java 常量 表达式,Java switch语句：需要常量表达式，但它是常量

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?

解决方案

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 a definition of what is required of a constant expression. 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.