Implementing a bitfield using java enums

问题

I maintain a large document archive and I often use bit fields to record the status of my documents during processing or when validating them. My legacy code simply uses static int constants such as:

static int DOCUMENT_STATUS_NO_STATE = 0
static int DOCUMENT_STATUS_OK = 1
static int DOCUMENT_STATUS_NO_TIF_FILE = 2
static int DOCUMENT_STATUS_NO_PDF_FILE = 4

This makes it pretty easy to indicate the state a document is in, by setting the appropriate flags. For example:

status = DOCUMENT_STATUS_NO_TIF_FILE | DOCUMENT_STATUS_NO_PDF_FILE;

Since the approach of using static constants is bad practice and because I would like to improve the code, I was looking to use Enums to achieve the same. There are a few requirements, one of them being the need to save the status into a database as a numeric type. So there is a need to transform the enumeration constants to a numeric value. Below is my first approach and I wonder if this is the correct way to go about this?

class DocumentStatus{

    public enum StatusFlag {

        DOCUMENT_STATUS_NOT_DEFINED(1<<0),
        DOCUMENT_STATUS_OK(1<<1), 
        DOCUMENT_STATUS_MISSING_TID_DIR(1<<2),
        DOCUMENT_STATUS_MISSING_TIF_FILE(1<<3),
        DOCUMENT_STATUS_MISSING_PDF_FILE(1<<4),
        DOCUMENT_STATUS_MISSING_OCR_FILE(1<<5),
        DOCUMENT_STATUS_PAGE_COUNT_TIF(1<<6),
        DOCUMENT_STATUS_PAGE_COUNT_PDF(1<<7),
        DOCUMENT_STATUS_UNAVAILABLE(1<<8);


        private final long statusFlagValue;

        StatusFlag(long statusFlagValue) {
            this.statusFlagValue = statusFlagValue;
        }

        public long getStatusFlagValue(){
            return statusFlagValue;
        } 

       }


    /**
     * Translates a numeric status code into a Set of StatusFlag enums
     * @param numeric statusValue 
     * @return EnumSet representing a documents status
     */
    public EnumSet<StatusFlag> getStatusFlags(long statusValue) {
        EnumSet statusFlags = EnumSet.noneOf(StatusFlag.class);
        StatusFlag.each { statusFlag -> 
            long flagValue = statusFlag.statusFlagValue
            if ( (flagValue&statusValue ) == flagValue ) {
               statusFlags.add(statusFlag);
            }
        }
        return statusFlags;
    }


    /**
     * Translates a set of StatusFlag enums into a numeric status code
     * @param Set if statusFlags
     * @return numeric representation of the document status 
     */
    public long getStatusValue(Set<StatusFlag> flags) {
        long value=0;
        flags.each { statusFlag -> 
            value|=statusFlag.getStatusFlagValue() 
        }
        return value;
    }

     public static void main(String[] args) {

        DocumentStatus ds = new DocumentStatus();
        Set statusFlags = EnumSet.of(
            StatusFlag.DOCUMENT_STATUS_OK,
            StatusFlag.DOCUMENT_STATUS_UNAVAILABLE);

        assert ds.getStatusValue( statusFlags )==258 // 0000.0001|0000.0010

        long numericStatusCode = 56;
        statusFlags = ds.getStatusFlags(numericStatusCode);

        assert !statusFlags.contains(StatusFlag.DOCUMENT_STATUS_OK);
        assert statusFlags.contains(StatusFlag.DOCUMENT_STATUS_MISSING_TIF_FILE);
        assert statusFlags.contains(StatusFlag.DOCUMENT_STATUS_MISSING_PDF_FILE);
        assert statusFlags.contains(StatusFlag.DOCUMENT_STATUS_MISSING_OCR_FILE);

    }

}

回答1:

your approach is exactly the way to do it.

回答2:

Instead of defining constructor parameters, you could simply use the internal ordinal() value to calculate this.

public enum StatusFlag {

    DOCUMENT_STATUS_NOT_DEFINED,
    DOCUMENT_STATUS_OK, 
    DOCUMENT_STATUS_MISSING_TID_DIR,
    DOCUMENT_STATUS_MISSING_TIF_FILE,
    DOCUMENT_STATUS_MISSING_PDF_FILE,
    DOCUMENT_STATUS_MISSING_OCR_FILE,
    DOCUMENT_STATUS_PAGE_COUNT_TIF,
    DOCUMENT_STATUS_PAGE_COUNT_PDF,
    DOCUMENT_STATUS_UNAVAILABLE;


    public long getStatusFlagValue(){
        return 1 << this.ordinal();
    } 

}

Please note that now you should abstain from reordering, inserting (other than at the end) or deleting entries, otherwise the flag values will change, and the meaning of your database contents will change.

回答3:

A slightly better way would be to store the result of 1 << this.ordinal() in a field when the enum values are constructed. This way, you don't have to provide each value manually, and the flag is only computed once.

public enum StatusFlag {

  DOCUMENT_STATUS_NOT_DEFINED,
  DOCUMENT_STATUS_OK, 
  DOCUMENT_STATUS_MISSING_TID_DIR,
  DOCUMENT_STATUS_MISSING_TIF_FILE,
  DOCUMENT_STATUS_MISSING_PDF_FILE,
  DOCUMENT_STATUS_MISSING_OCR_FILE,
  DOCUMENT_STATUS_PAGE_COUNT_TIF,
  DOCUMENT_STATUS_PAGE_COUNT_PDF,
  DOCUMENT_STATUS_UNAVAILABLE;

  public final int flag;

  StatusFlag() { 
    this.flag = 1 << this.ordinal();
  } 
}

EDIT: it could be useful to have isFlagSet method as well.

回答4:

Don't give your enums values. Use an EnumSet to combine them, and use Enum.ordinal() when persisting in order to convert to/from a single integer. You might also find Class.getEnumConstants() useful when reconstructing the set from the integer.

回答5:

I have made a complete library for this problem: http://claude-martin.ch/enumbitset/

The main goal was to store sets of enum types in bitfields. But it also supports other types.

With this you would not need any extra methods like your "getStatusFlags()". It can be used on any existing enum type simply by adding the interface EnumBitSetHelper (it is used like a "trait"). Each enum constant can then create an "EnumBitSet" which has all methods of Java's EnumSet and BitSet. Then you can work with these sets of enum constants and convert them to bitfield values.

It supports many formats such as BigInteger and long to easily store the value into a bit field. But note that this only works with Java version 8 and newer.

来源：https://stackoverflow.com/questions/5346477/implementing-a-bitfield-using-java-enums

标签

java

enums

enumset