Cannot access static field within enum initialiser

In this code I get a compiler error, see comment:

 public 开发者_StackOverflow社区enum Type {
   CHANGESET("changeset"),
   NEW_TICKET("newticket"),
   TICKET_CHANGED("editedticket"),
   CLOSED_TICKET("closedticket");

   private static final Map<String, Type> tracNameMap = new HashMap<String, Type>();

   private Type(String name) {
    tracNameMap.put(name, this); // cannot refer to static field within an initializer
   }

   public static Type getByTracName(String tn) {
    return tracNameMap.get(tracNameMap);
   }

  }

Is there a way to make this work, getting an enum value from a Map by one of its fields?

The map is probably overkill here. Unless you are planning on having many more than four enum values you can implement getByTracName(String tn) by simply iterating over the valid strings and returning the correct one. If the map keys are always the enum names then you can do:

public enum Type {
CHANGESET,
NEW_TICKET,
TICKET_CHANGED,
CLOSED_TICKET;

private static final Map<String, Type> tracNameMap = new HashMap<String, Type>();
static {
    for (Type t:Type.values()) {
        tracNameMap.put(t.name(), t);
    }
}
public static Type getByTracName(String tn) {
    return tracNameMap.get(tracNameMap);
}

}

or you can do:

public static Type getByTracName(String tn) {
  return Enum.valueOf(Type.class,tn);
}

Hah, funny! Just a few days ago, I stumbled across this.

From the Java Language Specification, Third Edition, Section 8.9:

It is a compile-time error to reference a static field of an enum type that is not a compile-time constant (§15.28) from constructors, instance initializer blocks, or instance variable initializer expressions of that type. It is a compile-time error for the constructors, instance initializer blocks, or instance variable initializer expressions of an enum constant e to refer to itself or to an enum constant of the same type that is declared to the right of e.

Discussion

Without this rule, apparently reasonable code would fail at run time due to the initialization circularity inherent in enum types. (A circularity exists in any class with a "self-typed" static field.) Here is an example of the sort of code that would fail:

enum Color {
        RED, GREEN, BLUE;
        static final Map<String,Color> colorMap = 
        new HashMap<String,Color>();
        Color() {
            colorMap.put(toString(), this);
        }
    } 

Static initialization of this enum type would throw a NullPointerException because the static variable colorMap is uninitialized when the constructors for the enum constants run. The restriction above ensures that such code won't compile.

Note that the example can easily be refactored to work properly:

enum Color {
        RED, GREEN, BLUE;
        static final Map<String,Color> colorMap = 
        new HashMap<String,Color>();
        static {
            for (Color c : Color.values())
                colorMap.put(c.toString(), c);
        }
    } 

The refactored version is clearly correct, as static initialization occurs top to bottom.

I'd use the Reversible Enum Pattern:

ReversibleEnum.java

/**
 * <p>
 * This interface defines the method that the {@link Enum} implementations
 * should implement if they want to have the reversible lookup functionality.
 * i.e. allow the lookup using the code for the {@link Enum} constants.
 * </p>
 * @author Atif Khan
 * @param < E >
 *          The value of Enum constant
 * @param < V >
 *          The Enum constant to return from lookup
 */
public interface ReversibleEnum< E, V >
{
  /**
   * <p>
   * Return the value/code of the enum constant.
   * </p>
   * @return value
   */
  public E getValue();

  /**
   * <p>
   * Get the {@link Enum} constant by looking up the code in the reverse enum
   * map.
   * </p>
   * @param  E - code
   * @return V - The enum constant
   */
  public V reverse( E code );
}

ReverseEnumMap.java

import java.util.HashMap;
import java.util.Map;

/**
 * <p>
 * A utility class that provides a reverse map of the {@link Enum} that is keyed
 * by the value of the {@link Enum} constant.
 * </p>
 * @author Atif Khan
 * @param < K >
 *          The class type of the value of the enum constant
 * @param < V >
 *          The Enum for which the map is being created
 */
public class ReverseEnumMap< K, V extends ReversibleEnum< K, V >>
{
  private final Map< K, V > mReverseMap = new HashMap< K, V >();

  /**
   * <p>
   * Create a new instance of ReverseEnumMap.
   * </p>
   * @param valueType
   */
  public ReverseEnumMap( final Class< V > valueType )
  {
    for( final V v : valueType.getEnumConstants() ) {
      mReverseMap.put( v.getValue(), v );
    }
  }

  /**
   * <p>
   * Perform the reverse lookup for the given enum value and return the enum
   * constant.
   * </p>
   * @param enumValue
   * @return enum constant
   */
  public V get( final K enumValue )
  {
    return mReverseMap.get( enumValue );
  }
}

You'd change Type.java as follows:

public enum Type implements ReversibleEnum< String, Type >  {
  CHANGESET( "changeset" ),
  NEW_TICKET( "new" ),
  TICKET_CHANGED( "changed" ),
  CLOSED_TICKET( "closed" );

  private String mValue;  

  private static final ReverseEnumMap< String, Type > mReverseMap = new ReverseEnumMap< String, Type >( Type.class );  

  Type(final String value)   
  {  
    mValue = value;  
  }  

  public final String getValue()   
  {  
    return mValue;  
  }  

  public Type reverse( final String value )  
  {  
    return mReverseMap.get( value );  
  } 
} 

How about this; doesn't require you to make code changes at two places which is kind of error prone IMO:

enum Type {

    CHANGESET("changeset"),
    NEW_TICKET("newticket"),
    TICKET_CHANGED("editedticket"),
    CLOSED_TICKET("closedticket");

    private static final Map<String, Type> tracNameMap =
                                      new HashMap<String, Type>();

    private final String name;

    public Type typeForName(final String name) {
        if (tracNameMap.containsKey(name)) {
            return tracNameMap.get(name);
        } else {
            for (final Type t : Type.values()) {
                if (t.name.equals(name)) {
                    tracNameMap.put(name, t);
                    return t;
                }
            }
            throw new IllegalArgumentException("Invalid enum name");
        }
    }

    private Type(String name) {
        this.name = name;
    }

}

My own workaround, though it requires repeating all the enum values:

    public enum Type {
        CHANGESET,
        NEW_TICKET,
        TICKET_CHANGED,
        CLOSED_TICKET;

        private static final Map<String, Type> tracNameMap = new HashMap<String, Type>();
        static {
            tracNameMap.put("changeset", CHANGESET);
            tracNameMap.put("newticket", NEW_TICKET);
            tracNameMap.put("editedticket", TICKET_CHANGED);
            tracNameMap.put("closedticket", CLOSED_TICKET);
        }
        public static Type getByTracName(String tn) {
            return tracNameMap.get(tracNameMap);
        }

    }