I would like to lookup an enum from its string value (or possibly any other value). I've tried the following code but it doesn't allow static in initialisers. Is there a simple way?
public enum Verbosity {
BRIEF, NORMAL, FULL;
private static Map<String, Verbosity> stringMap = new HashMap<String, Verbosity>();
private Verbosity() {
stringMap.put(this.toString(), this);
}
public static Verbosity getVerbosity(String key) {
return stringMap.get(key);
}
};
Use the valueOf method which is automatically created for each Enum.
Verbosity.valueOf("BRIEF") == Verbosity.BRIEF
For arbitrary values start with:
public static Verbosity findByAbbr(String abbr){
for(Verbosity v : values()){
if( v.abbr().equals(abbr)){
return v;
}
}
return null;
}
Only move on later to Map implementation if your profiler tells you to.
I know it's iterating over all the values, but with only 3 enum values it's hardly worth any other effort, in fact unless you have a lot of values I wouldn't bother with a Map it'll be fast enough.
You're close. For arbitrary values, try something like the following:
public enum Day {
MONDAY("M"), TUESDAY("T"), WEDNESDAY("W"),
THURSDAY("R"), FRIDAY("F"), SATURDAY("Sa"), SUNDAY("Su"), ;
private final String abbreviation;
// Reverse-lookup map for getting a day from an abbreviation
private static final Map<String, Day> lookup = new HashMap<String, Day>();
static {
for (Day d : Day.values()) {
lookup.put(d.getAbbreviation(), d);
}
}
private Day(String abbreviation) {
this.abbreviation = abbreviation;
}
public String getAbbreviation() {
return abbreviation;
}
public static Day get(String abbreviation) {
return lookup.get(abbreviation);
}
}
with Java 8 you can achieve with this way:
public static Verbosity findByAbbr(final String abbr){
return Arrays.stream(values()).filter(value -> value.abbr().equals(abbr)).findFirst().orElse(null);
}
@Lyle's answer is rather dangerous and I have seen it not work particularly if you make the enum a static inner class. Instead I have used something like this which will load the BootstrapSingleton maps before the enums.
Edit this should not be a problem any more with modern JVMs (JVM 1.6 or greater) but I do think there are still issues with JRebel but I haven't had a chance to retest it.
Load me first:
public final class BootstrapSingleton {
// Reverse-lookup map for getting a day from an abbreviation
public static final Map<String, Day> lookup = new HashMap<String, Day>();
}
Now load it in the enum constructor:
public enum Day {
MONDAY("M"), TUESDAY("T"), WEDNESDAY("W"),
THURSDAY("R"), FRIDAY("F"), SATURDAY("Sa"), SUNDAY("Su"), ;
private final String abbreviation;
private Day(String abbreviation) {
this.abbreviation = abbreviation;
BootstrapSingleton.lookup.put(abbreviation, this);
}
public String getAbbreviation() {
return abbreviation;
}
public static Day get(String abbreviation) {
return lookup.get(abbreviation);
}
}
If you have an inner enum you can just define the Map above the enum definition and that (in theory) should get loaded before.
And you can't use valueOf()?
Edit: Btw, there is nothing stopping you from using static { } in an enum.
In the Java Language Specification 7 there is an a example! that reflects your question on initialization of the map with self-references.
In case it helps others, the option I prefer, which is not listed here, uses Guava's Maps functionality:
public enum Vebosity {
BRIEF("BRIEF"),
NORMAL("NORMAL"),
FULL("FULL");
private String value;
private Verbosity(final String value) {
this.value = value;
}
public String getValue() {
return this.value;
}
private static ImmutableMap<String, Verbosity> reverseLookup =
Maps.uniqueIndex(Arrays.asList(Verbosity.values()), Verbosity::getValue);
public static Verbosity fromString(final String id) {
return reverseLookup.getOrDefault(id, NORMAL);
}
}
With the default you can use null, you can throw IllegalArgumentException or your fromString could return an Optional, whatever behavior you prefer.
Perhaps, take a look at this. Its working for me.
The purpose of this is to lookup 'RED' with '/red_color'.
Declaring a static map and loading the enums into it only once would bring some performance benefits if the enums are many.
public class Mapper {
public enum Maps {
COLOR_RED("/red_color", "RED");
private final String code;
private final String description;
private static Map<String, String> mMap;
private Maps(String code, String description) {
this.code = code;
this.description = description;
}
public String getCode() {
return name();
}
public String getDescription() {
return description;
}
public String getName() {
return name();
}
public static String getColorName(String uri) {
if (mMap == null) {
initializeMapping();
}
if (mMap.containsKey(uri)) {
return mMap.get(uri);
}
return null;
}
private static void initializeMapping() {
mMap = new HashMap<String, String>();
for (Maps s : Maps.values()) {
mMap.put(s.code, s.description);
}
}
}
}
Please put in your opinons.
You can define your Enum as following code :
public enum Verbosity
{
BRIEF, NORMAL, FULL, ACTION_NOT_VALID;
private int value;
public int getValue()
{
return this.value;
}
public static final Verbosity getVerbosityByValue(int value)
{
for(Verbosity verbosity : Verbosity.values())
{
if(verbosity.getValue() == value)
return verbosity ;
}
return ACTION_NOT_VALID;
}
@Override
public String toString()
{
return ((Integer)this.getValue()).toString();
}
};
since java 8 you can initialize the map in a single line and without static block
private static Map<String, Verbosity> stringMap = Arrays.stream(values())
.collect(Collectors.toMap(Enum::toString, Function.identity()));
You can use the Enum::valueOf() function as suggested by Gareth Davis & Brad Mace above, but make sure you handle the IllegalArgumentException that would be thrown if the string used is not present in the enum.
来源：https://stackoverflow.com/questions/1080904/how-can-i-lookup-a-java-enum-from-its-string-value