问题
I know that very similar questions have been asked and answered already, I read the ones I was able to locate and still not 100% clear.
Considering this code snippet:
public static void fooMethod {
while(<...>) {
....
final int temp = <something>;
....
}
}
No inner classes, nothing else special or unusual. Seems counter-intuitive to me.
Does declaring a local variable final in the above sample serve any purpose whatsoever?
Do I understand correctly that with or without final here compiler will produce exactly the same byte-code?
Am I missing something here? If it's an RTFM case, please point me in the right direction.
Follow-up Question (if I may)
What do I gain and/or lose by re-writing like this (with the understanding that temp does not have to be a primitive)?
public static void fooMethod2 {
int temp;
while(<...>) {
....
temp = <something>;
....
}
}
回答1:
In a few words: The final keyword, when used in local variables and parameters, does not make it to the generated bytecode (.class file) and, as expected, its use has no effect during runtime. (Compile-time, it could make a diference, though, check below.)
In those cases, when not enforced due to anonymous inner classes, it is merely a style choice, useful in documenting the intended scope of the variable.
The tests below confirm that information.
1: If the compiler can make something of it, using final makes difference:
Look at this snippet:
boolean zZ = true;
while (zZ) {
int xX = 1001; // <------------- xX
int yY = 1002; // <------------- yY
zZ = (xX == yY);
}
Two int variables, xX and yY. First time declared both as final and second time, took away the final from both. Here are the generated bytecodes (printed with javap -c):
Both final:
0: iconst_1 // pushes int 1 (true) onto the stack
1: istore_1 // stores the int on top of the stack into var zZ
2: goto 15
5: sipush 1001 // pushes 1001 onto the operand stack
8: istore_2 // stores on xX
9: sipush 1002 // pushes 1002 onto the operand stack
12: istore_3 // stores on yY
13: iconst_0 // pushes 0 (false): does not compare!! <---------
14: istore_1 // stores on zZ
15: iload_1 // loads zZ
16: ifne 5 // goes to 5 if top int (zZ) is not 0
19: return
Both non-final:
// 0: to 12: all the same
13: iload_2 // pushes xX onto the stack
14: iload_3 // pushes yY onto the stack
15: if_icmpne 22 // here it compares xX and yY! <------------
18: iconst_1
19: goto 23
22: iconst_0
23: istore_1
24: iload_1
25: ifne 5
28: return
In the case above, when they are final, the compiler knows that they are not equal and never compares them (false is generated in the bytecode wherever xX == yY is).
From this, we can conclude, bytecode-wise, the compiler does can do some optimization on the generated code when using final. (I'm not saying they are meaningful, but for sure final is not only a style choice here.)
2: If the compiler can't conclude anything, using final on local vars is just a design choice:
Now take the following code:
boolean zZ = true;
int aA = 1001;
int bB = 1002;
while (zZ) {
final int xX = aA; // <------- took away the "final" here, didnt matter
final int yY = bB; // <------- took away the "final" here, didnt matter
zZ = (xX == yY);
}
In this case, even using final, the compiler cannot tell compiler-time if xX and yY are equal, right?
Because of this, we can see: the generated bytecode is exactly the same (same MD5!) when we generate the class with or without final.
While, in the general case, some say and others disagree that there are performance benefits of using final, in local blocks, final is definitely only a style choice.
3: Local variables inside or outside loops - no difference at all:
The generated bytecode for this snippet...
boolean zZ = true;
int aA = 1001, bB = 1002;
while (zZ) {
int xX = aA; // <--- declaration is inside WHILE
int yY = bB;
zZ = (xX == yY);
}
...and the generated bytecode for this snippet...
boolean zZ = true;
int aA = 1001, bB = 1002;
int xX, yY; // <--- declaration is outside WHILE
while (zZ) {
xX = aA;
yY = bB;
zZ = (xX == yY);
}
...are exactly the same (only the line numbers changed, of course).
Other tests using objects (not only primitive typed variables) showed the same behaviour.
It is safe to conclude, then, if not used elsewhere, declaring local variables inside or outside loops is pretty much a design choice, with no bytecode effects.
Note: All tests were made under Oracle's JRE, version 1.7.0_13.
回答2:
final is a keyword for constant variables. Declaring it final prevents you from reassigning it later inside the loop.
temp will be re-declared on every iteration, regardless of whether it is final or not.
for example:
while (...)
{
final int temp = ...;
temp = 5; // compiler error
}
But if its not constant (final):
while (...)
{
int temp = ...;
temp = 5; // fine
}
回答3:
Consider this from a completly different perspective: In functional programming languages, it's the normal case that almost all assignments are final, and that the classes are immutable. That means that non-final assignments and/or mutable classes are the exception.
If your code was written in Scala, the IntelliJ IDE would show a hint "that this assignment can be changed to be final".
I really appreciate "finals", because if you read your code later, you see at the first glance that this assignment never changes some lines further down. And if you know that the instance is immutable, this will help as well.
Further, if you use "finals" consistently, the non-final ones will gain visibility, and these variables are normally the most important ones to observe.
来源：https://stackoverflow.com/questions/16755554/declaring-local-variable-as-final-in-a-loop