Why doesn't it work when I try to compare doubles in java?

Question

I get that this is a dumb question but I couldn't figure this out on google so this is my last resort.

Why is it that when I try to compare two doubles that are variables that I know are equal, it returns false? Here's an example that I hope is more clear:

double x = 323.23
double y = 323.23
System.out.println(x==y)

Output:

false

Why does this happen? Also, why does it print true if you just compare two doubles that are not variables or a double that is a variable and a double that is not? example:

double x = 323.23
System.out.println(x==323.23)
System.out.println(323.23==323.23)

output:

true
true

don't compare floats with `==`: http://www.cygnus-software.com/papers/comparingfloats/Comparing%20floating%20point%20numbers.htm — perreal, Jun 08 '14 at 06:48
Seriously dude, you should check your facts before posting Questions like this. Your code will not output "false" on any conformant implementation of Java. Now, you may be seeing strange results with floating point and `==` in other code ... but if you are going to post bogus examples like this instead of the real code, we can't help you. — Stephen C, Jun 08 '14 at 06:56
Contrary to the notice, I did not mark this as a duplicate. I voted to close for being unclear. — Patricia Shanahan, Jun 08 '14 at 10:02
It returns true when I print it out; could you share the whole document of code that you have to see if there is a problem somewhere else? — James, Jun 08 '14 at 06:47

score -7 · Answer 1 · answered Jun 08 '14 at 06:52

-7

You should not compare doubles on an objet level. Your sample prints true because the compile just optimizes your code so that x and y point to the same object. Instead use Double.equals(). If you think about how doubles can be represented even equal values can have different bit configurations. If checking lesser/greater use the Double.compareTo() method.

answered Jun 08 '14 at 06:52

Hannes

2,018
25
32

1

What does `Double` and objects have to do with this question? – Sotirios Delimanolis Jun 08 '14 at 06:56
In addition, `Double.compareTo()` has different semantics than `==`, `<=`, etc, so it can't be used exactly as a drop-in replacement. – awksp Jun 08 '14 at 15:33

Why doesn't it work when I try to compare doubles in java?

1 Answers1