Mapping between an enum and a string?

Question

So, I recently read this post and I want to do effectively the same thing.

I didn't really care about "ugliness", etc. so I implemented one of the methods like so:

public enum Day {

    Monday(1), 
    Tuesday(2), 
    Wednesday(3), 
    Thursday(4), 
    Friday(5), 
    Saturday(6), 
    Sunday(7);

    public final int id;

    Day(int id) {
        this.id = id;
    }

    public static Day getByID(int id) {
        Day d = null;
        for (Day dTemp : Day.values())
        {
            if (id == dTemp)
            {
                d = dTemp;
                break;
            }
        }

        return d;
    }

    public Day getNext() {
        return values()[(ordinal()+1)%values().length];
    }

    public Day getPrev() {
        return values()[(ordinal()-1)%values().length];
    }
}

But, the problem with it is in the if statement when I do:

if (id == dTemp)

It says that they are incompatible types. What should I do to fix it?

Were you mislead by c#, my child? (http://msdn.microsoft.com/en-us/library/cc138362.aspx) — The Nail, Jan 02 '12 at 22:27

Thomas · Accepted Answer · 2012-01-02T22:31:41.897

2

Use if ( id == dTemp.id ). Enums are classes and enum values are objects not ints and thus can't be cast (neither explicitly nor implicitly) to an int.

Alternatively, note that enums have an ordinal, i.e. an id of their position. In your example, Monday would have the ordinal 0 and Sunday would have the ordinal 6. You access the ordinal by calling the ordinal() method.

Thus if id is one-based you could do the following instead:

public static Day getByID(int id) {   
  //check id is between 1 and 7, I'll leave that for you
  return Day.values()[id - 1];
}

Note that you might want to cache Day.values() in a private static variable and access that cached array then.

Btw, where's the string you mention in your question?

edited Jan 02 '12 at 22:31

answered Jan 02 '12 at 22:24

Thomas

87,414
12
119
157

FYI, Bhaxy is already using `ordinal()`s to get the next and previous days. – The Nail Jan 02 '12 at 22:31
@TheNail yes, I saw that, too. I wonder why he's not using it in that static method though. – Thomas Jan 02 '12 at 22:33

Lion · Answer 2 · 2012-01-02T22:43:33.513

if(id == dTemp.id)

Should work.

Alternatively try with the ordinal() method of enum.

public final int ordinal()Returns the ordinal of this enumeration constant (its position in its enum declaration, where the initial constant is assigned an ordinal of zero). Most programmers will have no use for this method. It is designed for use by sophisticated enum-based data structures, such as EnumSet and EnumMap.

Returns: the ordinal of this enumeration constant. See.

Well, I copied your code and tested accordingly.

Both

if (id == dTemp.ordinal()+1)

and

if(id == dTemp.id)

worked fine and produced as expected. The statement

System.out.println(Day.Friday.getByID(1));

produces Monday.

Mapping between an enum and a string?

2 Answers2