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while(false)

 
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Hi
this code compiles and does not give error unreachable statement.
Why?
Can anyone please explain

public class ReachTest{
public static void main(String[] args){
boolean h = false;
while(h){
System.out.println("Hi");
h = true; // code compiles with or without this line.
}
}
}
 
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IntelliJ IDE Java
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Some answers here:
http://www.javaranch.com/ubb/Forum24/HTML/012797.html
http://www.javaranch.com/ubb/Forum24/HTML/008990.html
http://www.javaranch.com/ubb/Forum24/HTML/009732.html
http://www.javaranch.com/ubb/Forum24/HTML/010179.html
http://www.javaranch.com/ubb/Forum24/HTML/012859.html
...
And more if you try to search for "while false" using the search engine...
HIH
------------------
Valentin Crettaz
Sun Certified Programmer for Java 2 Platform
[This message has been edited by Valentin Crettaz (edited December 14, 2001).]
 
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try putting..
final boolean h = false;
 
Neha Sawant
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Valentin,
i have read all the forums but still fail to understand when it should compile and when not.
FEING,
I made it final and it is not compiling saying value of h cannot be changed.
I thought since it is not entering the loop it should not give this error.
All this should not come hopefully in my exams.
I have my exams tommorrow.
Bye
neha
 
Valentin Crettaz
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Read this then:
http://java.sun.com/docs/books/jls/second_edition/html/statements.doc.html#236365
And good luck for your exam !
------------------
Valentin Crettaz
Sun Certified Programmer for Java 2 Platform
 
Neha Sawant
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hi Valentin,
i have made a conclusion.
Pleaase tell me i am right
while(false){//some satemant} will not work since false is a constant.
whereas
if it is not a constant but an expression whose result is false will work
say
int x=1;
while(x>2){
//somecode }
this will compile
while(false){x=3;} compiles.
please help
neha
 
Neha Sawant
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sorry my last eg i meant to say wont compile
neha
 
Valentin Crettaz
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right you may have a variable statement being false as a condition to the while loop and it will work. However, if you write "false" or you give a constant (final variable) being false then it won't compile because the statement inside the while loop is unreachable and the compiler can figure that out during compilation.
See, you can change the value of a variable at runtime and that's why the compiler cannot complain about a false condition. However if you have a constant boolean expression whose value is false, the compiler knows that there is no way to change that value at runtime and that the while loop will never be executed, hence the unreachable statement. Try to think as the compiler does... You'll see there is logic in there
HIH
------------------
Valentin Crettaz
Sun Certified Programmer for Java 2 Platform
 
Neha Sawant
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thanks val,
but when i put final boolean h=false;
while(h){//some code)
then why does this compile.
since the compiler should know that h is final and cannot be changed
neha
 
Valentin Crettaz
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on my machine it doesn't compile

and says

What about your code ?

------------------
Valentin Crettaz
Sun Certified Programmer for Java 2 Platform
 
Neha Sawant
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it's compiling on mine.
class reactest{
public static void main(String args[]){
final boolean h=false;
int x=3;
while(h){
x=3;
}
}}
 
Valentin Crettaz
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weird because on mine (jdk1.4 b2 on Win2K) it says

then something must be wrong on your side because such code cannot compile according to the JLS

------------------
Valentin Crettaz
Sun Certified Programmer for Java 2 Platform
 
Neha Sawant
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fine val i will keep that in mind
neha
 
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Just my two cents.
if doesn't behave as while to this respect. "if(false) ...." will compile ok. This is so to allow conditional compilation of the sentences after if based on the variable tested.
 
Neha Sawant
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Thanks Jose.
i am aware of that
neha
 
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