Java Switch Statement: Constant Expression Required, But It Is Constant

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

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?

While they are constant from the perspective of any code that executes after the fields have been initialized, they are not a compile time constant in the sense required by the JLS; see §15.28 Constant Expressions for the specification of a constant expression¹. This refers to §4.12.4 Final Variables which defines a "constant variable" as follows:

We call a variable, of primitive type or type String, that is final and initialized with a compile-time constant expression (§15.28) a constant variable. Whether a variable is a constant variable or not may have implications with respect to class initialization (§12.4.1), binary compatibility (§13.1, §13.4.9) and definite assignment (§16).

In your example, the Foo.BA* variables do not have initializers, and hence do not qualify as "constant variables". The fix is simple; change the Foo.BA* variable declarations to have initializers that are compile-time constant expressions.

In other examples (where the initializers are already compile-time constant expressions), declaring the variable as final may be what is needed.

You could change your code to use an enum rather than int constants, but that brings another couple of different restrictions:

• You must include a default case, even if you have case for every known value of the enum; see Why is default required for a switch on an enum?
• The case labels must all be explicit enum values, not expressions that evaluate to enum values.

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^{1 - The constant expression restrictions can be summarized as follows. Constant expressions a) can use primitive types and String only, b) allow primaries that are literals (apart from null) and constant variables only, c) allow constant expressions possibly parenthesised as subexpressions, d) allow operators except for assignment operators, ++, -- or instanceof, and e) allow type casts to primitive types or String only.}

^{Note that this doesn't include any form of method or lambda calls, new, .class. .length or array subscripting. Furthermore, any use of array values, enum values, values of primitive wrapper types, boxing and unboxing are all excluded because of a).}

Constant expression required in Java switch statement

The error tells you exactly what is wrong.

Case labels must be compile-time constants. This:

case WorkflowKey.DOG_WORKFLOW.getKey():

has a method call, which is not known at compile time (according to the rules for determining what is constant).

Is WorkflowKey an enum type? Then you should just be switching on the enumerated constants:

case DOG_WORKFLOW:

You'll need a way to get the enum value from the 'process definition' instead of the String you're getting, of course.

Java swith case says Constant expression required for final field

This is not possible with a switch-statement. The switch-statement in Java is more constrained then most people know. What the comments suggest is true - your constants aren't "constant enough".
They may be final, but they are being initialized when the program starts. Java requires the switch label to be known at compile time.

The easiest solution would be to use if-else-if and I'd use the class for comparison directly instead of comparing the names character wise:

Class<?> def = definition.get(correctKey).getClass();
if (Integer.class.equals(def)) {
  // your code
} else if (Double.class.equals(def)) {
  // double code
} else {
  // Error, not found
}

If you "need" to use a switch, then you have to define an Enum with your constants and a function mapping your input to an enum constant.

It would be interesting to know what you're trying to achieve, maybe there is another solution entirely.

Constant expression required in switch statement

It will compile if you access your static final fields statically; e.g. case ColorManager.blue:. If you try and access then from a variable cm, then you're preventing the compiler recognising them as compile-time constants.

Constant expression required error in switch statement with strings

case statements have to be compile-time evaluable.

Something like Status.SUCCESS.toString() doesn't satisfy that. A string literal, on the other hand, does.

The obvious fix is to use an an if block.

constant expression required in switch with string

Two mistakes here:

1. The definition of the enum shouldn't have class in it. As a rule of thumb, the constants should be uppercase.

   enum RoleName {
       WORKER,
       BOSS;
   }
   

2. RoleName.Worker.name() isn't allowed as a case expression since it's not a constant expression. You could transform requestRole.getName() into a RoleName and use it as a switch expression.

   switch (RoleName.valueOf(requestRole.getName())) {
       case WORKER: /* ... */ break;
   }
   

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