hi expert, want to ask, in java if(NULL==variables) or if(variables==NULL) which will be more effective and what are the difference,
thanks
+9
A:
I would say that there is absolutely no difference in performance between those two expressions.
Interestingly enough however, the compiled bytecode (as emitted by Suns javac) looks a bit different for the two cases.
For boolean b = variable == null:
3: aload_1 // load variable
4: ifnonnull 11 // check if it's null
7: iconst_1 // push 1
8: goto 12
11: iconst_0 // push 0
12: istore_2 // store
For boolean b = null == variable:
3: aconst_null // push null
4: aload_1 // load variable
5: if_acmpne 12 // check if equal
8: iconst_1 // push 1
9: goto 13
12: iconst_0 // push 0
13: istore_2 // store
As @Bozho says, variable == null is the most common, default and preferred style.
For certain situations however, I tend to put the null in front. For instance in the following case:
String line;
while (null != (line = reader.readLine()))
process(line);
aioobe
2010-06-11 08:13:31
+1 good find... but performance-wise it's still the same? :)
Bozho
2010-06-11 08:21:38
I don't know. It would be cool to inspect the output of the JIT compiler for, say, the x86 implementation of suns java, and figure it out once and for all :D You know how to do that?
aioobe
2010-06-11 08:30:22
no. :) (I still wonder why this answer isn't more upvoted..)
Bozho
2010-06-11 08:31:49
I can downvote yours, Bozho, if that helps ;)And i can't prove it, but i believe different versions of the JDK would produce differently optimized byte code. I.e. i don't see why another compiler shouldn't produce the same byte code for both.
Stroboskop
2010-06-11 08:38:34
@Stroboskop, well, java compilers never put *that* much effort into optimizing the bytecode. Basically, most compilers just have a quite naive translation of java-code to bytecode, for the simple reason that the optimization should be deferred to the JIT compilation in which hardware etc is known. That is, I doubt the produced machine code will be different for the two cases. I mean, they're semantically equivalent, and simple enough to be compiled into the most efficient machine code possible anyway.
aioobe
2010-06-11 09:55:37
+6
A:
No difference.
if (variable == null) is (imo) a better programming style.
Note that null is lowercase in Java.
Bozho
2010-06-11 08:14:22
Bruno Rothgiesser
2010-06-11 08:17:05
+5
A:
no difference
(null == variables)
was sometimes used in good old times (C language)
to avoid writing:
(variable = NULL) by mistake
dfens
2010-06-11 08:15:57
+2
A:
The first is a hang over from C where it is perfectly legal to write if(variable = NULL)
Robert
2010-06-11 08:16:27
+5
A:
That's called "Yoda Conditions" and the purpose is to prevent you from accidentally using assignment (=) instead of equality checking (==).
scompt.com
2010-06-11 08:17:27
+2
A:
Short answer: no difference.
Longer answer: there is stylistic difference which is rather subjective. Some people argue constants should be in the left as a defensive style just in case you mistyped == into =. Some people argue constants should be in the right because it's more natural and readable.
A well designed language combined with a good compiler and static analysis tools, paranoia can be minimized, so you should write the most readable and natural code, which would be the constant on the right.
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polygenelubricants
2010-06-11 08:17:52
A:
My opinion: Don't care about such insignificant performance optimizations. If you have bad performance, find and target the real problems/bottlenecks in your code.
chiccodoro
2010-06-11 09:52:32