Why Does My Java Switch Statement Throw a 'Constant Expression Required' Error When Using Static Constants?

Java switch statement: Constant expression unexpected error when using static constants

When working with Java switch statements, it is important to ensure that the expressions used in the case labels are constant expressions. Constant expressions are expressions that are evaluated at compile time and produce a constant value. This is necessary because the compiler needs to know the exact value of the expression in order to determine which case to execute.

In the following example, we have a class with three static constants:

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

We then have a method that uses a switch statement 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 we try to compile this code, we get a constant expression required error on each of the three case labels. This is because, while the constants are constant from the perspective of any code that executes after the fields have been initialized, they are not compile time constants in the sense required by the Java Language Specification (JLS).

Specifically, the JLS defines a constant expression as an expression that can be evaluated at compile time to produce a constant value. This means that the expression cannot contain any variables that are not themselves constant, or any operations that are not themselves constant.

In our example, the Foo.BA* constants are not compile time constants because they do not have initializers. To fix this, we can simply initialize the constants 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;
    ...
}

Now, when we compile the code, it should compile successfully.

It is also important to note that enums can be used instead of int constants in a switch statement. However, enums have some restrictions that int constants do not. For example, enums must have a default case, even if you have case for every known value of the enum. Also, the case labels must all be explicit enum values, not expressions that evaluate to enum values.

The above is the detailed content of Why Does My Java Switch Statement Throw a 'Constant Expression Required' Error When Using Static Constants?. For more information, please follow other related articles on the PHP Chinese website!