Cast int to double then back to int in java
quick question.
Would this always be true?
int i = ...;
double d = i;
if (i == (int) d) ...
Or I need to 开发者_Python百科do rounding to be sure?
if (i == Math.round(d)) ...
Yes, all possible int
values can round-trip to a double
safely.
You can verify it with this code:
for (int i = Integer.MIN_VALUE; ; i++) {
double d = i;
if (i != (int) d) {
throw new IllegalStateException("i can't be converted to double and back: " + i);
}
if (i == Integer.MAX_VALUE) {
break;
}
}
Note that I'm not using a normal for
loop, because it would either skip Integer.MAX_VALUE
or loop indefinitely.
Note that the same is not true for int
/float
or for long
/double
!
If you're on a slow computer or don't have time to run the loop to check for yourself, the relevant part of the Java Language Specification is here § 5.1.2 Widening Conversions:
The following 19 specific conversions on primitive types are called the widening primitive conversions:
- byte to short, int, long, float, or double
- short to int, long, float, or double
- char to int, long, float, or double
- int to long, float, or double
- long to float or double
- float to double
Widening primitive conversions do not lose information about the overall magnitude of a numeric value. Indeed, conversions widening from an integral type to another integral type and from float to double do not lose any information at all; the numeric value is preserved exactly. [...]
(The following section § 5.1.3 Narrowing Primitive Conversions ensures that the way back, double -> int, doesn't loose any information either.)
A variation on Joachim's solution.
int i=Integer.MIN_VALUE;
do {
if(i != (int)(double) i) throw new AssertionError(i + " != (int)(double) "+i);
} while(i++ < Integer.MAX_VALUE);
To find the smallest value which causes an error for a conversion to float.
int i = 0;
do {
if(i != (int)(float) i) throw new AssertionError(i + " != (int)(float) "+i);
} while(i++ < Integer.MAX_VALUE);
prints
java.lang.AssertionError: 16777217 != (int)(float) 16777217
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