Two NaN values are defined in the java.lang package (Float.NaN and Double.NaN) : float « Java Source And Data Type « SCJP

1. SCJP
2. Java Source And Data Type
3. float

public class MainClass {
  public static void main(String[] argv) {
    System.out.println(Float.NaN);
    System.out.println(Double.NaN);

    float y = 0.8F;
    float x = (float) Math.sqrt(y); // where y may be negative
    if (x == Float.NaN) {
      x = 0.0F; // WRONG, always false
    }
    if (Float.isNaN(x)) {
      x = 0.0F;
      // the right way to detect NaN
    }
  }
}