Why Doesn't Java's Switch Statement Accept Seemingly Constant Variables as Case Labels?

Java Switch Statement Constant Expression Confusion

In Java, a switch statement requires constant expressions as case labels. However, developers often encounter errors complaining about non-constant expressions even when using seemingly constant variables.

Consider the following example:

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

    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";
        }
    }
}

When compiling this code, you may encounter a "constant expression required" error on each case label.

Why aren't Foo.BAR variables constant?

While Foo.BAR variables are effectively constant from an execution perspective, they are not "compile-time constants" as required by the Java Language Specification (JLS). Section 15.28 of the JLS defines a constant expression as one that:

• Refers to primitive types or String
• Only uses literals, constant variables, and parentheses
• Excludes assignment operators, , --, and instanceof
• Does not include type casting except to primitive types or String

In the example above, Foo.BAR variables are not declared with explicit initializers, therefore they do not qualify as compile-time constants.

Solution:

To resolve the error, initialize the Foo.BAR variables with compile-time constant expressions:

public static final int BAR = 1;
public static final int BAZ = 2;
public static final int BAM = 3;

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