Comparison With NaN Using == or !=

ID

java.test_nan_equality

Severity

high

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

Java

Tags

CWE:597, reliability

Description

Reports equality comparisons (== or !=) against Double.NaN or Float.NaN. By IEEE 754 specification, NaN is not equal to any value including itself, so x == Double.NaN is always false and x != Double.NaN is always true.

Rationale

This is a common and insidious bug. The comparison compiles without warnings but always yields the same result regardless of the value of x. The code path guarded by the comparison is either dead code (for ==) or always taken (for !=).

// Bad - ALWAYS false, even when x is NaN
if (x == Double.NaN) {
    handleNaN();  // unreachable
}

Remediation

Use the isNaN() methods from Double or Float.

// Good - correctly detects NaN
if (Double.isNaN(x)) {
    handleNaN();
}

References