not a constant in Enum | 易学教程

问题

I am using enumeration with switch case but I am getting the following error:

NEWS FEED is not a constant in FragmentName

This is my enum string constant,

 public enum FragmentName{
        FRAGMENT_NEWSFEED("NEWS FEED"),
        FRAGMENT_MESSAGES("MESSAGES"),
        FRAGMENT_EVENTS("EVENTS"),
        FRAGMENT_WHOISAROUDNME("WHOS AROUND");

        private final String text;
        private FragmentName(final String text) {
            this.text = text;
        }
        @Override
        public String toString() {
            return text;
        }
    }

//This is my function from where i check for corresponding enum constant

 public void changeTitle(String title) {
        switch (Enums_String.FragmentName.valueOf(title)) {
            case FRAGMENT_NEWSFEED:
                System.out.println("1");
                break;
            case FRAGMENT_EVENTS:
                System.out.println("2");
                break;
            case FRAGMENT_MESSAGES:
                System.out.println("3");
                break;
            case FRAGMENT_WHOISAROUDNME:
                System.out.println("4");
                break;
        }
    }

When I call

     changeTitle("NEWS FEED");

it creates an exception in the changeTitle function even the value passed is same, so any help would be appreciated as I have tried my every effort to solve this.

回答1:

Add this code to your enum

private static final Map<String, FragmentName> map = new HashMap<>();
static {
    for (FragmentName en : values()) {
        map.put(en.text, en);
    }
}

public static FragmentName valueFor(String name) {
    return map.get(name);
}

Now instead of valueOf use valueFor

switch (Enums_String.FragmentName.valueFor(title))
//                                ^^^^^^^^

回答2:

The valueOf

Returns the enum constant of the specified enum type with the specified name. The name must match exactly an identifier used to declare an enum constant in this type. (Extraneous whitespace characters are not permitted.

What you want do id get the enum by a member value for that you have write a function to do so like fromString below

 public enum FragmentName {

    FRAGMENT_NEWSFEED("NEWS FEED"),
    FRAGMENT_MESSAGES("MESSAGES"),
    FRAGMENT_EVENTS("EVENTS"),
    FRAGMENT_WHOISAROUDNME("WHOS AROUND");

    private final String text;

    private FragmentName(final String text) {
        this.text = text;
    }

    @Override
    public String toString() {
        return text;
    }

    public static FragmentName fromString(String value) {
        for (FragmentName fname : values()) {
            if (fname.text.equals(value)) {
                return fname;
            }
        }
        return null;
    }
}

and replace your switch case like

    switch (FragmentName.fromString(title)) {

回答3:

Create a method like this :

public static FragmentName getFragmentNameByText(String text) {
    for (FragmentName fragment : values()) {
      if (fragment.text.equals(text)) {
         return fragment;
      }
    }
    return null;
}

and call this instead of valueOf().

回答4:

You can change your function to compare the string values passed in:

public void changeTitle(String title) {
        if(title.equals(FRAGMENT_NEWSFEED.toString())) {
            System.out.println("1");
        } else if(title.equals(FRAGMENT_MESSAGES.toString())) {
            System.out.println("2");
        } else if(title.equals(FRAGMENT_EVENTS.toString())) {
            System.out.println("3");
        } else if(title.equals(FRAGMENT_WHOISAROUDNME.toString())) {
            System.out.println("4");
        } else {
            // throw an error
        }
    }

You can't operate an switch off of a function call, so you have to use an if-else block.

来源：https://stackoverflow.com/questions/31271061/not-a-constant-in-enum

标签

java

enums

enumeration