Comparing float and double primitives in Java

Question

I came across a strange corner of Java.(It seems strange to me)

double dd = 3.5;          
float ff = 3.5f;
System.out.println(dd==ff);   

o/p: true

double dd = 3.2;
float ff = 3.2f;
System.out.println(dd==ff);

o/p: false

I observed that if we compare any two values (a float and a double as I mentioned in the example) with .5 OR .0 like 3.5, 234.5, 645.0 then output is true i.e. two values are equal otherwise output is false though they are equals.

Even I tried to make method strictfp but no luck. Am I missing out on something.

Answer 1

The difference is that 3.5 can be represented exactly in both float and double - whereas 3.2 can't be represented exactly in either type... and the two closest approximations are different.

Imagine we had two fixed-precision decimal types, one of which stored 4 significant digits and one of which stored 8 significant digits, and we asked each of them to store the number closest to "a third" (however we might do that). Then one would have the value 0.3333 and one would have the value 0.33333333.

An equality comparison between float and double first converts the float to a double and then compares the two - which would be equivalent to converting 0.3333 in our "small decimal" type to 0.33330000. It would then compare 0.33330000 and 0.33333333 for equality, and give a result of false.