Code Correctness: Comparison with NaN

Abstract

與 NaN 進行比較一律是錯誤。

Explanation

對 NaN 進行比較時，一律會估算為 false，除了 != 運算子之外，這個運算子一律會估算為 true，因為 NaN 處於未排序狀態。

範例 1：以下將嘗試確定變數不是 NaN。


  ...
  if (result == Double.NaN){
    //something went wrong
    throw new RuntimeException("Something went wrong, NaN found");
  }
  ...

這會嘗試驗證 result 不是 NaN，不過使用 == 運算子搭配 NaN 一律會產生值 false，因此這項檢查永不會拋出異常。

References

[1] NUM07-J. Do not attempt comparisons with NaN CERT

[2] Java Language Specification Chapter 4. Types, Values, and Variables Oracle

[3] INJECT-9: Prevent injection of exceptional floating point values Oracle

[4] Standards Mapping - Common Weakness Enumeration CWE ID 486

desc.structural.java.code_correctness_comparison_with_nan