== behaviour

[Micheal John]

http://www.danchisholm.net/oct1/mybook/chapter3/exam1.html

class EBH011 {
  public static void main (String[] args) {
    float a = Float.POSITIVE_INFINITY;
System.out.println("a ::"+a);
    double b = Double.POSITIVE_INFINITY;
System.out.println("b ::"+b);
    double c = Double.NaN;
System.out.println("c ::"+c);
    System.out.print((a == b)+","+(c == c)+","+(c != c));
}}

output:
a ::Infinity
b ::Infinity
c ::NaN
true,false,true

how (c==c) is false?
how (a==b) is true?

[Mat Austin]

Hi,

The rule for NaN (as defined in the language specification) is "The equality operation == returns false if either operand is NaN". So (NaN == anything) will return false.

When you compare two primitives of different type then (where possible) the smaller primitive type will be promoted to the l;arger type for the purpose of the comparison. In this example, a will be promoted to a double. Having established this, it is just a fact of the language that POSITIVE_INFINITY == POSITIVE_INFINITY.

[kiran reddy]

The positive infinity of type float is promoted to the positive infinity of type double. NaN is not equal to anything including itself.

[Sanjeev Singh]

Mat,
Can you locate the JLS for the same.

[Mat Austin]

The JLS 15.21.1 states that "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." Therefore if you set both values to the one-and-only POSITIVE INFITIY then an equality comparison will return true.

If the question about NaNs is also answered as part of that same sub-section. However, my earlier quote was from the last paragraph of 4.2.3 of the language specification.

[Joe San]

Nan is equal to nothing and you cannot compare nothing to nothing.