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Vishal Arora
Greenhorn

Joined: Jul 29, 2005
Posts: 9
Hi,
Given below is the questions that i got in one of the mock exams and the output of it is given as:
"equal unequal equal equal equal" but no explanation is given.
Can somebody explain me why the output is like that. Why it is not "unequal equal equal equal equal ".

public class Test041
{
static double d;
static float f;

public static void main(String args[])
{
compare(Long.MAX_VALUE, Long.MAX_VALUE );

compare(Integer.MAX_VALUE, Integer.MAX_VALUE );

compare(Character.MAX_VALUE, Character.MAX_VALUE);

compare(Short.MAX_VALUE, Short.MAX_VALUE );

compare(Byte.MAX_VALUE, Byte.MAX_VALUE );
}

static void compare(double d, float f)
{
if (f == d) System.out.print(" equal" );
else System.out.print(" unequal");
}
}
Philip Heller
author
Ranch Hand

Joined: Oct 24, 2000
Posts: 119
Interesting question! I would expect both of the first two comparisons to be equal. After all, they look like they compare equal values.

Of course, weird stuff happens at the boundaries of anything, and numeric types are no exception. So here I have to say that this is definitely beyond the scope of what you need to know for the exam. Beware of mock exams that test your knowledge of trivia ... the exam isn't a trivia test.

But anyway, in the 2nd "unequal" result, how can comparing Integer.MAX_VALUE to itself come out unequal? I don't know, but I can offer what I can rule out. When the params are sent to the method, they are converted to a double and a float. There are static methods that tell us the bit pattern of any double or float, so I tried this:




The results are identical: 41dfffffffc00000.

So we know the problem isn't in the method-call conversion. It must be in the comparison. I added these lines:


I was surprised to see that both comparisons result in false! What really happens in the comparison? The float is promoted to a double. Could (double)maxValAsFloat be different from maxValAsDouble? I tried the converting-to-bits trick again, and the bit patterns are identical. So I'm stumped!

-- Phil


Consultant to SCJP team.<br />Co-designer of SCJD exam.<br />Co-author of "Complete Java 2 Certification Study Guide".<br />Author of "Ground-Up Java".
Jim Yingst
Wanderer
Sheriff

Joined: Jan 30, 2000
Posts: 18671
First, let me reiterate what Phil said: this is definitely beyond the scope of what you need to know for the exam. Partly because it depends on obscure details, and partly because it would require a lot more careful calculation than is expected for SCJP problems. So if you read this thread and still don't understand the problem, and you just want to do well on the test: don't worry about it. Just move on.

But if you want more detail anyway...

[B][Phil]:


The results are identical: 41dfffffffc00000.[/B]

That's because of a cut-and-paste error on the last line of the code above. The results aren't really identical.

[B][Phil]:

I was surprised to see that both comparisons result in false![/B]

Well, that's because the equality operator is symmetric. x == y if and only if y == x.

Vishal: what's actually happening here is that when we convert from an integral type to a floating-point type, some roundoff may occur. This means that the value you get may not be exactly what you were expecting, and == may return unexpected results. It doesn't always return strange results, as some integral numbers are already equal to the closest number they would round to in floating-point. And sometimes both left and right sides of the equation get rounded to the same floating-point number. Figuring out exactly how and when this happens requires a good understanding of exactly how floating-point numbers are represented in Java; I'm not going to go into it in detail since it's definitely beyond the scope of the exam. For our purposes, it's enough to say that it appears to happen more or less at random.

This is really just a special case of a general principle when dealing with floating point: they often aren't exactly what you expect them to be. Writing code that depends on exact equality is usually a bad idea where floating-point numbers are concerned. This is why, for example, JUnit has no assertEquals(float, float) method - but it has an assert(float, float, float) where the third parameter describes the maximum allowable difference between the first two parameters. (You could set this third parameter to zero I suppose, but it's probably a bad idea nless you're sure you know what you're doing.) Just remember that == is unreliable for floating point - that's all you really need to know here.
[ August 07, 2005: Message edited by: Jim Yingst ]

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