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Out-of-Bounds literals in Java

A few days ago I answered this Java question on Stack Overflow, and since then it seems to have garnered a lot of attention, so I thought I’d write a quick post on the topic touched by the question.

To start, note that the int type in Java can hold integral values from -2147483648 to 2147483647, inclusive. Now, The question can be reformulated as follows:

How come the Java compiler rejects

int n = 2147483648;

but accepts

int n = 2147483647 + 1;

Isn’t 2147483647 + 1 a constant expression that “evaluates” to 2147483648 and, therefore, subject to rejection just like the literal 2147483648 in the first snippet? Or, shouldn’t the fact that we are unable to assign an out-of-range value in the first snippet apply also for the second? The answers of course are “no”.

The real issue here is that the 2147483648 in the first snippet is simply an invalid literal; it isn’t syntactically correct. From JLS §3.10.1:

It is a compile-time error if a decimal literal of type int is larger than 2147483648 (231), or if the decimal literal 2147483648 appears anywhere other than as the operand of the unary minus operator (§15.15.4).

(It’s really the bit after the “or” that applies here.) 2147483647 + 1, on the other hand, is perfectly valid; we’re just adding two numbers and there just happens to be an integer overflow.

I appreciate that the answers to these types of questions could all potentially be “because the language specification says so”, but hopefully this has provided some insight beyond that.