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help..

 
Greenhorn
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1: Double a = new Double(Double.NaN);
2: Double b = new Double(Double.NaN);
3:
4: if( Double.NaN == Double.NaN )
5: System.out.println("True");
6: else
7: System.out.println("False");
8:
9: if( a.equals(b) )
10: System.out.println("True");
11: else
12: System.out.println("False");


ok the first expression is false. But why the second one is true if the equals() compare the Double Value?
 
Ranch Hand
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Because == doesn't compare the value. Double.NaN is a single class instance, so == will return true because you have the same instance on both sides of the comparison.
Equals has been defined to return false always when at least one of the items being compared is NaN, according to IEEE conventions.
 
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Please use a meaningful topic title!
 
Marcos Vilela
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but if you compile the code you will see that equals() will return true!
 
Greenhorn
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Hi,

I think == method can be used in Primitive datatypes and not objects.

The moment it becomes Double and not double it becomes an object and plus
NaN is a static value. Since you initialized to both the objects to a constant value it evaluates to true. Replace this with Double.NaN with 1.0
it will evaluate to the same result.



Originally posted by Marcos Vilela:
1: Double a = new Double(Double.NaN);
2: Double b = new Double(Double.NaN);
3:
4: if( Double.NaN == Double.NaN )
5: System.out.println("True");
6: else
7: System.out.println("False");
8:
9: if( a.equals(b) )
10: System.out.println("True");
11: else
12: System.out.println("False");


ok the first expression is false. But why the second one is true if the equals() compare the Double Value?

 
Marcos Vilela
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Posts: 18
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== is false because NaN is not equal to anything, not even itself.

The point is: why a.equals(b) returns true?

Thanks..
 
Jagan Tirumalai
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This might be of help from Java Docs 1.4 about NaN under Class Double

NaN
public static final double NaN

A constant holding a Not-a-Number (NaN) value of type double. It is equivalent to the value returned by Double.longBitsToDouble(0x7ff8000000000000L).

See Also:
Constant Field Values
 
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