Is `1/0` a constant expression in Java? [duplicate]

Question

As far as I understand the Java 8 JLS the expression (1/0) is considered a constant expression, but when I try to compile the following program with OpenJDK 8 I get an error

public class Switch {     public static void main(String[] args) {         switch(42) {             case (1/0):                 return;             default:                 return;         }     } } 

The error says (1/0) isn't a constant expression

Switch.java:4: error: constant expression required             case (1/0):                  ^ 1 error 

Am I missing something? Or is it a bug in OpenJDK 8?

Answer 1

The compiler is doing constant folding (precomputing trivial literal expressions). This is a case where the expression "completes abruptly", to use the JLS verbiage, disqualifying it from meeting the definition of "constant expression". So it's not a bug, it's consistent with the JLS.

And yes, the expression doesn't evaluate to a value either (warning the user trying to do something like this that the result will not be a constant expression), but the compiler doesn't know that until it tries. Not evaluating to a value and completing abruptly would seem to go hand-in-hand.

Adding a variable declaration like

int x = 1 / 0; 

doesn't cause a compiler error, it's the switch that forces the expression to be evaluated at compile time.

By the way I checked that this happens for version 7 of the Oracle and IBM JDKs too, it's not OpenJDK or JDK8 specific.

Answer 2

A constant expression must be able to evaluate to a value, since the compiler must reconduce that expression to a value.

1/0 doesn't have any value.

From JSL §15.28:

A compile-time constant expression is an expression denoting a value of primitive type or a String that does not complete abruptly and is composed using only the following: