Why Can't Java's `switch` Statement Use Non-Compile-Time Constants?

Java switch Statement: Understanding Constant Expression Requirement

In Java, switch statements require constant expressions for case labels. While constants like Foo.BAR may seem constant, they are not considered compile-time constants as defined by the Java Language Specification (JLS). According to JLS §15.28, a constant expression must be known at compile time.

Why Foo.BA_ is not a compile-time constant:

Although Foo.BA_ variables are effectively constant after field initialization, they lack compile-time constant initializers. To create compile-time constants, initialize the variables explicitly with constant expressions. For example:

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

Alternatives to Switch Statements:

Consider using enums instead of int constants. However, enums impose additional constraints, such as requiring a default case even if the switch statement covers all enum values. Moreover, case labels must be explicit enum values rather than expressions evaluating to enum values.

Restrictions on Constant Expressions in Switch Statements:

Constant expressions used in switch statements have specific limitations:

• Only primitive types and String are allowed.
• Primaries are limited to literals, constant variables, and parentheses around constant expressions.
• Operators are allowed except for assignment operators, , --, or instanceof.
• Type casts are only permitted to primitive types or String.

In summary, when using switch statements in Java, ensure that the expressions in case labels are compile-time constants. Consider enums as an alternative, but be aware of their unique restrictions. Understanding these requirements will enhance the reliability and clarity of your code.

The above is the detailed content of Why Can't Java's `switch` Statement Use Non-Compile-Time Constants?. For more information, please follow other related articles on the PHP Chinese website!