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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.
Related
I have a program I need to implement that has the following code:
for (int n = 1024; true; n+=n)
I cannot find any other examples of java loops having such a format. What does this mean? I've tried to research it, but I don't even know what to search for - it's totally foreign to me.
The basic for statement is described in the language spec:
for ( [ForInit] ; [Expression] ; [ForUpdate] ) Statement
You are asking about the case when Expression is true. (The square brackets above mean it is optional).
The meaning of that is described just below, in Sec 14.14.1.2:
If the Expression is not present, or it is present and the value resulting from its evaluation (including any possible unboxing) is true, then the contained Statement is executed.
...
If the Expression is present and the value resulting from its evaluation (including any possible unboxing) is false, no further action is taken and the forstatement completes normally.
So, Expression is present, and evaluates to true (because true evaluates to true). Hence, Statement is executed, and will continue to be executed because Expression remains true.
As such, it is an infinite loop (unless there is a break, return, throw or System.exit inside the loop).
Why you dont use another loop? Use for example do-while instead of
for (int n = 1024; true; n+=n)
You can make a work around with:
int n=1024;
do{
//your code
n+=n;
}while(condition==false);
This question already has answers here:
If statement executing all conditions
(9 answers)
Closed 4 years ago.
Got a basic if statement question, so I have an if statement in Java as shown here:
if (
!isOmitted(word,map.get(word.length()+1)) &&
!isInserted(word,map.get(word.length()-1)) &&
!isTransposed(word,map.get(word.length())) &&
!isSubstituted(word,map.get(word.length())) &&
!isCapital(word,map.get(word.length())))
{
noSuggestion=true;
}
where each individual method works perfectly as desired. Is there any way for java to check all conditions even when one is false? I know that the nature of the && operator is that as soon as a condition does not hold true, there is no point in checking the remaining conditions, as the entire condition is going to be set to false, but I was hoping I could do something like this in order to keep my code someone cleaner. I know I can use boolean variables, and assign the returned value to 5 different variables, but is there any other work around to force every condition to be checked? Thanks a lot in advanced
A single & is a non-short-circuit operand - ie both sides are evaluated irrespective of whether required.
More info here - https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.22.2
However, this sounds like a poor design. In fact, some code analysis tools would automatically flag these operators as suspicious for this reason. Typically you would want to call each method as they make some change to the state of your objects, which is not something you would expect in an if statement. But your method names do not suggest this. What are you trying to achieve?
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.
I have a project that reads: "There will be no conditional statements (if, switch, or,…)." I'm not sure if this includes for and while loops, since both technically run on conditions. Could I get away with saying that they're "conditional loops" instead?
It would probably be acceptable to use loops (for, while,...), but you would need to check with the project author. I tend to treat loops and conditional statements separately, as they usually have different purposes...
Conditional statements like if and switch will make a choice out of a list of options. They only run once.
Loops like for and while are typically designed to run a piece of code multiple times.
Of course this is only a generalisation, and everyone probably has a different opinion, but I certainly treat them differently because they have different primary purposes.
For extra credit, Wikipedia seems to agree. The If Statement is a conditional operator, and the For Loop is an iteration statement.
for and while loops use (terminating) conditions, not conditional statements, so on that basis loops are OK.
Apart from loops, another option would be the ternary operator ? - it's not a statement, it's an operator, and you may be able to code some conditional flow using these, ie this code:
int x;
if (<some condition>)
x = 1;
else
x = 2;
may be coded using the ternary operator as:
int x = <some condition> ? 1 : 2;
i saw somewhere in GWT code , it was something like this
assert display instanceof Widget : "display must extend Widget";
The assert keyword, as the name implies, makes an assertion about the code. It is used to specify something that holds true all the time -- or that, at least, should be true!
The assert keyword is followed by a boolean value (true or false), or an expression, to be evaluated at runtime, that returns a boolean.
assert true;
assert 1 == 1;
If, for any reason, the boolean expression evaluates to false, then an AssertionError is thrown.
// this will throw an AssertionError:
int x = 1;
assert x == 2;
When you use it, you make a clear statement about the state of your program on a given point, which can make it easier for readers to follow through your code.
There's a programming paradigm called program by contract, in which pieces of code make statements about the pre-conditions that must hold true for them to execute properly, and the post-conditions, that are guaranteed to hold true after their execution. You can use the assert keyword to implement this.
For example, if you write a method that calculates the square root of a number, it will only work for numbers that are greater than or equal to zero, and the result is guaranteed to satisfy the same conditions:
public double sqrt(final double x) {
assert x >= 0 : "Cannot calculate the square root of a negative number!"
double result = ...;
assert result >= 0 : "Something went wrong when calculating the square root!"
return result;
}
The most interesting aspect of assertions is that you can ask the compiler to remove them from the bytecode (by means of the -disableassertion argument), so that you won't get any kind of performance penalty at runtime on production. For this precise reason, it is of fundamental importance that the expression to be evaluate does not cause side-effects, that is, the expression should look like a pure mathematical function. Otherwise, the behavior of your program could change if the compiler removed your assertions.
Finally, if the assertions are compiled into the bytecode, they can be read by a software that will automatically generate tests that will try to break your code. It can be useful to find bugs earlier!
The assert keyword was introduced in 1.4 (follow that link for a complete description). It is a shorthand to throw an exception at runtime if a condition is not satisfied.
Think of it as
assert condition : message
as
if ( ! condition ) {
throw new AssertionError ( message ) ;
}
The idea is to give developers an easy way to help users (in your case GWT API users) to detect common errors/pitfalls
When it was introduced, the assert statement became a reserved word and that caused a few compilation issues when old code was recompiled for I.4. Especially for JUnit test suites where there was a much used assert() method. JUnit reacted by replacing assert with assertTrue()
It means that if display isn't an object of type Widget, you'll get an AssertionError with the text string that follows the assertion. Assertions are helpful for debugging.
assert keyword is used to simplify the userdefined exception.what happens,to define a userdefined exception we have to create our own exception class by defining the exception causing condition first then we have to throw that in our program.
but from java 1.5 onwards we have a keyword as assert where only we have to write assert(condition) if condtion is true it executes the other part of the program or else if it is false the it creates the object of AssertionError class and we have to handle it.
so no need to define our userdefind error.
The following text (emphasis mine) explains various forms of assertions clearly:
The assertion statement has two forms.
The first, simpler form is:
assert Expression1 ;
where Expression1 is a boolean expression. When
the system runs the assertion, it evaluates Expression1 and if it is
false throws an AssertionError with no detail message.
The second form of the assertion statement is:
assert Expression1 : Expression2 ; (Your example falls here)
where: Expression1 is a boolean expression. Expression2 is an expression that
has a value. (It cannot be an invocation of a method that is declared
void.) Use this version of the assert statement to provide a detail
message for the AssertionError. The system passes the value of
Expression2 to the appropriate AssertionError constructor, which uses
the string representation of the value as the error's detail message.
Also, refer the following oracle link for detailed information:
http://docs.oracle.com/javase/7/docs/technotes/guides/language/assert.html