Infinity is equal to Infinity in Java

Question

I wanted to test if infinity is equal to infinity in Java:

Double.POSITIVE_INFINITY == Double.POSITIVE_INFINITY

I was surprised by the result when it turned out to be true. My question is how can two infinite values be equal?

An unquantifiable amount can be equal to itself as also an unquantifiable amount. — px06, Oct 14 '16 at 08:15
Infinity behaves as a regular number. https://en.wikipedia.org/wiki/Ordinal_arithmetic#Large_countable_ordinals As opposed to `NaN`, which by definiiton is _not_ a number. — Marko Topolnik, Oct 14 '16 at 08:15

score 7 · Accepted Answer · answered Oct 14 '16 at 08:13

Because Double.POSITIVE_INFINITY represents a specific number, so comparing it to itself using == should return true.

This behaviour is specified explicitly in JLS Sec 15.21.1:

Floating-point equality testing is performed in accordance with the rules of the IEEE 754 standard:

...

Otherwise, two distinct floating-point values are considered unequal by the equality operators.

In particular, there is one value representing positive infinity and one value representing negative infinity; each compares equal only to itself, and each compares unequal to all other values.

Infinity is equal to Infinity in Java

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