Double.NaN Float.NaN

Double.NaN  Float.NaN
double i = 0.0/0.0;
double i = Double.NaN;

equals

http://docs.oracle.com/javase/7/docs/api/java/lang/Double.html 

public boolean equals(Object obj)
Compares this object against the specified object. The result is true if and only if the argument is not null and is a Double object that represents a double that has the same value as the double represented by this object. For this purpose, two double values are considered to be the same if and only if the method doubleToLongBits(double) returns the identical long value when applied to each. Note that in most cases, for two instances of class Double, d1 and d2, the value of d1.equals(d2) is true if and only if
d1.doubleValue() == d2.doubleValue()
also has the value true. However, there are two exceptions:
  • If d1 and d2 both represent Double.NaN, then the equals method returns true, even though Double.NaN==Double.NaN has the value false.
  • If d1 represents +0.0 while d2 represents -0.0, or vice versa, the equal test has the value false, even though +0.0==-0.0 has the value true.
This definition allows hash tables to operate properly.

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