This question already has answers here:
Semicolon at end of 'if' statement
(18 answers)
Closed 9 years ago.
By "empty if-statement", I mean something like this (note the semicolon):
if (condition);
I'm having trouble thinking of an application for this. With a while loop you can do this:
while (callUntilReturnsFalse());
But there's no such application for an if-statement. What's more, the Java compiler doesn't issue an error or a warning when confronted with such a statement. This can lead to large and silent problems, especially with a long and convoluted statement:
if ((functionA() && functionB(getFoo()) ||
checkForComplexCondition(arg1, arg2, getBar(getFoo())));
{
doStuff();
}
My question is: why is this allowed in Java? And, more importantly, can I enable an option to cause a warning when this happens?
(This question was asked before with regards to C#, which does issue a warning, but I was hoping to find a way to cause a warning with Java.)
why is this allowed in Java?
See Java Language Specification (14.6. The Empty Statement):
An empty statement does nothing.
It's simply allowed and it's equivalent to (and will be translated to):
if (condition) { }
Which means, if the condition is true, do nothing.
If you're using eclipse, you can look here, you might find something useful (I'm not sure there exists such an option for semicolon terminator):
Window → Preferences → Java → Compiler → Error/Warnings
EDIT
As #nullptr pointed out in his answer, there exist an IDE warning for this, you need to set warning on Empty statement.
I don't think this is truly relevant to the intent of the question but I think it should be stated as it is relevant to the essence of the question.
There is an effect of an:
if(variable);
if the variable is volatile. It''s effect is to cause a memory barrier to be honoured between the current thread and any other threads accessing the variable.
public volatile variable;
....
if(variable);
See here for a more detailed discussion.
I cannot imagine any real value to putting this kind of statement in your code but I felt it important to note that there is a real effect to this statement in this very specific situation.
There's one construct that I use fairly frequently which the "null statement" makes clearer and easier to understand. Here's an example:
for (int i=0; i < argc; i++)
{
if (argv[i]=="left")
hpos++;
else if (argv[i]=="right")
hpos--;
else if (argv[i]=="up")
;
else if (arv[i]=="down")
;
else fprintf(stderr, "Unknown option \"%s\\n".", argv[i]);
}
In this case, I still want to check for the existence of certain options, while only executing code for some of them. In this case, using the null statement, as above, makes the function and structure of the code more readable and comprehensible to the next guy who has to come along and maintain it.
There are certainly ways to restructure this code to not require the null statement. But I don't believe that its intention will be as clear as in the code snippet.
I found a warning for this in Eclipse as Empty statement:
Thanks to Maroun Maroun for putting me on the right track.
I don't see so much danger in the possibility of an if with an empty statement. The rationale behind it resides in the grammar of the Java language, which allows the empty statement ;:
Block:
{ BlockStatements }
BlockStatements:
{ BlockStatement }
BlockStatement:
LocalVariableDeclarationStatement
ClassOrInterfaceDeclaration
[Identifier :] Statement
LocalVariableDeclarationStatement:
{ VariableModifier } Type VariableDeclarators ;
Statement:
Block
;
Identifier : Statement
StatementExpression ;
if ParExpression Statement [else Statement]
assert Expression [: Expression] ;
switch ParExpression { SwitchBlockStatementGroups }
while ParExpression Statement
do Statement while ParExpression ;
for ( ForControl ) Statement
break [Identifier] ;
continue [Identifier] ;
return [Expression] ;
throw Expression ;
synchronized ParExpression Block
try Block (Catches | [Catches] Finally)
try ResourceSpecification Block [Catches] [Finally]
Mind that this is true for almost all imperative languages.
I mean it can be dangerous and difficult to find as every other empty body in case you forgot any implementation, certainly nothing I would lose the sleep for. In a long and convoluted statement you could get problems because of a ( ) closing the wrong pair of expressions or even for thinking your condition wrong (especially with many && and ||).
I'm mostly a C# developer, although I have a little Java background. But I think my answer applies to both. I suspect it's not an intentional feature, but more of an emergent feature. The grammar of the language goes (roughly)
if (*condition*)
*statement*
Unfortunately the below are both valid statements (I checked, you can drop as many into C# as you like and the compiler doesn't complain):
;
{
}
Therefore the construct that you highlighted is allowed.
The condition could be a function call with side effects. It wouldn't be correct to treat it as an error or warning.
In the statement
if (eval) { //pseudo-code
}
Sometimes data is actually changed in evaluation of (eval). For example, in
while (someIterator.next()) {
}
Calling next() actually changes the state of the someIterator object.
And of course there is the classic example that usually happens from a typo (and is not recommended)
int x;
if (x = getNumberOfWidgets() > 5) {
}
Conventional wisdom advises against coding this way, as it is harder to tell what is going on. However, the statements are legal and so that is one reason why such an 'if' statement is allowed.
I believe that they left it in because it can increase code readability. Even if nothing should be done for a case you may still want to let people know that the case is important.
Related
This question already has answers here:
Semicolon at end of 'if' statement
(18 answers)
Closed 7 years ago.
Just a general Java question. Why does the null variable exist? I'm working with some introduction to CS students and one of the most common mistakes is semi-colons where they are not supposed to be. For example
if(isTired());{
sleep(10);
}
The misplaced semi-colon before the open parenthesis keeps the if statement from working correctly, and I was wondering why the null line did in a Java program. In this example null seems to be a detriment and was wondering when someone would use it when coding.
The null statement is useful because there are some places in Java where you want to do nothing. They are not frequent, but they exist. Often they are the 'compulsory' statements that you have to put in as part of a construct. For example, if you want to write a for loop that only terminates on a 'break', then you want there to be nothing in the 'conditional' part of the for:
for (int i=0;;i++) {
int j = complexCalculation(i);
if (j<0 && complexCondition(j)) {
break;
}
}
If the null statement wasn't allowed, that wouldn't compile. If it was allowed there, but not in other places, that would be inconsistent (and make life more difficult for compiler writers).
The reality is that, once you get to be fairly proficient with the language, errors caused by accidentally adding null statements are rare.
This question already has answers here:
Is it ok if I omit curly braces in Java? [closed]
(16 answers)
Closed 9 years ago.
I am using if condition without braces in java something like
if(somecondition)
//Only one line Business logic
but some told use braces always even one line statement something like this
if(somecondition){
//Only one line Business logic
}
What is the better way according to java sandard?
there's no real "standard". i prefer always using braces because if you dont you risk someone adding an innocent looking logging statement turning your code from
if(somecondition)
//Only one line Business logic
into
if(somecondition)
log.debug("condition was true");
//Only one line Business logic
and then things stop working :-)
That's a matter of taste. I would use braces or else no braces but write all code in one line to improve readability.
Also you might consider using a ternary operator
booleanExpression ? value1 : value2
In addition to #radai answer, if you are a real evil mind, when you see a if with no braces you can do something that will make you ennemies by adding a semi-colon on the same line of the if but at the 800th column of the line(or something).
like
if(condition) /*a loooot of whitespace*/ ;
//Only one line Business logic that will get executed whatever is the condition
This is why i prefer to use braces and recommend people to use them
No naked if statements. You're just asking for trouble. Always use { }
it is better to use braces when checking for errors or updating the code.
imagine.
if(answer.equals("add"))
addedValue += Scanner.readInt();
but you have a new requirement to add only the absolute value, so you change to.
if(answer.equals("add2))
valueToBeAdded = Scanner.readInt();
if(valueToBeAdded < 0) valueToBeAdded = - valueToBeAdded;
addedValue += valueToBeAdded;
this is not a really correct algorithm, is just an example of what can happens.
Using if statement with braces is better way to java standard, because it increase the readability and reduce unwanted error.
The two statements have exactly the same effect but I have suffered so often from the lack of braces that I also always comment that there should be braces even around 1 line statements. This makes the code easier to maintain and can save a lot of headache. My experience shows that one line if statements often turn into multi-line statements on later iterations so what you save by not writing two { the first time, you will give later on.
According to java standard braces are better because if they are not there compiler has to work around more and also would be performance issue.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Does Java evaluate remaining conditions after boolean result is known?
When executing code with multiple && statements in java, does the compiler continue to resolve additional boolean comparisons if the first one resolves to false?
if (1==2 && 3==4 && 4==5) { Do Something }
Once determining 1==2 is false, will the compiler immediately break out of the if statement or will it continue to resolve 3==4 and 4==5 after?
In the case of && it'll stop evaluating the moment it detects that one of the conditions is false. This is called short circuit evaluation.
"SHORT CIRCUIT EVALUATION IN PROGRAMMING LANGUAGES"
In the case of any logical expression most compiler stop evaluation expression as soon as result evaluated. its not only in Java but in almost in every language. (but not all e.g. VB6)
You can also check it as follows:
i = 0 , j =1;
i && ++j;
system.out.print(" j=" + j);
The value of j will be 1, it means ++j was not executed.
does the compiler continue to resolve additional boolean comparisons if the first one resolves to false
The short answer. No! The compiler javac and the JIT analyses statically all the code. It doesn't take short cuts like this. e.g. if the second condition doesn't compile, you will always get a compilation error regardless of the first condition.
What you may have intended to ask is; will the program execute the other conditions if the first or second one is false In this case, it will not.
This doesn't make a difference for the compiler -- all it does is resolve the boolean comparisons into machine code, represented by one or more .class files.
As far as the actual runtime... If I remember correctly from Computer Science class, both Tarun's and Lews's answers are correct -- the comparison will short-circuit as soon as it gets to an expression that isn't true.
IF "1==2" is false than the compiler will immediately break out the if statement.
&&
- The above is a short-circuit AND operator. If the 1st condition results in false the 2nd condition is not evaluated.
Eg:
if ((a==false) && (b==true)){
}
The above will stop after the first evaluation.
- If you want to force the evaluation of both the conditions, then try Non-Short Circuit AND (&)
A sidenote to the keyword 'Efficiency' in your title :- as stated by other answers your basic question is easily answered, so it's not a question of efficiency, but a question if you need for some reason all expressions to be evaluated, for example when executing code which changes state (which would be by the way not good coding at all).
But if you think of 'Efficiency' in terms of executing speed, then you almost never have to optimize on that level, either the compiler will do it for you or you wouldn't notice the difference. It may be important in some time critical operation or in operations which run in very big loops, but in nearly every other case, it's much more important to write clean code, so that another human can read it well.
Basically I'm quite new to Java and just been given some code which reads:
if (n > 1)
l--;
m = l;
Although I'm wondering whether this would be equivalent to either one of these, and if so which and why?
#1
if (n > 1) {
l--;
m = l;
}
OR
#2
if (n > 1) {
l--;
}
m = l;
It's equivalent to the second one. The if statement executes the next statement if its expression evaluates to true. It doesn't matter to the compiler if the next statement is a single statement (as it is in your original code sample) or a block (as it is in your second revision).
It's the same as the second block of code.
When you don't see braces following a "grouping" statement (the if statement, in your example), it means that only the next line falls within the scope of that grouping statement.
Going beyond what the question asks, languages such as Java, C/C++, and C# use braces to declare blocks of code, whereas languages such as Python use whitespace. You can think of line of code is a block by itself. Blocks can be incrementally built by combining more blocks. This is done by grouping blocks; in Java, this is done through curly braces. When the if statement is evaluated (or a for loop, or a while loop, etc), the next outermost block falls under that statement.
the 2nd option.
If there's no { }, only the following statement is part of the If case
Obviously #2, because a if statement only takes the first expression after it into account. That is why it is always important to use brackets around your statements, both for clarity and fewer bugs. There is nothing more frustrating than spending hours wondering why code doesn't work because you forgot to add a bracket somewhere
I had an if statement checking some value, And encountered a weird bug(Not sure!). My code was incorrect syntactically and in result it produced a wrong result, however eclipse didn't raised any error while compiling. Why My below code worked?
if((this.trackPointList.get(point).getTurnOutId().equals(seg.getSegRef().getTurnOut())) && seg.getSegRef().getKind().equals("arc")); // <---- See here I have semicolon
{
... code to run ...
}
Above code check only first condition and ignores seg.getSegRef().getKind().equals("arc") but I guess this should raised an issue at compile time, Am I right? My logic worked once I debugged it by skimming line by line and found this semicolon. I will appreciate if someone could explain, if it is a valid syntax.
Enlighten Me, Please!
The ; makes Java think that the body of the if conditional is complete, even if there is no other code preceding it. In effect, the code in the if statement is executed, but no body exists because the ; is there.
The { ...code to run...} is just a code block that executes, and anything declared inside that block is not visible outside the block. It will always run here because it's not part of the if block.
edit: here's another stack overflow question about the { } blocks: What do curly braces in Java mean by themselves?
The code is syntactically correct. You can write ifs without braces, like:
if(condition) statement;
Having empty statements is also valid. For instance this code is valid:
int a = 0;;
;;;
So an empty if is valid as well, although it doesn't make much sense :)
if(condition);
An if statement followed by a semicolon is called an "empty if statement".
It rarely is of any use, but it's syntactically legal.
You can write something like this
if ( doSomethingThatReturnsABoolean() )
; // Empty statement
else
doSomeOtherThing()
but it would be better to write
if ( !doSomethingThatReturnsABoolean() )
doSomeOtherThing()
Regarding your observation that only the first condition gets checked:
If the first condition returns false the second condition will not get checked, because
(false && secondCondition)
always equals to false, so the value of secondCondition is irrelevant.
The ; after the if(..) statement represents an empty statement that is executed conditionally when the if(..) evaluates to true. The { .. } represents a code block that always executes with it's own scope.
The second condition may be ignored if the first condition is false due to short-circuit evaluation.
if(condition); is equivalent to if(condition){}
same thing with for loop & while loop:
for(;condition;); is equivalent to for(;condition;){}
while(condition); is equivalent to while(condition){}