How to avoid duplicate enum values in java?

Question

I have enum which corresponds to encodings.I need to check that enum don't have repeating number values for encodings.

public enum EncodingsEnum
{

ISO8859_1("ISO-8859-1",0), ISO8859_2("ISO-8859-2",1),
    ISO8859_3("ISO-8859-3",2), ISO8859_4("ISO-8859-4",3),
    ISO8859_5("ISO-8859-5",4), ISO8859_6("ISO-8859-6",5),
    ISO8859_7("ISO-8859-7",6), ISO8859_8("ISO-8859-8",7),
    ISO8859_9("ISO-8859-9",8), ISO8859_11("ISO-8859-11",9),
    ISO8859_13("ISO-8859-13",10),ISO8859_15("ISO-8859-15",11),
    UTF_8("UTF-8",11);

    public static final int ENCODINGS_COUNT = EncodingsEnum.values().length;
    private final String encodingName;
    private final int encodingNumber;

    EncodingsEnum(final String encodingName,int encodingNumber)
    {
        ReferenceChecker.checkReferenceNotNull(encodingName);

        this.encodingName = encodingName;
        this.encodingNumber = encodingNumber;
    }

    public static String getEncodingNameByNumber(int number)
    {
        for(EncodingsEnum encoding : EncodingsEnum.values())
        {
            if(encoding.encodingNumber == number)
            {
                return encoding.getEncodingName();
            }
        }
        throw new RuntimeException("Encoding with this number isn't supported:" + number);

    }

    public static int getEncodingNumberByName(final String name)
    {
        for(EncodingsEnum encoding : EncodingsEnum.values())
        {
            if(encoding.encodingName.equals(name))
            {
                return encoding.getEncodingNumber();
            }
        }
        throw new RuntimeException("Encoding with this name isn't supported:" + name);
    }

    public String getEncodingName()
    {
        return this.encodingName;
    }

    public int getEncodingNumber()
    {
        return this.encodingNumber;
    }
}

There is a problem that I can create encoding with the same number as one of the existing encodings,so I need to check that enum contains element with this number and throw exception.But I dont know how to do that.Any idea?Thanks.

Answer 1

This shouldn't be checked at runtime: it would be too late. Add a unit test that iterates through the enum values, and check that they all have a different number. And make sure to always execute the unit tests, and check that they pass, before generating a new version of your application/library.

@Test
public void encodingNumbersMustBeUnique() {
    Set<Integer> numbers = new HashSet<Integer>();
    for (EncodingsEnum e : EncodingsEnum.values()) {
        assertFalse(numbers.contains(e.getEncodingNumber()));
        numbers.add(e.getEncodingNumber());
    }
}

Answer 2

In the constructor you can iterate through the encodingNumbers of the enum and throw an exception if you find a duplicate. Something like this:

public boolean isPresent(String type){
            ClassType[] typeArray = ClassType.values();
            for(ClassType cType: typeArray){
                if(cType.absoluteName/*this is some private field of the class*/.equalsIgnoreCase(type)){
                    return true;
                }
            }
            return false;
        }

Put the above check just before you are assigning number and name in your constructor. If the check returns true, throw an exception else not.