Java using enum with switch statement

Question

I've looked at various Q&As on SO similar to this question but haven't found a solution.

What I have is an enum which represents different ways to view a TV Guide...

In the NDroid Application class

static enum guideView {
    GUIDE_VIEW_SEVEN_DAY,
    GUIDE_VIEW_NOW_SHOWING,
    GUIDE_VIEW_ALL_TIMESLOTS
}

...when the user changes the view an event handler receives an int from 0-2 and I'd like to do something like this...

In an Android Activity onClick(DialogInterface dialog, int which) event handler

// 'which' is an int from 0-2
switch (which) {
    case NDroid.guideView.GUIDE_VIEW_SEVEN_DAY:
    ...
    break;
}

I'm used to C# enums and select/case statements which would allow something like the above and I know Java does things differently but I just can't make sense of what I need to do.

Am I going to have to resort to if statements? There will likely only ever be 3 choices so I could do it but I wondered how it could be done with switch-case in Java.

EDIT Sorry I didn't completely expand on the issue as I was looking at it as being a generic Java issue. I've added to the question to explain a bit further.

There isn't anything that's Android specific which is why I didn't tag it as Android but the enum is defined in the Application class and the code where I wan't the switch is in an Activity. The enum is static as I need to access it from multiple Activities.

Solution 1

The part you're missing is converting from the integer to the type-safe enum. Java will not do it automatically. There's a couple of ways you can go about this:

1. Use a list of static final ints rather than a type-safe enum and switch on the int value you receive (this is the pre-Java 5 approach)
2. Switch on either a specified id value (as described by heneryville) or the ordinal value of the enum values; i.e. guideView.GUIDE_VIEW_SEVEN_DAY.ordinal()

3. Determine the enum value represented by the int value and then switch on the enum value.

   enum GuideView {
       SEVEN_DAY,
       NOW_SHOWING,
       ALL_TIMESLOTS
   }
   
   // Working on the assumption that your int value is 
   // the ordinal value of the items in your enum
   public void onClick(DialogInterface dialog, int which) {
       // do your own bounds checking
       GuideView whichView = GuideView.values()[which];
       switch (whichView) {
           case SEVEN_DAY:
               ...
               break;
           case NOW_SHOWING:
               ...
               break;
       }
   }
   

   You may find it more helpful / less error prone to write a custom valueOf implementation that takes your integer values as an argument to resolve the appropriate enum value and lets you centralize your bounds checking.

Solution 2

If whichView is an object of the GuideView Enum, following works well. Please note that there is no qualifier for the constant after case.

switch (whichView) {
    case SEVEN_DAY:
        ...
        break;
    case NOW_SHOWING:
        ...
        break;
}

Solution 3

The enums should not be qualified within the case label like what you have NDroid.guideView.GUIDE_VIEW_SEVEN_DAY, instead you should remove the qualification and use GUIDE_VIEW_SEVEN_DAY

Solution 4

I like a few usages of Java enum:

1. .name() allows you to fetch the enum name in String.
2. .ordinal() allow you to get the integer value, 0-based.
3. You can attach other value parameters with each enum.
4. and, of course, switch enabled.

enum with value parameters:

    enum StateEnum {
        UNDEFINED_POLL  ( 1 * 1000L,       4 * 1000L),
        SUPPORT_POLL    ( 1 * 1000L,       5 * 1000L),
        FAST_POLL       ( 2 * 1000L,  4 * 60 * 1000L),
        NO_POLL         ( 1 * 1000L,       6 * 1000L); 
        ...
    }

switch example:

private void queuePoll(StateEnum se) {
    // debug print se.name() if needed
    switch (se) {
        case UNDEFINED_POLL:
            ...
            break;
        case SUPPORT_POLL:
            ...
            break;

Solution 5

Short associative function example:

public String getIcon(TipoNotificacao tipo)
{
    switch (tipo){
        case Comentou : return "fa fa-comments";
        case ConviteEnviou : return "icon-envelope";
        case ConviteAceitou : return "fa fa-bolt";
        default: return "";
    }
}

Like @Dhanushka said, omit the qualifier inside "switch" is the key.

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Comments

[trashgod]

"Nested enum types are implicitly static. It is permissable to explicitly declare a nested enum type to be static."—JLS §8.9.

[SystemParadox]

@trashgod, indeed- but I would always use the implicit declaration to avoid confusion as that's actually more obvious IMO. A global static enum (which is what I assume this is) is probably wrong in most cases.

[Squonk]

Many thanks. It took me a while to work it into my code but it's working nicely now with the example code you posted. 6 years with C# and 1 year with Java - it's not often I come across something that stumps me. So many similarities but the occasional one like this which is so very different. I won't forget this one in a hurry. :-)

[haridsv]

You need the unqualified enum names in the case statement, so case GuideView.SEVEN_DAY: gives a compilation error, it should be case SEVEN_DAY:.

[WORMSS]

That only works if you are not getting your data externally. Since OP said which returns value 1, 2, 3 not a TYPE your method would not work without a reversed switch case that takes 1, 2, 3 and returns a TYPE before going into the switch case he listed..