Why Doesn't My Java Switch Statement Accept Static Constants from an Abstract Class?

Java Switch Statement: Troubleshooting Constant Expression Requirement

In the scenario presented, a switch statement attempts to match an integer constant with static constants defined in an abstract class. However, the compilation fails with a "constant expression required" error.

Understandably, compile-time constant expressions are necessary for switch statements. Yet, the Foo.BAR, Foo.BAZ, and Foo.BAM constants seem constant.

Why aren't these constants treated as compile-time constant expressions?

According to the Java Language Specification (JLS) §15.28, a compile-time constant expression consists of only certain primitive types and literals, excluding object references. In this case, Foo.BAR is not a compile-time constant expression because it refers to a static field, which is initialized at runtime, not compile-time.

Resolving the issue:

To resolve this issue, the Foo.BA* variables must be initialized with compile-time constant expressions:

public abstract class Foo {
    ...
    public static final int BAR = 1;
    public static final int BAZ = 2;
    public static final int BAM = 3;
    ...
}

With these initializers, the compiler can determine the values of Foo.BA* at compile time, making them valid for use in a switch statement.

Alternative approach using enums:

Another option to ensure compile-time constant values is to use enums:

public enum FooConstants {
    BAR,
    BAZ,
    BAM
}

However, using enums imposes additional restrictions, such as requiring a default case and prohibiting case labels that evaluate to expressions.

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