closes #mBody, was opened by Sidebar --> | ||
| </body> | ||
| </html> |
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