[java] Java switch statement: Constant expression required, but it IS constant

So, I am working on this class that has a few static constants:

public abstract class Foo {
    ...
    public static final int BAR;
    public static final int BAZ;
    public static final int BAM;
    ...
}

Then, I would like a way to get a relevant string based on the constant:

public static String lookup(int constant) {
    switch (constant) {
        case Foo.BAR: return "bar";
        case Foo.BAZ: return "baz";
        case Foo.BAM: return "bam";
        default: return "unknown";
    }
}

However, when I compile, I get a constant expression required error on each of the 3 case labels.

I understand that the compiler needs the expression to be known at compile time to compile a switch, but why isn't Foo.BA_ constant?

I got this error on Android, and my solution was just to use:

public static final int TAKE_PICTURE = 1;

instead of

public static int TAKE_PICTURE = 1;

If you're using it in a switch case then you need to get the type of the enum even before you plug that value in the switch. For instance :

SomeEnum someEnum = SomeEnum.values()[1];

switch (someEnum) {
            case GRAPES:
            case BANANA: ...

And the enum is like:

public enum SomeEnum {

    GRAPES("Grapes", 0),
    BANANA("Banana", 1),

    private String typeName;
    private int typeId;

    SomeEnum(String typeName, int typeId){
        this.typeName = typeName;
        this.typeId = typeId;
    }
}

I recommend you to use enums :)

Check this out:

public enum Foo 
{
    BAR("bar"),
    BAZ("baz"),
    BAM("bam");

    private final String description;

    private Foo(String description)
    {
        this.description = description;
    }

    public String getDescription()
    {
        return description;
    }
}

Then you can use it like this:

System.out.println(Foo.BAR.getDescription());

Because those are not compile time constants. Consider the following valid code:

public static final int BAR = new Random().nextInt();

You can only know the value of BAR in runtime.

I recommend using the following way:

public enum Animal {
    DOG("dog"), TIGER("tiger"), LION("lion");
    private final String name;

    @Override
    public String toString() {
        return this.name;
    }
}


public class DemoSwitchUsage {

     private String getAnimal(String name) {
         Animal animalName = Animal.valueOf(name);
         switch(animalName) {
         case DOG:
             // write the code required.
             break;
         case LION:
             // Write the code required.
             break;
         default:
             break;
         }
     }
}

Sometimes the switch variable can also make that error for example:

switch(view.getTag()) {//which is an Object type

   case 0://will give compiler error that says Constant expression required

   //...
}

To solve you should cast the variable to int(in this case). So:

switch((int)view.getTag()) {//will be int

   case 0: //No Error

   //...
}

Below code is self-explanatory, We can use an enum with a switch case:

/**
 *
 */
enum ClassNames {
    STRING(String.class, String.class.getSimpleName()),
    BOOLEAN(Boolean.class, Boolean.class.getSimpleName()),
    INTEGER(Integer.class, Integer.class.getSimpleName()),
    LONG(Long.class, Long.class.getSimpleName());
    private Class typeName;
    private String simpleName;
    ClassNames(Class typeName, String simpleName){
        this.typeName = typeName;
        this.simpleName = simpleName;
    }
}

Based on the class values from the enum can be mapped:

 switch (ClassNames.valueOf(clazz.getSimpleName())) {
        case STRING:
            String castValue = (String) keyValue;
            break;
        case BOOLEAN:
            break;
        case Integer:
            break;
        case LONG:
            break;
        default:
            isValid = false;

    }

Hope it helps :)

You can use an enum like in this example:

public class MainClass {
enum Choice { Choice1, Choice2, Choice3 }
public static void main(String[] args) {
Choice ch = Choice.Choice1;

switch(ch) {
  case Choice1:
    System.out.println("Choice1 selected");
    break;
 case Choice2:
   System.out.println("Choice2 selected");
   break;
 case Choice3:
   System.out.println("Choice3 selected");
   break;
    }
  }
}

Source: Switch statement with enum

This was answered ages ago and probably not relevant, but just in case. When I was confronted with this issue, I simply used an if statement instead of switch, it solved the error. It is of course a workaround and probably not the "right" solution, but in my case it was just enough.

Edit: 2021.01.21

This Answer is a bit misleading, And I would like to clarify it.

1. Replacing a switch statement with an if should not be considered as a goto solution, there are very good reasons why both concepts of switch and if exist in software development, as well as performance matters to consider when choosing between the two.
2. Although I provide a solution to the presented error, My answer sheds no light on "why" the problem occurs but instead offers a way around the problem.

In my specific case, using an if statement instead was just enough to solve the problem. Developers should take the time and decide if this is the right solution for the current problem you have at hand.

Thus this answer should be considered solely as a workaround in specific cases as stated in my first response, and by no means as the correct answer to this question

In my case, I was getting this exception because

switch (tipoWebServ) {
                            case VariablesKmDialog.OBTENER_KM:
                                resultObtenerKm(result);
                                break;
                            case var.MODIFICAR_KM:
                                resultModificarKm(result);
                                break;
                        }

in the second case I was calling the constant from the instance var.MODIFICAR_KM: but I should use VariablesKmDialog.OBTENER_KM directly from the class.

Got this error in Android while doing something like this:

 roleSpinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
        @Override
        public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {

            switch (parent.getItemAtPosition(position)) {
                case ADMIN_CONSTANT: //Threw the error

            }

despite declaring a constant:

public static final String ADMIN_CONSTANT= "Admin";

I resolved the issue by changing my code to this:

roleSpinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
        @Override
        public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {

            String selectedItem = String.valueOf(parent.getItemAtPosition(position));
            switch (selectedItem) {
                case ADMIN_CONSTANT:

            }

You get Constant expression required because you left the values off your constants. Try:

public abstract class Foo {
    ...
    public static final int BAR=0;
    public static final int BAZ=1;
    public static final int BAM=2;
    ...
}