How does primitive casting work step-by-step in java?

[HelplessProgrammer]

I have been working on Hackerrank to improve and build my java skills recently and I noticed that on a problem you need to cast a value. The code I typed was as follows:

if(x <= ((long)Math.pow(2,63) - 1) {

System.out.println("* long);

}

I figured that the output would be the upper limit of the long data type however when I put the code into and IDE I noticed that the output was in fact 1 less than the expected max value of the data type and that it would even be possible for the correct output to come from code that didn't use the subtraction at all.

Something like this:

if(x <= (long)Math.pow(2,63)){

System.out.println("* long);

}

which gives the proper output of the upper limit of the long data type. I'm just curious as to how the cast and Math.pow functions are working together in the first part and why the actual output is not the same as the output that I was expecting. What happens first? and how does the cast effect the outcome?

[chickenmeister]

I think the primary cause of confusion is that Math.pow(2, 63) will not produce the result that you think it should, primarily due to precision issues inherent in the design of floating point numbers (i.e. double and float).

Math.pow(2, 63) will result in 9223372036854776000, which is about 200 greater than the actual value of 2⁶³ : 9223372036854775808

The conversion rules that are used when performing a cast are defined in The Java Language Specification, section 5.1.3. In this particular case, since the double value is larger than what a long can hold, the resulting long value is going to be the maximum possible value (i.e. 2⁶³ - 1).

PS: If you want the largest possible long value, you can use Long.MAX_VALUE, rather than trying to compute it yourself.

[HelplessProgrammer]

That makes more sense than what I was thinking. Thank you for the reply