Is it possible for a==a to evaluate false for some variable a in Java?
I've been posed this slightly obscure but interesting question about the开发者_如何学Go behavoir of Java. Any ideas?
Yep, Try this...
public class Main
{
public static void main(String[] args)
{
double a = Double.NaN;
if( a == a ) System.out.println("equal");
}
}
http://www.ideone.com/K0d2j
Yes, for float
or double
NaN
s (but not Float
or Double
). Section 4.2.3 of the JLS 3rd Ed. I believe IEEE 754 defines the operation that way. Those are the only cases.
As giddy said, concurrency can mess it up. I just tried this:
public class Madness {
private static volatile long a = 0;
public static void main(String[] args) {
long good = 0;
long bad = 0;
new Thread() {
{
setDaemon(true);
}
public void run() {
while (true)
a++;
}
}.start();
long print = System.currentTimeMillis() + 1000;
while (true) {
if (a == a)
good++;
else
bad++;
if (System.currentTimeMillis() > print) {
System.out.println(String.format("%d / %d", good, bad));
print = System.currentTimeMillis() + 1000;
}
}
}
}
Output was:
19936409 / 382
38360780 / 640
56895813 / 898
75827635 / 1159
94500958 / 1423
113184503 / 1701
131711068 / 1960
150423573 / 2239
168898106 / 2509
Admittedly, this is a case specifically designed to cause this. Removing the volatile on a
changes things. I see some initial "bad" hits, but then it seems to be all good. I haven't investigated, but I have a feeling it's something to do with hot spot optimizing the code after some number of iterations.
The code for Double.isNaN()
static public boolean isNaN(double v) {
return (v != v);
}
This sounds like an interview questions. A similar questions when is x == x + 0
false which has more answers. ;)
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